WO2018015224A1 - Oil recovery method - Google Patents

Abstract

A method for recovering crude oil from a reservoir wherein the reservoir has a temperature in the range of 20 to 150°C and the formation water has a pH of less than 7.5 and a concentration of dissolved hydrogen sulfide of less than 10 mg/L, (0.29 mmol/L), the method comprising: injecting an aqueous displacement fluid into the reservoir and recovering crude oil from the reservoir characterized in that the aqueous displacement fluid has: (a) (i) at least one zinc salt, (ii) at least one chloride salt, and optionally, (iii) at least one bromide salt dissolved therein; (b) a concentration of dissolved zinc in the range of 100 to 3,750 mg/L, preferably, 175 to 3,750 mg/L, wherein the dissolved zinc is in the form of hydrated Zn²⁺ ions, one or more hydrated [ZnCl_4-n]^n-2 species wherein n is an integer selected from 0, 1, 2 and 3, and, optionally, one or more hydrated [ZnBr_4-n]^n-2 species wherein n is an integer selected from 0, 1, 2 and 3; (c) a mol% of bromide anions of from 0 to 10 mol% based on the total molar concentration of chloride and bromide anions; (d) a pH of less than 7.5; and (e) a mole fraction of the sum of the hydrated [ZnCl_4-n]^n-2 species of at least 0.4, preferably, at least 0.5, more preferably, at least 0.75, most preferably, at least 0.9 based on the total molar concentration of dissolved zinc species.

Description

OIL RECOVERY METHOD

The present invention relates to a method for injecting an aqueous solution of a zinc salt into a reservoir for recovery of crude oil therefrom.

Background

It has long been known that only a portion of the oil can be recovered from an oil- bearing reservoir as a result of the natural energy of the reservoir. So-called secondary recovery techniques are used to recover additional oil from a reservoir, the simplest method of which is by direct replacement with another medium, for example, water (referred to in the art as "waterflooding"). Enhanced oil recovery techniques involve adding additives such as surfactants or polymers to a waterflood to further increase the recovery of oil from a reservoir.

US 5,123,488 relates to a method for improved displacement efficiency where two horizontal wells are used to remove hydrocarbonaceous fluids from a formation oil reservoir. A first liquid immiscible with hydrocarbonaceous fluids contained in the formation is injected into a lower horizontal well. This first liquid has a specific gravity greater than that of the hydrocarbonaceous fluids which causes the hydrocarbonaceous fluids to be displaced upwardly in the formation. Thereafter, a second liquid having a specific gravity greater than the first liquid is injected into a lower horizontal well. This causes the first liquid and hydrocarbonaceous fluids to be displaced upwardly toward a second horizontal well. Thereafter, hydrocarbonaceous fluids are removed from the formation by the upper horizontal well. Suitable displacing liquids are said to include seawater, brackish water, brine solutions, and mixtures thereof. It is also taught that zinc chloride and zinc bromide may be used to form salt solutions. Table 1 of US 5,123,488 lists examples of suitable high density fluids including: ZnCl₂ solutions having

concentrations of 2, 4, 8, 12 and 70 (sat.) % by weight corresponding to specific gravities at 20°C of 1.0167, 1.0350, 1.0715, 1.1085 and 1.9620; sodium chloride brines having concentrations of between 2 and 26 (sat.) % by weight corresponding to specific gravities at 20°C of between 1.0144 and 1.2025; potassium chloride brines having concentrations of between 2 and 24 (sat.) % by weight corresponding to specific gravities at 20°C in the range of 1.0110 and 1.1623; and calcium chloride brines having concentrations of between 2 and 40 (sat.) % by weight corresponding to specific gravities at 20°C in the range of 1.0148 and 1.3957. The method of US 5,123,488 therefore relies on the high densities of the first and second immiscible liquids to displace the hydrocarbonaceous fluids upwardly in the reservoir.

It has now been found that incremental oil recovery may be achieved by injecting an aqueous solution of a zinc salt into an oil-bearing reservoir wherein the amount of incremental oil recovery is dependent on the form of the dissolved zinc species which, in turn, is dependent on a number of parameters including: (a) the reservoir temperature; (b) the pH of the formation water; (c) the concentration of dissolved hydrogen sulfide in the formation water; (d) the pH of the injected aqueous solution; (e) the concentration of any dissolved hydrogen sulfide in the aqueous solution; (f) the concentration of chloride anions and of any bromide anions in the aqueous solution; (g) the concentration of any inorganic zinc-complexing anions (other than chloride and bromide) in the aqueous solution; (h) the concentration of any organic zinc-complexing anions in the aqueous solution; and (i) the concentration of any organic zinc-complexing ligands in the aqueous solution.

Summary of the Invention

According to the present invention, there is provided a method for recovering crude oil from a reservoir comprising a porous and permeable rock having crude oil and formation water in the pore space thereof wherein the reservoir has a temperature in the range of 20 to 150°C and the formation water has a pH of less than 7.5 and a concentration of dissolved hydrogen sulfide of less than 10 mg/L, (0.29 mmol/L), the method comprising injecting an aqueous displacement fluid into the reservoir and recovering crude oil from the reservoir wherein the aqueous displacement fluid comprises a solution of a zinc salt in an aqueous solvent having (i) at least one chloride salt, and optionally, (ii) at least one bromide salt dissolved therein, characterized in that the aqueous displacement fluid has: (a) a concentration of dissolved zinc in the range of 100 to 3,750 mg/L (1.52 to 57.35 mmol/L), preferably, 175 to 3,750 mg/L (2.66 to 57.35 mmol/L), wherein the dissolved zinc is in the form of hydrated Zn²⁺ ions, one or more hydrated [ZnCl_4-n]^n~2 species wherein n is an integer selected from 0, 1, 2 and 3, and, optionally, one or more hydrated [ZnBr_4-n]^n~ species wherein n is an integer selected from 0, 1, 2 and 3;

(b) a mol% of bromide anions of from 0 to 10 mol% based on the total molar concentration of chloride and bromide anions;

(c) a pH of less than 7.5; and (d) a mole fraction of the sum of the hydrated [ZnC -n]"^"2 species of at least 0.4, preferably, at least 0.5, more preferably, at least 0.75, most preferably, at least 0.9 based on the total molar concentration of dissolved zinc species.

It has been found, from coreflood experiments, that incremental oil recovery is achieved when the aqueous displacement fluid has a minimum dissolved zinc

concentration of at least 100 mg/L (1.52 mmol/L), preferably, at least 175 mg/L(2.66 mmol L), more preferably, at least 200 mg/L (3.058 mmol/L), most preferably, at least 400 mg/L (6.08 mmol/L), in particular, at least 500 mg/L (7.60 mmol/L).

Coreflood experiments have also shown that incremental oil recovery declines significantly when the mole fraction of the sum of the hydrated [ZnCl_4-n]^n"2 species (which may be either a measured value a predicted value determined from geochemical modelling studies) in the aqueous displacement fluid is less than about 0.4, in particular, less than 0.35, at the temperature of the coreflood experiment.

Without wishing to be bound by any theory, it is believed that the one or more hydrated [ZnCl_4-n]^n"2 species (wherein n is an integer selected from 0, 1, 2 and 3) are more active in releasing incremental oil from a reservoir rock than hydrated Zn ions.

Accordingly, a larger bank of mobile oil is swept through the reservoir towards the production well than would occur if the dissolved zinc was predominantly in the form of hydrated Zn²⁺ ions. It is also believed that a further advantage of the aqueous displacement fluid employed in the method of the present invention is that degree of loss of the hydrated [ZnCl_4-n]^n"2 species (wherein n is an integer selected from 0, 1, 2 or 3) to the reservoir is significantly lower than the degree of loss of hydrated Zn²⁺ ions to the reservoir.

Definitions

By "hydrated [ZnCl₄._n]^n"2 species" is meant dissolved [ZnCl_4-n]^n"2 species having a solvation shell comprised of water molecules, also referred to in the art as a hydration shell. Without wishing to be bound by any theory, the [ZnC -J^11"2 species are thought to have both chloride ligands and water ligands, with the possible exception of the [ZnCl₄]^" species which are deemed to be tetrahedral complexes having only chloride ligands.

By "hydrated Zn²⁺ ions" is meant dissolved Zn ⁺ ions having a solvation shell comprised only of water molecules. Without wishing to be bound by any theory, hydrated Zn²⁺ ions are believed to exist in aqueous solutions in the form of the hexaaqua complex [Zn(¾0)₆]²⁺. Hereinafter, the one or more hydrated [ZnCl_4-n]^n"2 species wherein n is an integer selected from 0, 1, 2 or 3 will be referred to as "[ZnCi_4-n]^n~2 species" and the hydrated Zn²⁺ ions will be referred to as "Zn ions".

The person skilled in the art will understand that the unit "mg/L" correspond to the unit "ppmv" (parts per million on a volume of water basis).

By "formation water" is meant the water associated with the reservoir rock, whether by inclusion in pores or between grains or otherwise. This formation water may comprise connate water, any invading aquifer water and any previously injected water.

By "produced water" is meant water separated from oil at a production facility.

The term "bank of mobile oil" is well known to the person skilled in the art and refers to a portion of the reservoir where the oil saturation is increased because of the application of an improved oil recovery method.

Detailed Description

It has been found from geochemical modelling of zinc speciation, that the

concentrations of the desired [ZnCl_4-n]^n~2 species in the aqueous displacement fluid, and hence the mole fraction of the sum of the [ZnCl₄._n]^n"2 species, are dependent upon:

(1) P¾

(2) Temperature;

(3) The chloride anion concentration of the aqueous displacement fluid;

(4) The concentration of any bromide anions in the aqueous displacement fluid;

(5) The concentration of any zinc-complexing inorganic anions in the aqueous

displacement fluid, i.e., anions that compete with chloride anions and the optional bromide anions to form water-soluble complexes with zinc, for example, sulfate anions;

(6) The concentration of any zinc-complexing inorganic anions that form water- insoluble complexes with zinc, for example, hydro sulfide (HS^") anions;

(7) The concentration of any organic zinc-complexing anions, for example, organic carboxylate anions, that compete with chloride anions and the optional bromide anions to form water-insoluble or water-soluble complexes with zinc; and

(8) The concentration of organic zinc-complexing ligands, including both

monodentate and polydentate ligands in the aqueous displacement fluid.

The influence of each of these parameters on zinc speciation is discussed in more detail below.

Geochemical modelling has shown that a portion of the dissolved zinc may precipitate from solution under alkaline conditions (for example, in the form of zinc hydroxide). Geochemical modelling of zinc speciation has also shown that pH values of less than 7.5, in particular, less than 7.0, are optimal for forming the desired [ZnCl₄._n]^n_2 species whilst minimizing the concentrations of other complexed zinc species such as [ZnHC0₃]⁺,

[ZnOH]⁺ and [Zn(OH)₂]. In particular, [ZnHC0₃]⁺, [ZnOH]⁺ and [Zn(OH)₂] species are present in solution in only trace amounts at pH values of less than 7.0, especially at pH values of less than 6.5 and may be ignored when modelling zinc speciation.

The person skilled in the art will understand that the injected aqueous displacement fluid will eventually buffer to the pH of the formation water.

Accordingly, the concentration of the desired [ZnCl_4-n]^n"2 species is optimized when:

(a) the aqueous displacement fluid, prior to injection into the reservoir, has a pH of less than 7.5, preferably, less than 7.0, more preferably, less than 6.5, most preferably, in the range of 4.5 to 6.5; and

(b) the aqueous displacement fluid is injected into a reservoir having a formation water with a pH of less than 7.5, preferably, less than 7.0, more preferably, less than 6.5, most preferably, in the range of 4.5 to 6.5. Typically, the reservoir may comprise a sandstone rock comprising a formation water having a pH in the range of 4.5 to 6.5.

The pH of a formation water may be determined using a sample of formation water separated from a sample of produced fluid (water, oil and gas) taken downhole in a production well, at a wellhead, from a flow line or at a production facility. The person skilled in the art will understand that the pH of the separated formation water is determined immediately after the water has been reduced to atmospheric conditions. Alternatively, the pH of a formation water may be estimated using a suitable software package (for example, ScaleChem™ supplied by OLI Systems, Inc.) that combines a geochemical model with a PVT (Pressure- Volume- Temperature) model.

Geochemical modelling of zinc speciation has also shown that bromide species are less favored than chloride species owing to the weaker binding of bromide to zinc than chloride to zinc. Accordingly, [ZnCl_4-n]^n~2 species predominate over the corresponding [ZnBr_4-n]^n"2 species for aqueous displacement fluids containing both chloride and bromide anions (when the concentration of bromide anions is up to 10 mol% based on the total molar concentration of chloride and bromide anions). Thus, the aqueous displacement fluid employed in the method of the present invention contains a negligible amount of [ZnBr_4-n]^n"2 species at reservoir conditions (reservoir temperature and pH of the formation water). By a "negligible amount of [ZnBr_4-n]^n~2 species" is meant less than about 1 mol% of the total dissolved zinc is in the form of [ZnBr_4-n]^n"2 species.

Geochemical modelling has also shown that zinc chloride species predominate over zinc sulfate species when the mol% of sulfate anions in an aqueous solution is less than 0.5 mol% (based on chloride anion concentration). The impact of sulfate anion concentration on zinc speciation is discussed in more detail below.

Zinc speciation for aqueous fluids may be modelled using any suitable geochemical modelling software, for example, PHREEQC software, provided by the US Geological Survey (USGS) or Geochemist's Workbench provided by Aqueous Solutions LLC. The data that is inputted into the geochemical model includes: temperature; pH; solvent (water); the concentration of chloride salt(s) selected from chloride salts of Group IA and Group IIA metals (in particular, sodium chloride, potassium chloride, calcium chloride and magnesium chloride); the concentration of zinc salt(s); the concentration of optional bromide salt(s) selected from bromide salts of Group IA and Group IIA metals (in particular, sodium bromide, potassium bromide, calcium bromide and magnesium bromide); the concentration of any other optional dissolved salt(s) selected from sulfate salt(s) and salts of zinc-complexing organic anions such as carboxylate salt(s); the concentration of any other optional dissolved species such as zinc-complexing organic ligands and hydrosulfuric acid; and, experimentally determined chemical equilibria for chemical species, including both zinc species and non-zinc species, that may form under the modelled conditions, these chemical equilibria being inputted as equations that are a function of temperature and pH.

The geochemical model uses the experimentally determined chemical equilibria to calculate the concentrations of the individual [ZnCl_4-n]"^"2 species (and, where the modelled aqueous solution contains a bromide salt, the concentrations of the individual [ZnBr_4-n]^n"2 species) at the modelled temperature and pH. Where the modelled aqueous solution contains any optional dissolved solids the model uses the experimentally determined equilibria to calculate the concentrations of any additional zinc species, for example, zinc sulfate species, zinc carboxylate species or zinc species comprising coordinated organic ligands. Typically, the model may perform a number of iterations of these calculations until all chemical species have reached their equilibrium concentrations. Typically, the geochemical model repeats these calculations at a plurality of temperatures that span the reservoir temperature range of 20 to 150°C, for example, at the upper and lower temperature limits of 20 and 150°C and at temperatures separated by intervals of between 10 to 30°C, preferably, intervals of between 15 to 25°C, for example, at 20°C intervals. Where the modelled aqueous fluid contains only chloride salt(s) and the zinc salt is zinc chloride, modelled average numbers of coordinated chloride ligands, Nci, for the hydrated zinc species may be determined using the geochemical model. Thus, average Nci values may be determined using Equation 2 below:

([ZnCl]⁺) + ( ZnCl2]*2 + (.[ZnCl3]^~*3) + ([ZnCU]^2~*4) . . _n .

: (Equation 1 )

[Zn]total ^ ^J

Conversely, if the modelled aqueous fluid contains only bromide salt(s) and the zinc salt is zinc bromide, average values for the number of coordinated bromide ligands, N_Br may be determined using Equation 3 below:

_ ([ZnBr]⁺) + ([ZnBr2]*2) + ([Zn5r3]^""*3) + ([ZnBr4]²~*4) _a^_on2) B^r [Zn]total ^ ^^Ua '

Equations 1 and 2 are therefore based on a modelled aqueous fluid that does not contain any additional inorganic zinc-complexing anions (for example, sulfate or hydro sulfide anions) or any organic zinc-complexing anions (for example, organic carboxylate anions) or organic zinc-complexing ligands, in particular, chelating ligands (for example, ethylenediaminetetraacetic acid (EDTA)).

The zinc speciation geochemical model may be validated using experimentally derived data obtained using any suitable analytical technique including X-ray Absorption (XAS) spectroscopy, for example, Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy and X-ray Absorption Near Edge Structure (XANES) spectroscopy, as described in, "Structural characterization of zinc(II) chloride in aqueous solution and in the protic ionic liquid ethyl ammonium nitrate by X-ray absorption spectroscopy", P D'Angelo et al, The Journal of Chemical Physics 135, 154509 (2011) and "An XAS study of speciation and thermodynamic properties of aqueous zinc bromide complexes at 25-150°C, W Liu et al, Chemical Geology 298-299 (2012) 57-69. Typically, average chloride coordination numbers to zinc, Nci, may be determined from experimental EXAFS data obtained for various aqueous displacement fluids comprising at least one dissolved zinc salt and at least one dissolved chloride salt (in the absence of any dissolved bromide salt and in the absence of any additional inorganic zinc-complexing anions, any organic zinc- complexing anions or organic zinc-complexing organic ligands, and at various

temperatures. Similarly, average bromide coordination numbers to zinc, Νβ_Γ, may be detemiined from experimental EXAFS data obtained for various compositions of aqueous displacement fluids comprising at least one dissolved zinc salt and at least one dissolved bromide salt (in the absence of any dissolved chloride salt, and in the absence of any additional inorganic zinc-complexing anions, any organic zinc-complexing anions or any organic zinc-complexing ligands, and at various temperatures.

Typically, the aqueous displacement fluid employed in these EXAFS tests is a solution of a zinc salt in a synthetic brine and may therefore contain dissolved inorganic solids other than zinc and chloride (or zinc and bromide) such as Group IA metal cations (in particular, sodium and/or potassium cations), or Group IIA metal cations (in particular, calcium and/or magnesium cations). These Group I A and Group IIA metal cations are believed to have no influence on zinc speciation.

The zinc speciation model may then be validated by comparing modelled values of average chloride coordination numbers to zinc (Nci) or average bromide coordination numbers to zinc (Ne_r) with the values derived from the EXAFS data. The modelled average zinc coordination values were found to be a good fit for the experimentally derived values.

The validated geochemical model may then be used to determine the mole fraction of the sum of the [ZnCl_4-n]^n~2 species as a function of increasing chloride concentration (moles/litre) at a plurality of temperatures spanning the reservoir temperature range of 20 to 150°C. These calculations may be performed for aqueous displacement fluids having a constant concentration of dissolved zinc selected from one or more values spanning the range of 100 to 3,750 mg/L and having varying concentrations of chloride anions

(dissolved chloride) and Group IA or Group IIA metal cations and, optionally, containing varying amounts of additional dissolved solids (for example, sulfate anions, zinc- complexing organic anions, zinc-complexing organic ligands or mixtures thereof). The calculations may be initially performed for a number of aqueous fluids that differ only in their dissolved chloride concentrations, for example, aqueous fluids having a constant dissolved zinc concentration with bromide concentration set to zero in the model.

Similarly, the concentrations of other inorganic zinc-complexing anions (such as sulfate and hydrosulfide anions) and of organic zinc-complexing anions (such as carboxylate anions) or organic zinc-complexing ligands (such as ethylenediaminetriacetic acid, EDTA) may be set to zero in the model. Thus, the zinc salt inputted into the model is zinc chloride and the chloride salt inputted into the model is a salt of a Group IA metal cation or a Group IIA metal cation. Preferably, the mole fractions of the sum of the [ZnCl_4-n]^n~2 species, as a function of increasing chloride concentration (moles/litre), are determined at the upper and lower reservoir temperature limits of 20 and 150°C and at temperatures separated by intervals of between 10 to 30°C, preferably, intervals of between 15 to 25°C, for example, at 20°C intervals. In this case, the mole fractions of the sum of the [ZnCl_4-n]^n"2 species are based on the total equilibrium molar concentrations of the hydrated [ZnCl_4-n]^n"2 species and hydrated Zn²⁺ ions, since other zinc species, such as [ZnHC0₃]⁺, [ZnOHf and [Zn(OH)₂] species, are predicted to exist in only trace amounts at the modelled conditions.

The validated geochemical model may be used to determine the mole fraction of the sum of the [ZnCl_4-n]^n"2 species for various aqueous displacement fluids employed in a plurality of coreflood experiments. In each case, the geochemical model was run by inputting the concentrations of salts for the aqueous displacement fluid used in the coreflood experiment and running the model at the temperature of the coreflood experiment and either at a fixed H, for example, at a pH of 5.5 or with a "floating" pH that reaches an equilibrium value of, typically, less than 6.5. The aqueous displacement fluids employed in the coreflood experiments comprised solutions of zinc chloride in synthetic brines. These synthetic brines did not contain any bromide, sulfate or hydrosulfide anions, or any organic zinc-complexing anions or ligands. It was found that incremental oil recovery started to decline significantly in the coreflood experiments for aqueous displacement fluids having a mole fraction of the sum of the [ZnCl_4-n]^n~2 species at or below about 0.4, in particular, below 0.35.

The minimum molar concentration of chloride anions, [Chloride]_mi_n, that achieves the mole fraction of sum of the [ZnC -n]"^"2 species of about 0.4 at a plurality of different modelled temperatures (with bromide anions, other inorganic zinc-complexing anions, organic zinc-complexing anions and organic ligands set to zero in the model) may then be determined. It was found that the minimum chloride anion concentration, [Chloride]_mi_n, varied with reservoir temperature according to Equation 3 below:

[Chloride]_min (mole fraction of 0.4) = 1.9677exp(-0.035 x T) Equation 3 wherein "exp" is the natural exponential function, and T is the reservoir temperature in degrees Centigrade (°C). Thus, significant incremental oil may be recovered from the reservoir provided that the concentration of chloride anions in the aqueous displacement fluid is maintained at or above the value of [Chloride] _mi_n at the reservoir temperature (when the aqueous displacement fluid does not contain any additional zinc complexing anions or organic ligands).

Suitably, the molar concentration of chloride anions in the aqueous displacement fluid may be selected so as to optimize the concentration of [ZnCi_4-n]^n"2 species in the aqueous displacement fluid at the reservoir temperature and hence minimize the concentration of Zn²⁺ ions. It is preferred that the aqueous displacement fluid has a chloride anion concentration that is at or above a minimum chloride anion concentration that is predicted to provide a mole fraction of the sum of the [ZnCl_4-n]^n~2 species at the reservoir temperature of 0.5, more preferably, 0.75, most preferably 0.9 (with bromide anion, other inorganic zinc-complexing anions, organic zinc-complexing anions and organic zinc-complexing ligands set to zero in the model). It has been found, by geochemical modelling of zinc speciation in various aqueous fluids, that the minimum chloride anion concentrations that are predicted to provide mole fractions of the sum of the [ZnCl_4-n]^n"2 species of 0.5, 0.75 and 0.9 vary with reservoir temperature according to Equations 4, 5, and 6 respectively:

[Chloride]_min (mole fraction of 0.5) = 2.3392exp(-0.031 x T) Equation 4

[Chloride]_mj_n (mole fraction of 0.75) 3.2209exp(-0.022 x T) Equation 5 [Chloride]_min (mole fraction of 0.9) = 3.6758exp(-0.014 x T) Equation 6 wherein "exp" and "T" are as defined above. Accordingly, it is preferred that the molar concentration of chloride anions in the aqueous displacement fluid is at or above a value of [Chloride]_mi_n determined using Equation 4, more preferably, at or above a value of

[Chloride]_mi_n determined using Equation 5, and, most preferably, at or above a value of [Chloridej_mi_n determined using Equation 6.

Equations 3 to 6 were obtained using the maximum dissolved zinc concentration of 3750 mg/L. However, Equations 3 to 6 remain valid for all zinc concentrations in the range of 100 to 3750 mg/L. This is because geochemical modelling has shown that the amount of chloride required to reach a given target mole fraction of the sum of the [ZnCl_4- n]^n"2 species increases with increasing zinc concentration. This means that the [Chloride] _mj_n determined at the maximum dissolved zinc concentration of 3750 mg/L will also be effective in achieving at least the desired target mole fraction for the sum of the [ZnCl_4-n]^n"2 at the minimum dissolved zinc concentration of 100 mg/L.

The value of [Chloride] _mj_n determined using Equation 3 that is predicted to give a mole fraction of the sum of the [ZnC -J"^"2 species of about 0.4 (and the preferred values of [Chloride]_rai_n determined using Equations 4, 5 and 6 that are predicted to give mole fractions of the sum of the [ZnCl_4-n]^n_2 species of about 0.5, 0.75 and 0.9 respectively) increases with decreasing temperature. The person skilled in the art will understand that the injected aqueous displacement fluid and any previously injected aqueous fluids are typically at a lower temperature than the reservoir. However, owing to the high specific heat capacity of the reservoir rock, the aqueous displacement fluid will generally equilibrate to the reservoir temperature. Accordingly, the ability to control zinc speciation by adjusting the temperature of the aqueous displacement fluid is limited.

As discussed above, binding of bromide to zinc is significantly weaker than binding of chloride to zinc. This is reflected in the geochemical modelling of zinc speciation which shows that for aqueous fluids containing both chloride and bromide anions, [ZnCl_4-n]^n~2 species predominate over [ZnBr_4-n]^n"2 species. In particular, geochemical modelling of various aqueous fluids having constant halide anion concentrations (wherein halide is selected from chloride and bromide) but varying proportions of bromide anions (with other inorganic zinc-complexing anions, organic zinc-complexing anions and organic zinc- complexing ligands set to zero in the model) shows that insignificant amounts of dissolved zinc are in the form of [ZnBr_4-n]^n"2 species when the bromide anion concentration is at or below 10 mol percent (mol%), based on the total molar amount of chloride and bromide anions. By "insignificant amounts of [ZnBr_4-n]^n"2 species" is meant values of up to 1 mole percent of [ZnBr_4-n]^n~2, based on total moles of dissolved zinc (equivalent to a mole fraction of the sum of the [ZnBr_4-n]^n~2 species of up to 0.01). Accordingly, the minimum chloride anion concentrations determined using Equations 3 to 6 remain valid provided that the bromide anion concentration of the aqueous displacement fluid is less than or equal to 10 mol % based on the total molar concentration of chloride and bromide anions. Thus, a concentration of bromide anions of up to 10 mol% may be tolerated in the aqueous displacement fluid.

Geochemical modelling has shown that sulfate anions compete with chloride anions (and optional bromide anions) to form water-soluble complexes with zinc. Thus, when sulfate anion concentration is set in the model to 5 mol% (based on the molar chloride anion concentration), there is a reduction in the mole fraction of [ZnCl_4-n]^n"2 species across all modelled temperatures. The value of 5 mol% of sulfate (based on the molar chloride anion concentration) inputted into the model was selected as this is a typical sulfate concentration for seawaters and estuarine waters. These modelling studies were performed with bromide anions, inorganic zinc-complexing anions (other than sulfate), organic zinc- complexing anions and organic zinc-complexing ligands set to zero in the model. It was found that the desired mole fractions of the sum of the [ZnCl_4-„]^n"2 species of 0.4, 0.5, 075 and 0.9 respectively may be achieved, across the entire reservoir temperature range of 20 to 150°C, when the aqueous displacement fluid has a concentration of dissolved chloride that is at least about 20% greater than the minimum chloride concentrations determined using Equations 3 to 6 respectively.

It has also been found that the minimum chloride anion concentrations determined using Equations 3 to 6 remain valid provided that the sulfate concentration of the aqueous displacement fluid is less than or equal to 0.5 mol% (based on the molar concentration of chloride anions). Thus, a concentration of sulfate anions of less than or equal to 0.5 mol%, preferably, less than 0.25, more preferably, less than 0.1 mol% (based on the molar concentration of chloride anions) may be tolerated in the aqueous displacement fluid.

The person skilled in the art will understand that formation waters have a relatively low concentration of sulfate anions and therefore mixing of the aqueous displacement fluid with the formation water within the reservoir has a negligible impact on zinc speciation. Thus, the formation water typically has a sulfate anion concentration of less than 100 mg/L, preferably, less than 40 mg/L, for example, less than 10 mg/L.

In order to mitigate the risk of forming water-soluble zinc species of other inorganic zinc-complexing anions, it is preferred that the concentration of chloride and optional bromide anions in the aqueous displacement is substantially in excess of the concentration of other inorganic zinc-complexing anions (such as sulfate and carboxylate anions) that are capable of forming water-soluble zinc species. Thus, the amount of chloride and optional bromide anions is preferably at least 99.5 mol%, more preferably, at least 99.75 mol%, most preferably, at least 99.90 mol%, based on the total molar concentration of inorganic anions (provided that bromide anions comprises at most 10 mol% of the chloride and bromide anions).

Preferably, the concentration of organic zinc-complexing anions in the aqueous displacement fluid (prior to injection into the reservoir) is less than 1 mEq/L, more preferably, less than 0.1 mEq/L, most preferably, less than 0.05 mEq/L wherein the organic zinc-complexing anions have one or more zinc-complexing anionic functional groups, and the equivalent concentration is based on the number of such anionic functional groups in the organic zinc-complexing anion(s). By "mEq/L" is meant "milliEquivalents per litre" which is a term well known to the person skilled in the art. Naturally occurring waters (other than untreated produced waters) that may be used as solvent for the aqueous displacement fluid typically have concentrations of organic zinc-complexing anions of less than 1 mEq/L.

Preferably, the concentration of organic zinc-complexing ligands (including both monodentate and polydentate ligands, in particular, bidentate and tridentate ligands) in the aqueous displacement fluid (prior to injection into the reservoir) is less than 1 mEq/L, more preferably, less than 0.1 mEq/L, most preferably, less than 0.05 mEq/L, in particular, less than 0.01 mEq/L wherein the equivalent concentration is based on the denticity (number of donor groups) of the ligand(s). In particular, it is preferred that additives that chelate zinc (polydentate ligands) such as EDTA are not added to the aqueous

displacement fluid as these polydentate ligands may reduce the concentration of the desired [ZnCl_4-n]^n_2 species in the aqueous displacement. Naturally occurring waters (other than untreated produced waters) that may be used as solvent for the aqueous displacement fluid typically have concentrations of organic zinc complexing ligands of less than 1 mEq/L.

Preferably, the aqueous displacement fluid (prior to injection into the reservoir) contains only trace amounts of inorganic zinc-complexing anions that form insoluble zinc precipitates (hereinafter "precipitate precursor anions"). These precipitate precursor anions include dissolved hydrosulfide, carbonate, hydroxide and phosphate anions. Preferably, the formation water of the reservoir also contains only trace amounts of precipitate precursor anions, for example, concentrations of precipitate precursor anions of less than 2 mg/L.

The formation water of a reservoir may contain dissolved hydrogen sulfide (H₂S). At a pH of less than 7.5, in particular, less than 7.0, the dissolved hydrogen sulfide contained in the formation water is substantially in the form of dissolved hydrosulfide anions.

Injection of the aqueous displacement fluid into a reservoir may therefore lead to precipitation of insoluble zinc sulfide within the reservoir upon dispersive mixing of the aqueous displacement fluid (containing dissolved zinc) with the formation water

(containing dissolved hydrosulfide anions). The person skilled in the art will understand that a low amount of precipitation of zinc sulfide may be tolerated, for example, an amount of precipitation of less than 5 mol% of the initial dissolved zinc. It is therefore preferred that the aqueous displacement fluid is injected into a reservoir having a formation water with a concentration of dissolved hydrogen sulfide of less than 10 mg/L (0.290 mmol/L), preferably less than 5 mg/L (0.145 mmol/L), more preferably less than 2 mg/L, (0.058 mmol/L), most preferably, less than 1 mg/L (0.029 mmol/L).

The person skilled in the art will understand that the concentration of dissolved hydrogen sulfide in the formation water may be determined by analysis of a sample of formation water that is separated from a sample of produced fluids taken at the wellhead, from a flow line or at a production facility. The formation water is analyzed, using techniques well known to the person skilled in art, for both total dissolved sulfur concentration (for example, by inductively coupled plasma atomic emission spectroscopy) and sulfate anion concentration (for example, by ion chromatography). The dissolved hydrogen sulfide concentration is then determined by subtracting the sulfate anion concentration from the total dissolved sulfur concentration. The person skilled in the art will understand that the dissolved hydrogen sulfide concentration of the formation water is determined immediately after separation of the formation water from the associated oil and gaseous phases.

Preferably, the aqueous displacement fluid, prior to injection into the reservoir, contains either no dissolved hydrogen sulfide or trace amounts of hydrogen sulfide of less than 10 mg/L (0.290 mmol/L), preferably less than 5 mg/L (0.145 mmol/L), more preferably less than 2 mg/L, (0.058 mmol/L), most preferably, less than 1 mg/L (0.029 mmol/L), for example, less than 0.5 mg/L (0.0145 mmol/L).

Typically, the aqueous displacement fluid may be formed by adding a zinc salt to a naturally occurring saline water that contains chloride anions and optionally bromide anions and sulfate anions. Typically, the total dissolved solids (TDS) content of the saline water is at least 5,000 mg/L, preferably, at least 10,000 mg/L, more preferably, at least 20,000 mg/L, yet more preferably, at least 30,000 mg/L. Suitably, the TDS content of the saline water is less than 300,000 mg/L preferably, less than 250,000 mg/L. Suitably, the saline water has a total dissolved solids content in the range of 5,000 to 250,000 mg/L, preferably, 10,000 to 250,000 mg/L, more preferably, 15,000 to 225,000 mg/L, most preferably, 30,00 to 200,00 mg/L, in particular, 35, 000 to 100,000 mg/L.

Suitably, the naturally occurring saline water may be a seawater, estuarine water, produced water or aquifer water containing only a trace amount of dissolved hydrogen sulfide ("trace amount" is as defined above). Seawater and estuarine water typically have a bromide anion concentration of about 0.3 mol% (based on total moles of chloride and bromide) which is significantly below the upper limit for the mol% of bromide that may be tolerated in the aqueous displacement fluid. As discussed in more detail below, the person skilled in the art will also understand that seawater and estuarine water contain only trace amounts of carboxylate anions and organic ligands. However, the concentration of sulfate anions in seawater and estuarine water is typically significantly higher than 0.5mol%

(based on the chloride anion content in these waters). Thus, seawaters and estuarine waters typically have concentrations of sulfate anions of from 3.0 to 7.0 mol%, preferably, 4.0 to 6.5 mol%, in particular, 4.5 to 5.5 mol% (based on chloride). If necessary, a chloride salt may be added to the seawater or estuarine water to increase its chloride concentration to a value at least 20% higher, preferably at least 25% higher, than the value of [chloride]_mi_n determined using Equation 3, or to a value at least 20% higher, preferably, at least 25% higher than the values of [Chloride]_mj_n determined using any one of Equations 4 to 6 thereby ensuring that the desired mole fractions of the sum of the [ZnCl_4-n]^n~2 species are achieved. The person skilled in the art would understand that when the chloride concentration of the seawater or estuarine water is already at least 20% higher, preferably, at least 25% higher than the calculated value of [Chloride]_mj_n required to achieve the desired mole fraction of the sum of the [ZnCU-J"^"2 species, there is no requirement to increase the chloride concentration. Suitably, the chloride salt that is added to the seawater or estuarine water may selected from chloride salts of Group IA and Group IIA metals.

Seawater and estuarine water contain zinc at concentrations significantly below the threshold zinc concentration for the aqueous displacement fluid of the present invention of 100 mg/L. For example, the concentration of zinc in seawater is less than 10 nM, in particulai', less than 5 nM such that naturally occurring dissolved zinc does not provide any significant contribution to the total dissolved zinc concentration of an aqueous

displacement fluid that uses seawater, estuarine water as solvent.

The water used as solvent for the aqueous displacement fluid may also be a naturally occurring saline water having a mol% of sulfate anions of less than 0.5 (based on the molar concentration of chloride anions), a mol% of bromide anions of less than or equal to 10 mol% (based on the total molar concentration of chloride and bromide anions) and containing either no hydrogen sulfide or a trace amount of hydrosulfide anion (as defined above). Suitable saline waters may include saline produced waters, saline aquifer waters and mixtures thereof.

The concentration of sulfate anions in produced waters and saline aquifer waters is typically significantly less than 0.5 mol% (based on the molar concentration of chloride anions). Typically, the sulfate concentration of a produced water or saline aquifer water is less than 0.25 mol %, in particular, less than 0.1 mol% (based on the chloride anion concentration). Produced waters and saline aquifer waters are available having naturally occumng sulfate concentrations of less than lOOmg/L, preferably, less than 40mg/L, for example, in the range of 1 to 10 mg/L. The concentration of bromide anions in produced waters and saline aquifer waters is typically significantly less than 10 mol%, for example, is less than or equal to 1 mol% (based on the total molar amounts of chloride and bromide anions). Accordingly, produced waters and saline aquifer waters have acceptable levels of sulfate and bromide anions for use as solvent for the aqueous displacement fluid. If necessary, a chloride salt may be added to the produced water or saline aquifer water to increase the chloride concentration to above the value of [Chloridejmin determined using Equation 3 or to above a preferred value of [Chloride]min determined using one of Equations 4 to 6. However, it is preferred that a produced water or aquifer water is selected having naturally occurring chloride anion concentration above the value of

[Chloridej_mi_n.

Typically, a produced water is subjected to a number of treatments before being used as solvent for the aqueous displacement fluid, including: de-oiling (removal of dispersed oil), and suspended solids removal (for example, removal of particles of sand). The produced water is typically also subjected to disinfection (removal of microorganisms and algae), soluble organics removal and dissolved gas removal (removal of light hydrocarbon gases, carbon dioxide and hydrogen sulfide). Soluble organics removal reduces the concentration of naturally occurring organic zinc-complexing anions (for example, formate, acetate, n-propionate, n-butyrate, n-pentanoate, n-hexanoate, malonate, oxalate, benzoate, alkybenzoates and anions of naphthenic acids) that form complexes with zinc. Typically, the concentration of organic zinc complexing anions in the treated produced water is reduced to a value of less than 0.1 mmol/L such that the water is of acceptable quality for use as solvent for the aqueous displacement fluid. Typically, the concentration of dissolved hydrogen sulfide in the treated produced water is less than 10 mg/L (0.290 mmol/L), preferably less than 5 mg/L (0.145 mmol/L), more preferably less than 2 mg/L, (0.058 mmol/L), most preferably, less than 1 mg/L (0.029 mmol/L), for example, less than 0.5 mg/L (0.0145 mmol/L) such that the water is of acceptable quality for use as solvent for the aqueous displacement fluid.

Soluble organic compounds may be removed from a produced water using techniques well known to the person skilled in the art. For example, adsorption is a widely accepted technology for the removal of soluble hydrocarbons (organic compounds) from produced water. Typically, the produced water may be passed through an adsorption column packed with a porous solid material (adsorbent) such that the hydrocarbons present in the produced water adhere onto the surface of adsorbent and are retained within the porous structure. The effluent from the adsorption column (treated produced water) contains little or no soluble organics. Suitable adsorbents include activated carbon, nutshell media, and modified organoclays.

The concentrations of dissolved zinc in saline produced waters may be in the range of 0.01 to less than 100 mg/L or 0.01 to less than 50mg/L, for example, 0.01 to 5 mg/L. The amount of zinc salt added to the saline water should therefore take into account the presence of any naturally occurring dissolved zinc in the saline produced water.

It has been found that the presence of nitrite and nitrate anions in naturally occurring saline waters is not of concern because: (a) the corresponding zinc nitrite and zinc nitrate species have significantly lower formation constants than the [ZnCl_4-n]^n~2 species and

[ZnBr_4-n]^n"2 species; and, (b) halide anions (chloride and bromide anions) predominate over nitrite and nitrate anions in these naturally occurring waters. Thus, the concentration of nitrate in naturally occurring waters is typically less than 1 ppmv. Similarly, the concentration of nitrite in naturally occurring water is typically less than 1 ppmv.

Saline waters (other than untreated produced waters) also typically contain negligible amounts of naturally occurring zinc-complexing organic anions such as carboxylate anions, for example, less than 0.1 mEq/L, more preferably, less than 0.05 mEq/L, most preferably, less than 0.001 mEq/L of such organic anions. For example, seawater typically has a maximum concentration of zinc-complexing organic anions of 0.00083 mEq/L.

Where the aqueous displacement fluid is formed by dosing a zinc salt into a naturally occurring saline water, it is preferred that the saline water contains negligible amounts of water-soluble zinc-complexing organic ligands, for example, less than 0.1 mEq/L, preferably, less than 0.05 mEq/L, most preferably, less than 0.01 mEq/L, in particular, less than 0.001 mEq/L of such ligands. Typically, naturally occurring saline waters (other than produced waters) contain insubstantial amounts of naturally occurring water-soluble zinc- complexing organic ligands. For example, the concentration of naturally occurring zinc- complexing organic ligands in seawater is less than 5 nM, in particular, less than 2 nM (see Kenneth W Bruland, "Complexation of zinc by natural organic ligands in the central North Pacific, Limnol Oceanogr., 32(2). 1989, 269-285).

The person skilled in the art would understand that when an aqueous displacement contains levels of sulfate anions of greater than 40 mg/L, in particular, greater than 100 mg/L, there is a risk of souring the reservoir (the souring risk increasing with increasing sulfate concentration). Souring of a reservoir is believed to arise from the production of hydrogen sulfide by sulfate-reducing bacteria (SRB). Souring of a reservoir is detrimental to the method of the present invention owing to the risk of formation of insoluble precipitates of zinc sulfide upon mixing of the aqueous displacement fluid (containing dissolved zinc) with the formation water (containing dissolved hydrosulfide anions).

Souring of a reservoir may be mitigated by the presence of nitrate anions in the aqueous displacement fluid as the nitrate anions are believed to stimulate nitrate-reducing, sulfide-oxidizing bacteria (NR.- SOB) and heterotrophic nitrate-reducing bacteria (hNRB) that compete with SRB for biodegradable oil organics. As discussed above, naturally occuning saline waters contain relatively low amounts of nitrate anions of less than lmg/L, for example, less than 0.1 mg/L. It may therefore be necessary to add one or more nitrate salts to an aqueous displacement having a concentration of sulfate greater than 40 mg/L, in particular, greater than 100 mg/Lin order to control souring. Typically, a nitrate salt(s) is added to the aqueous displacement fluid in an amount that gives a nitrate anion

concentration of at least 5 mg/L(0.08 mmol/L), preferably at least 10 mg/L(0.16 mmol/L). Preferably, the nitrate salt(s) is added to the aqueous displacement fluid in an amount that gives a nitrate anion concentration in the range of 10 to 500 mg/L(0.16 to 8 mmol/L, more preferably, in the range of 10 to 100 mg/L (0.16 to 1.6 mmol/L). It has been found that the formation constant for zinc nitrate species is significantly lower than the formation constants for zinc chloride and zinc bromide species in the temperature range of 20 to 150°C such that the aqueous displacement fluid contains negligible amounts of zinc nitrate species. Accordingly, zinc nitrate species may be ignored when determining the mole fraction of sum of the [ZnCl_4-n]^n~2 species in the aqueous displacement fluid.

However, souring of a reservoir may also be mitigated by dosing a zinc salt into a water having a concentration of sulfate of less than or equal to 100 mg/L, preferably, less than or equal to 40 mg/L. For example, the aqueous displacement fluid may be fonned by adding the zinc salt to a low salinity water such as river water, lake water, low salinity aquifer water, low salinity produced water or mixtures thereof having a natural

concentration of sulfate of less than or equal to lOOmg/L, preferably, less than or equal to 40mg/L. If the chloride concentration of the resulting solution is below the value of

[Chloridej_mi_n (as determined using Equation 3 or the prefened values determined using Equations 4 to 6), a chloride salt may be added to the low salinity water to increase the chloride concentration to above the calculated value of [Chloride]_mi_n. Typically, the chloride salt that is added to the aqueous displacement fluid is selected from the group consisting of Group IA metal chloride salts (preferably, sodium chloride, potassium chloride, or mixtures thereof) and Group IIA metal chloride salts (preferably, calcium chloride, magnesium chloride, strontium chloride or mixtures thereof). Preferably, the chloride salt is selected from sodium chloride and potassium chloride.

The aqueous displacement fluid may also be formed by dosing a zinc salt, preferably, zinc chloride, into a "sulfate reduced saline water". By "sulfate reduced saline water" is meant a naturally occurring saline water that has been treated to remove sulfate and has a residual sulfate concentration of less than or equal to 100 mg/L, preferably, less than or equal to 40 mg/L.

As discussed in more detail below, the naturally occurring saline water may be treated to remove sulfate anions using nanofiltration, reverse osmosis or precipitation processes. Suitable sources of the naturally occurring saline waters include seawater, estuarine water, saline produced waters, saline aquifer waters, and mixtures thereof.

However, most produced waters do not require treatment to remove sulfate as they have naturally occurring sulfate concentrations of below 100 mg/L.

In the case of a sulfate reduced saline water, it is preferred that halide ions (selected from chloride and bromide anions) comprise at least 99.5 mol%, more preferably, at least 99.75 mol%, most preferably, at least 99.90 mol% of the total inorganic anions in the aqueous displacement fluid (with the proviso that the concentration of bromide anions is at most 10 mol% based on the total molar concentration of chloride and bromide anions).

The use of a sulfate reduced saline water also has the advantage of reducing the risk of mineral scale formation that may occur upon injection of an aqueous displacement fluid containing sulfate anions into a reservoir having a formation water that contains precipitating precursor cations for sulfate anions such as barium cations. The use of a sulfate reduced saline water also means that the sulfate concentration has no impact on the zinc speciation in the aqueous displacement fluid.

A sulfate reduced saline water may be produced by contacting a naturally occurring saline water (feed water) having a relatively high sulfate concentration, with a

nanofiltration (NF) membrane that selectively excludes sulfate anions whilst allowing monovalent ions such as Group IA metal ions (e.g. sodium ions) and halide ions (e.g.

chloride ions and bromide ions) to pass therethrough thereby producing a sulfate reduced saline water stream (NF permeate) having a lower concentration of sulfate anions and a retentate having a higher concentration of sulfate anions than the feed water. Thus, the permeate stream (sulfate reduced saline water) removed from a nanofiltration membrane typically has a sulfate concentration of less than or equal to 100 mg/L, preferably, less than or equal to 40 mg/L.

It is also possible to treat a naturally occurring saline water using reverse osmosis (RO) to generate a treated water (i.e., an RO permeate that passes through the RO membrane) that is substantially free of sulfate anions, organic anions and organic ligands. However, the RO permeate would also be substantially free of chloride anions and bromide anions. It may therefore be necessary to increase the concentration of chloride anions in the RO permeate to a value at or above the minimum value determined using Equation 3 or to a value at or above a preferred minimum value of [Chloride]_mi_n determined using one of Equations 4, 5 or 6. This may be achieved by blending the RO permeate with an NF permeate.

Any water-soluble zinc-complexing organic anions and water-soluble zinc- complexing organic ligands may also be removed from the naturally occurring saline water using membrane separation processes wherein the membrane is either a nanofiltration membrane that has a molecular weight cut-off that excludes the zinc-complexing anions and ligands or is a reverse osmosis membrane that excludes substantially all dissolved solids. Typically, a sulfate reduced water that is produced using a membrane separation process has a concentration of zinc-complexing anions and ligands of less than 1 mg/L.

It is also envisaged that sulfate anions may be removed from a naturally occurring saline water by adding a precipitating counter-cation to the water such as barium cations thereby resulting in the formation of insoluble barium sulfate which may then be separated from the water by filtration. Any organic anions in the naturally occurring saline water may also be precipitated from solution by addition of precipitating cations.

Typically, the zinc salt is dosed into a saline water (preferably, a sulfate reduced saline water) having a chloride anion concentration in the range of 2,250 to 125,000 mg/L (0.06 to 3.52 moles/litre), preferably, 5,000 to 100,000 mg/L (0.144 to 2.82 moles/litre), more preferably, 15,000 to 25,000 mg/L(0.42 to 0.70 moles/L). If necessary, a chloride salt may be dosed into the naturally occurring saline water (preferably, a sulfate reduced saline water) to increase the chloride anion concentration of the aqueous displacement fluid to a value at or above [Chloride]_mi_n determined using Equation 3 or to increase the chloride anion concentration to a value at or above a preferred value of [Chloride]_mi_n determined using one of Equations 4 to 6. Suitable chloride salts are listed above. Typically, a saline water (preferably, a sulfate reduced saline water) is selected having a naturally occurring concentration of chloride anions that results in optimal concentrations of the [ZnCl_4-n]^n"2 species in the aqueous displacement fluid. In particular, in the case of sulfate reduced saline waters, where there is a choice of naturally occurring saline waters that may be used as feed to a sulfate removal process, in particular, a nanofiltration process, it is preferred to select a naturally occurring saline water having a chloride concentration that provides the highest mole fraction of the sum of the [ZnC -nP^"2 species at the reservoir temperature. It is to be understood that the saline water should have a naturally occurring bromide concentration of at most 10 mol% (based on the total moles of chloride and bromide anions).

The zinc salt that is dosed into the saline water to form the aqueous displacement fluid may be selected from zinc hydroxide, basic zinc carbonate, zinc sulfate, zinc nitrite, zinc nitrate, a zinc halide (in particular, zinc chloride and zinc bromide), and mixtures thereof, most preferably, zinc chloride. However, the skilled person will understand that zinc sulfate should not be employed if the saline water is a sulfate reduced water. Where the zinc salt is zinc hydroxide or basic zinc carbonate, this would necessitate the addition of an acid to render these salts soluble in the saline water. Typically, the pH of the aqueous displacement fluid is reduced to a value of less than 7.0, preferably, less than 6.5 to reduce the risk of precipitation of insoluble zinc salts prior to injection of the aqueous displacement fluid into the reservoir.

Insoluble zinc complexes, for example, zinc carbonate or zinc hydroxide, may precipitate from solution if the pH of the aqueous displacement fluid is at or above 7.5. In order to avoid any precipitation of insoluble zinc species, the pH of the aqueous displacement fluid is typically adjusted to a value of less than 7.5, preferably, less than 7.0, more preferably, less than 6.5, most preferably, less than 6.0, prior to injection of the fluid down an injection well and into the oil-bearing reservoir. Accordingly, an acid may be added to the saline water either before or after the addition of zinc chloride in order to prevent precipitation of insoluble zinc complexes. Alternatively, the acid and zinc chloride may be added simultaneously to the saline water. For ease of adjustment of the pH, it is preferred to add the acid to the saline water before addition of the zinc chloride. Typically, the concentration of acid in the aqueous displacement fluid is less than 0.5% by weight, preferably, less than 0.25% by weight, more preferably, less than 0.1% by weight, for example, less than 0.05% or less than 0.025% by weight. The use of excess acid should be avoided owing to the risk of corrosion of pipework and downhole equipment and of dissolving acid soluble material present in the reservoir such as carbonate cements.

Typically, the pH of the aqueous displacement fluid is adjusted to a value in the range of 4.0 to 6.5, preferably 4.5 to 6.0. However, as discussed above, after injection of the aqueous displacement fluid into the oil-bearing reservoir, the pH of the aqueous displacement fluid is believed to buffer to the pH of the formation water.

Preferably, the acid is a protic acid. Preferred protic acids include hydrochloric acid, hydrobromic acid, sulfuric acid and nitric acid. Mixtures of acids may be used to adjust the pH of the aqueous displacement fluid. Preferably the acids are used in the form of aqueous solutions. Hydrochloric acid or nitric acid is preferred for adjusting the pH of the saline water as these are readily available as concentrated aqueous solutions, for example, as 5% by weight aqueous solutions. Protic acids such as sulfuric acid and hydrobromic acid are preferably avoided as these contribute additional zinc-complexing anions to the aqueous displacement fluid that may interfere with the formation of the desired [ZnCl_4-n]^n~2 species. The use of organic acids such as formic acid, acetic acid, and propionic acid is preferably avoided as the organic carboxylate anions may complex zinc thereby interfering with the formation of the desired [ZnCl _-n]^n~2 species.

Percentage incremental oil production is defined herein as:

[(Sor - Sor tSoi - SorVj x l OO

wherein S_or is the residual oil saturation achieved with a baseline saline water, Sor¹ is the residual oil saturation achieved with the aqueous displacement fluid, and S₀i is the initial oil saturation. Typically, the baseline saline water is of the same composition as the aqueous displacement fluid but omits the zinc salt.

Typically, the incremental oil production that can be achieved using the method of the present invention is at least 2%, preferably at least 3%, more preferably, at least 5%, in particular, at least 7.5%, for example, at least 10% above that achieved or predicted to be achieved when waterflooding the reservoir with the baseline saline water in the absence of the added zinc salt.

Suitably, the reservoir comprises at least one oil-bearing layer of reservoir rock that is penetrated by at least one injection well and the aqueous displacement fluid is injected into the reservoir from the injection well. Suitably, the oil-bearing layer(s) of the reservoir is penetrated by at least one production well and the crude oil is recovered by being produced from the reservoir into the production well.

Without wishing to be bound by any theory, it is believed that there may be loss of zinc to the reservoir. However, as discussed above, it is believed that losses of [ZnCi_4-n]^n~2 species to the reservoir are lower than loss of Zn²⁺ ions such that the [ZnCl_4-n]^n~2 species propagate further through the reservoir than Zn²⁺ ions. Accordingly, modelling of zinc speciation may be used to select a concentration of chloride anions in the aqueous displacement fluid that optimizes the mole fraction of the sum of the [ZnCl_4-n]^n"2 species at the reservoir temperature. Suitably, zinc speciation modelling may be used to select a naturally occurring brine from a plurality of naturally occurring brines having a concentration of chloride anions that optimizes the mole fraction of the sum of the [ZnCl₄_ _n]^n"2 species at the reservoir temperature. Also, zinc speciation modelling may be used to determine an amount of chloride salt, in particular, a chloride salt selected from sodium chloride, and potassium chloride that may be added to the aqueous displacement fluid to optimize the mole fraction of the sum of the [ZnCl_4-n]^n"2 species at the reservoir temperature. Also, as discussed above, zinc speciation modelling may also be used to determine tolerable concentrations of additional inorganic complexing anions (in particular, bromide and sulfate anions) in the aqueous displacement fluid.

The zinc salt may be dosed into the naturally occurring saline water in the form of a powder, for example, using a metered hopper system. The optional chloride salt may also be dosed into the naturally occurring saline water in the form of a powder, preferably, using a separate metered hopper system. Preferably, the powder is stored under a blanket of a dry inert gas such as nitrogen, in order to mitigate risks associated with powder handling or with hydration of the powder.

Alternatively, a highly concentrated solution of the zinc salt may be dosed into the saline water to form the aqueous displacement fluid (hereinafter "zinc concentrate"). Similarly, the optional chloride salt may be dosed into the saline water in the form of a highly concentrated solution of the chloride salt (hereinafter "chloride concentrate"). The zinc concentrate and chloride concentrate are preferably separately dosed into the saline water, for example, using metered pump systems.

Typically, the aqueous displacement fluid is injected into the reservoir from an injection well. It is envisaged that the zinc concentrate (and, optionally, the chloride concentrate) may be mixed with the saline water to form the aqueous displacement fluid within the injection well thereby mitigating the risk of precipitation of zinc salts prior to injection of the aqueous displacement fluid into the reservoir. Typically, the injection well has a wellhead and injection tubing extending from the wellhead to a location in the injection well at or immediately above an oil-bearing zone of the reservoir. Typically, a packer is arranged in an annulus formed between the injection tubing and the wall of the injection well. The saline water may be injected into the injection well down the injection tubing. The injection well may also have an injection line, for example, a coiled tubing, arranged either within the annulus formed between the injection tubing and the wall of the injection well or within the injection tubing. Typically, the injection line extends from the wellhead to a region in the wellbore at or immediately below the lower end of the injection tubing, for example, within 1 metre below the lower end of the injection tubing. The person skilled in the art will understand that when the injection line lies within the injection tubing, it may be concentrically or acentrically arranged in the mjection tubing. The person skilled in the art will also understand that when the injection line is located in the annulus, the injection line extends through the packer. The zinc concentrate may be injected into the injection well down the injection line. The amount of zinc concentrate mixed into the saline water to form the aqueous displacement fluid may be controlled by adjusting the volumetric flow rate for the zinc concentrate in the injection line, the volumetric flow rate for the saline water in the injection tubing or both. Other known methods for injecting a chemical into an injection well may also be employed.

If desired, the chloride concentrate may also be mixed with the saline water within the injection well. Thus, the chloride concentrate may be mixed with the zinc concentrate to form a mixed concentrate stream that may be injected down the injection line. The person skilled in the art will understand that the volumetric ratio in which the concentrates are mixed will be dependent upon the concentrations of zinc salt and chloride salt in the zinc concentrate and chloride concentrate respectively, and the amount of zinc salt and chloride salt that is to be dosed into the saline water. The amount of the mixed concentrate that is mixed with the saline water to form the aqueous displacement fluid may be controlled by adjusting the volumetric flow rate for the mixed concentrate in the injection line, the volumetric flow rate for the saline water in the mjection tubing or both.

Alternatively, a dedicated injection line for the zinc concentrate and a dedicated injection line for the chloride concentrate may be arranged either within the annulus formed between the injection tubing and the wall of the injection well or within the injection tubing. It is envisaged that one of the lines may be arranged within the annulus and the other within the injection tubing. Typically, the dedicated injection lines for the zinc concentrate and chloride concentrate extend from the wellhead to a region in the wellbore at or

immediately below the lower end of the injection tubing, for example, within 1 meter below the lower end of the injection tubing. The amounts of zinc concentrate and of chloride concentrate mixed into the saline water may be controlled by adjusting the volumetric flow rate for the zinc concentrate and chloride concentrate in the dedicated injection lines, the volumetric flow rate for the saline water in the injection tubing or both.

Suitably, the zinc concentrate has a concentration of zinc salt of at least 10,000 mg/L, preferably, at least 15,000 mg/L, for example, a concentration of zinc salt in the range 20 to 75% by weight, preferably, 30 to 70% by weight, for example, 40 to 70% by weight. The upper limit for the concentration of zinc salt in the concentrate is the saturation concentration at the storage conditions for the concentrate. Suitably, the concentrate is stored in a vessel at the injection site. Depending on the concentration of the zinc salt, the zinc concentrate may be corrosive. Accordingly, it may be desirable to form the vessel from corrosion resistant steel such as a steel alloy having a minimum 10.5% by weight chromium content. Alternatively the internal surface of the vessel may be provided with a liner or a coating formed from a corrosion resistant material, for example, a metal such as titanium or a polymeric material. The zinc concentrate may be prepared using fresh water as solvent. The zinc concentrate may also be prepared using a saline water as solvent, for example, a produced water as solvent. The saline water used as solvent for the zinc concentrate may have a concentration of sodium chloride up to the saturation concentration of sodium chloride at the ambient temperature at which the concentrate is stored at the injection site. Preferably the saline water used as solvent for the zinc concentrate has a sulfate concentration of less than or equal to 100 mg/L, preferably, less than or equal to 40 mg/L. Where the zinc salt is zinc hydroxide or basic zinc carbonate, it is essential that an acid is added to the concentrate in order to acidify the concentrate thereby dissolving the zinc salt.

Suitably, the chloride concentrate has a concentration of chloride salt of at least 10,000 mg/L, preferably, at least 15,000 mg/L, for example, a concentration in the range 20 to 36% by weight, preferably, 20 to 35% by weight. The upper limit for the concentration of chloride salt in the concentrate is the saturation concentration at the storage conditions for the concentrate. Suitably, the concentrate is stored in a vessel at the injection site. The chloride concentrate may be prepared using fresh water as solvent. The chloride concentrate may also be prepared using a saline water as solvent, for example, a water having a TDS concentration in the range of 1,000 to 50,000 mg/L. Preferably the saline water used as solvent for the chloride concentrate has a sulfate concentration of less than or equal to 100 mg/L, preferably, less than or equal to 40 mg/L.

Preferably, the zinc salt and chloride salt (or zinc concentrate and chloride concentrate) are separately dosed into the saline water thereby allowing the amounts of zinc salt and chloride salt that are dosed into the saline water to be independently varied.

Suitably, the amount of the zinc salt powder or of the zinc concentrate (and optional chloride salt powder or concentrate) that is dosed into the saline water may be controlled, for example, to maintain the concentration of dissolved zinc in the aqueous displacement fluid at or near a target concentration, for example, within ± 10% of a target concentration. The dosing of the zinc salt powder or zinc concentrate (and optional chloride salt powder or concentrate) into the saline water is preferably automated, for example, using a metering system that is controlled via a computer. Preferably, the powder is dosed into the saline water flowing through an injection header. Suitably, the zinc concentrate (and optional chloride concentrate) is either dosed into a saline water flowing through an injection header or, as discussed above, is dosed into the saline water within an injection well.

Suitably, the amount of the optional chloride salt powder or chloride concentrate that is dosed into the saline water may be controlled so as to maintain the concentration of chloride in the aqueous displacement fluid at a value that optimizes the mole fraction of the sum of the [ZnCl_4-n]^n"2 species at the reservoir temperature to bring the mole fraction above a preferred value of 0.5, 0.75 or 0.9. However, owing to the economic cost of adding chloride to a brine, it is preferred to select a brine composition that is a close match for the desired chloride concentration thereby either eliminating the requirement to add a chloride salt to the brine or reducing the amount of chloride salt that is added to the brine.

After injection of the aqueous displacement fluid into the reservoir, an aqueous drive fluid may be injected to sweep the aqueous displacement fluid (and hence the bank of mobile oil) through the reservoir to the production well or to maintain the pressure in the reservoir. Suitably, this aqueous drive fluid may be seawater, estuarine water, brackish water, saline produced water, saline aquifer water, or mixtures thereof. Where injection of sulfate anions is to be avoided, the aqueous drive fluid may be a sulfate reduced water. Preferably the aqueous drive fluid is the saline water that is used to prepare the aqueous displacement fluid.

The viscosity of the aqueous displacement fluid is close to that of the saline water. This is because the zinc chloride species (and optional zinc bromide species) are present in the aqueous displacement fluid at a relatively low concentration. Typically, the aqueous displacement fluid has a viscosity in the range of 1.00 to 2.00 centipoise (cP), preferably, 1.00 to 1.50 cP, in particular, 1.00 to 1.25 cP, for example, in the range of 1.00 to 1.15 cP, when measured at the "standard temperature and pressure" (STP) of the International Union of Pure and Applied Chemistry (IUPAC), that is, a temperature of 273.15K and an absolute pressure of 100 kPa. For avoidance of doubt, the viscosity of the aqueous displacement fluid is preferably determined in the absence of any dissolved gases. In contrast, it has been reported that in highly concentrated zinc chloride solutions with a 2.T chloride: zinc ratio, the [Ζη¾]^" and [ZnCl₄]^2" complexes form by sharing the chloride ions between two zincs resulting in high viscosity solutions (D J Harris et al, Molecular Physics, 2001, Vol 99, No. 10, 825-833).

Preferably, the aqueous displacement fluid has a density in the range of 1.000 to 1.210 g/cm³ when measured at the standard temperature and pressure (STP) of the

International Union of Pure and Applied Chemistry (IUPAC), that is, a temperature of 273.15K and an absolute pressure of 100 kPa.

In the method of the invention, the aqueous displacement fluid is preferably injected, under pressure, into at least one injection well and produced fluids are recovered from at least one production well. The aqueous displacement fluid passes from the injection well into and through at least one oil-bearing layer of the reservoir. The passage of the aqueous displacement fluid through the oil-bearing layer of the reservoir displaces oil from the reservoir rock and forces the displaced oil ahead of it, towards the production well from which the oil is recovered. Preferably, the injection well and production well are not overlying.

However, the aqueous displacement fluid may also be used in a process where a well penetrates at least one oil-bearing layer of a reservoir and this well serves as both the injection well and production well, that is, the aqueous displacement fluid is injected into the well and then the well is subsequently put onto production (known in the industry as a "huff and puff process).

The method of the present invention may be used in secondary recovery mode which may occur at commencement of oil production from the reservoir (omitting primary recovery) or after primary recovery of oil under the natural pressure of the reservoir.

Alternatively, the method of the present invention may be used in tertiary recovery mode (for example, after a waterflood with a high salinity water or a low salinity water).

The person skilled in the art will understand that in tertiary recovery, injection of the original fluid is stopped and a different fluid is injected into the reservoir for enhanced oil recovery. Thus, the fluid that is injected into the reservoir during tertiary recovery is the aqueous displacement fluid, and the fluid that has previously been injected into the formation during secondary recovery may be a water that does not contain a zinc salt or contains insubstantial amounts of zinc salts, for example, naturally occurring levels of dissolved zinc such as less than 10 mg/L, in particular, less than 5 mg/L. Typically, the previously injected water has a TDS of at least 5,000 mg/L, preferably, a TDS of at least 10,000, most preferably, a TDS of at least 30,000 mg/L, in particular, a TDS in the range of 30,00 to 250,00 mg/L. Suitably, the previously injected water may be selected from seawater, estuarine water, brackish water, produced water, aquifer water, or a mixture thereof.

The present invention will now be illustrated by reference to the following Examples and Figures.

Coreflood Facilities

The following studies utilized a coreflood facility operated at non-reservoir conditions (referred to in the art as "reduced conditions") of temperatures up to 75°C, a pore pressure of 20 bar gauge (2 MPa gauge) and a confining pressure of 60 bar gauge (6MPa gauge). Hereinafter "bar gauge" is referred to as "barg". The coreflood facility employed dead fluids (oil and brine having no dissolved gas at the conditions of the test). Core Preparation

Core plug samples, nominally 3 " long by 1.5" in diameter were used for the studies. However, the person skilled in the art will understand that different sized core plug samples may also be used. The samples were first restored i.e. the samples were cleaned using miscible solvents (for example, methanol and toluene) such that they were as close to being in a "water wet" condition as possible. After cleaning, the samples were placed into hydrostatic coreholders and the samples were saturated with a high salinity brine by flowing the water through the core plugs under a back pressure. After a throughput of approximately 10 pore volumes of brine, the samples were removed from the hydrostatic coreholders and the initial water saturation was set up in each sample using the procedure described below. The composition of the high salinity brine is given in Table 1 below. Acquisition of Initial Water Saturation (SwT)

It was essential that each core plug sample had a representative initial water saturation (S_w0 value. The initial water saturation for each sample was achieved by a confined porous plate de-saturation technique, using the strongly non- wetting gas, nitrogen. This technique is well known to the person skilled in the art and will not be discussed further here. Once the initial water saturations were acquired, the samples were loaded into hydrostatic core holders and saturated with a refined oil under back pressure. A dispersion test (discussed below) was then performed to confirm the value of S_wj acquired.

Ageing of core samples

The core plug samples were then loaded into coreholders and slowly raised in pressure and temperature to the test conditions.

The refined oil was then miscibly displaced at the test conditions by crude oil via a 0.5 PV slug of toluene. Thus, a slug of toluene is injected into the sample before injecting the crude oil. The toluene is miscible with both the refined oil and the crude oil and therefore allows the refined oil to be readily displaced by the crude oil. After the differential pressure across the core sample had stabilized, the crude oil viscosity and effective permeability of the core sample to the crude oil were measured using techniques well known to the person skilled in the art. The core sample was then aged in the crude oil for one week. During the ageing period the crude oil was replaced once immediately before commencing waterflooding of the core samples. A minimum of one Pore Volume of crude oil was injected and a sufficient amount of crude oil was used to achieve a constant pressure drop (differential pressure) across the sample.

Coreflood test method All corefloods were performed under unsteady state conditions using procedures well known to the person skilled in the art.

A secondary waterflood was performed using a high salinity brine. This brine was injected into the core sample at a typical injection rate of nominally 4 ml/hour (which is a typical frontal advance rate for a waterflood in a reservoir). During injection of the brine, the differential pressure across the sample was recorded and the volume of oil produced from the sample was measured. Oil production was determined by collecting the effluent produced from the core sample using a volumetric sampler into which has been loaded a plurality of sample bottles. The mass of oil contained in each of the sample bottles and hence the total mass of oil produced from the core sample could then be determined. The total volume of produced oil could then be calculated from the density of the oil.

The waterflood with the high salinity brine was allowed to continue until the differential pressure, and oil production readings reached equilibrium. Equilibrium was taken to be reached when no oil was observed in the effluent that was removed from the core sample. Permeability of the core plug sample to water at residual (remaining) oil saturation was then measured using a technique well known to the person skilled in the art.

In coreflood tests that employed an aqueous displacement fluid comprising a zinc salt dissolved in a brackish water (that did not contain any sulfate anions), a tertiary waterflood using a brackish water was run at the same injection rate as for the secondary waterflood with the high salinity brine. This tertiary waterflood was continued until the differential pressure, and oil production readings reached equilibrium. Permeability of the core plug samples to water at residual (remaining) oil saturation was then measured. The tertiary waterflood using a brackish water was then followed by a further tertiary waterflood using an aqueous displacement fluid prepared by adding a zinc salt to the brackish water. The tertiary waterflood with this aqueous displacement fluid was also continued until the differential pressure, and oil production readings reached equilibrium.

In coreflood tests that employed a displacement fluid comprising a zinc salt dissolved in a high salinity base fluid (that also did not contain any sulfate anions), the brackish base waterflood was omitted. A tertiary waterflood was run at the same injection rate as for the secondary waterflood with the high salinity brine using a test aqueous displacement fluid prepared by adding a zinc salt to the high salinity brine. The tertiary waterflood with the test aqueous displacement fluid was also continued until the differential pressure, and oil production readings reached equilibrium.

During the sequence of core floods, samples of oil and produced aqueous effluent were collected and the amount of oil produced under different waterflood conditions was determined.

At the end of the sequence of waterfloods, the final oil saturation of the core sample was determined by means of a dispersion test (as described below) thereby ensuring effective mass balancing during the waterflooding sequence.

Dispersion tests

Dispersion tests were used at different stages of the preparation of the core samples and the coreflood studies. The objective of a dispersion test is to measure the volume of fluid within the core sample at different stages of the experiments. For example, a dispersion test earned out on a core sample that is at 100% water saturation will provide the pore volume and porosity of the core sample, a dispersion carried out on the core sample when at initial water saturation (S_Wi) or at residual oil saturation (S_or) will provide a measure of these saturation values. Thus, the dispersion test results provide quality assurance for the volumetric data that are obtained during the coreflood studies.

In a dispersion test, an undoped fluid located in the pore space of the core sample may be miscibly displaced by doped fluid or a doped fluid located in the pore space of the core sample may be displaced by an undoped fluid. Thus, an undoped aqueous fluid may be miscibly displaced by an 'iodide' doped aqueous fluid (or vice versa) while an undoped oleic fluid may be miscibly displaced by an 'iododecane' doped oleic fluid (or vice versa). The doped oleic fluid may be a doped refined oil. The density of the miscibly displaced fluid (effluent) is determined over time by taking samples of the effluent. The densities of the samples of effluent are normalized to the density of the doped fluid (p_sampie ⁼

Peffiuent/pdo ed fluid) and the normalized densities are plotted against the volume of effluent displaced from the core when each of the samples of effluent were taken. The volume of the mobile fluid (aqueous or oleic fluid) within the core sample is then calculated from the volume of the displaced fluid present in the effluent samples. Thus, the volume of mobile fluid may be deduced from the densities and volumes of each of the effluent samples. The volume of mobile fluid within the pore space of the core sample is also the volume of effluent that has been displaced from the core sample when the normalized density of the displaced fluid is 0.5. When a dispersion test is performed for a core sample at 100% water saturation, aqueous fluids are used and the test gives a value for the total pore volume of the core sample.

When a dispersion test is performed with a core sample at initial water saturation, S_Wi, part of the pore volume of the core sample is occupied by a mobile oil phase, with the remainder of the pore volume occupied by an immobile water phase. The dispersion test therefore uses an iododecane doped oil (oleic fluid) as the displacement fluid.

Accordingly, the volume of aqueous fluid in the pore space of the core sample at S_w; is:

Volume of Aqueous Fluid = Total Pore Volume - Volume of oil.

When a dispersion test is performed with a core sample at residual oil saturation, S_or, part of the pore volume of the core sample is occupied by a mobile aqueous phase with the remainder of the pore space occupied by an immobile oil phase. The dispersion test therefore uses an iodide doped aqueous fluid. Accordingly, the volume of oil in the pore space of the core sample at S_or is:

Volume of Oil = Total Pore Volume - Volume of Aqueous Fluid.

Thus, provided the total pore volume of the core sample has been determined, the volume of oil remaining in the core sample can be determined.

As discussed above, during the sequence of core floods, samples of produced oil and produced aqueous effluent were collected and the volume of oil produced under different waterflood conditions was determined.

Percentage incremental oil production was then calculated from:

[(S₀r - S₀r V Soi - Sor I x l OO.

Soi (the initial oil saturation) was calculated from:

S₀ (hydrocarbon pore volume)/total pore volume.

Hydrocarbon pore volume was determined from dispersion tests carried out with the core sample at initial water saturation, S_Wi. Total pore volume was determined from dispersion tests with all of the pore volume of the core sample filled with water i.e. before acquiring S_Wi.

Residual oil saturation. S_or, was then calculated from the volume of oil produced during the secondary waterflood:

Sor ⁼ Soi - [oil produced from secondary waterflood/total pore volume]. Sor¹ was determined using the volume of oil produced during the tertiary waterflood:

Sor^{1 =} S_or-[oil produced in the tertiary waterflood/total pore volume].

The total amount of oil produced was compared with the final residual oil saturation of the cores, as determined from dispersion tests, to ensure effective mass balancing during the waterfloods.

Table 1 : Brine compositions used in tests

Fluid 1 Fluid 2 Fluid 3 Fluid 4 Fluid 5

(Synthetic (Brackish (Highly

Seawater) Water) Saline

Brine)

Salt mg/L mg/L mg/L mg/L mg/L

(ppmv) (ppmv) (ppmv) (ppmv) (ppmv)

NaHC0₃ 191.4 6.3 202.4 1864.2 27.5

Sulfate 0 0 0 0 0

CaCl₂.2H₂0 1467.1 48.1 46463.0 218.8 7700.2

MgCl₂.6¾0 10639.8 349.0 25395.0 125.5 2576.3

KC1 724.6 23.8 0.0 34.3 148.7

Bromide 0 0 0 0 0

NaCl 23478.1 770.1 147111.5 14591.4 41928.4

Total 18,992.0 622.9 120423.9 9094.7 32318.0

Chloride

Total 30490 1000 194304 16834 52381

Dissolved

solids Table 2, below, shows the modelled concentrations of zinc species for the brine compositions at the temperature of the coreflood tests.

Table 2— Zinc speciation

Comparative Example 1

A coreflood test was carried out under reduced conditions (at a temperature of 72°C, a pH of 5.5 and a pore pressure of 20 barg) with Reservoir Core 1 using the test method described above. Reservoir Core 1 was a sandstone rock having a quartz content of 78.5% by weight and a total clay content of 10.2% by weight as measured by X-ray diffraction. An aqueous displacement fluid formed by adding 100 ppmv of zinc chloride to Fluid 2 (the brackish water of Table 1) was injected into the core following a secondary waterflood with Fluid 1 (the synthetic seawater of Table 1) and a tertiary waterflood with Fluid 2. The results are presented in Table 3 below. It can be seen that there was little incremental oil recovery compared with the baseline secondary waterflood with Fluid 1 and with the tertiary waterflood with Fluid 2. Table 3

a. Experimental Ν_α value derived from EXAFS data performed on Fluid 2 containing 1000 ppmv

ZnCl₂ at a temperature of 70°C.

b. Nci value derived by running the geochemical model at a temperature of 70°C with the brine composition of Fluid 2 and a ZnCl₂ concentration of 1000 ppmv as input data,

c. Mole fraction is based on the total moles of hydrated [ZnCl₄.,,]^n"2 species and hydrated Zn²⁺ ions.

Example 1

A reduced condition coreflood test was carried out at a temperature of 72°C, a pH of 5.5 and a pore pressure of 20 barg with Reservoir Core 3. Reservoir Core 3 was a sandstone rock having a quartz content of 81.6% by weight and a total clay content of 7.4%) by weight, as measured by X-ray diffraction. A secondary waterflood using Fluid 1 (synthetic seawater of Table 1) was followed by a tertiary waterflood using Fluid 1 containing 1000 ppmv of zinc chloride. The results are shown in Table 4 below. It can be seen that there was incremental oil recovery compared with the baseline secondary waterflood with Fluid 1 and a higher percentage of incremental oil recovery than for Comparative Example 1.

Table 4

a. Experimental N_cl value derived from EXAFS data performed on Fluid 1 containing 1000 ppmv ZnCl₂ at a temperature of 70°C.

b. Nci value derived by running the geochemical model at a temperature of 70°C with the brine composition of Fluid 1 and a ZnCl₂ concentration of 1000 ppmv as input data.

c. Mole fraction is based on the total moles of hydrated [ZnCl₄._n]^n"2 species and hydrated Zn²⁺ ions.

Example 2

A coreflood test was carried out under reduced conditions at a temperature of 55°C, a pH of 5.5 and a pressure of 20 bar gauge with Reservoir Core 4 using the test method described above. Reservoir Core 4 was a sandstone rock having a quartz content of 79.6% by weight and a total clay content of 10.8% by weight, as measured by X-ray diffraction. A secondary waterflood using Fluid 3 (highly saline brine of Table 1) was followed by a tertiary waterflood using an aqueous displacement fluid formed by adding 400 ppmv of zinc chloride (about 200 ppmv zinc) to Fluid 3. The results are shown in Table 5 below. It can be seen that there was significant incremental oil recovery compared with the baseline secondary waterflood with Fluid 3 and a higher percentage oil recovery than for

Comparative Example 1. Table 5

a. Experimental Ν_α value derived from EXAFS data performed on Fluid 3 containing 1000 ppmv ZnCl₂ at a temperature of 50°C.

b. N_Ci value derived by running the geochemical model at a temperature of 50°C with the brine composition of Fluid 3 and a ZnCl₂ concentration of 1000 ppmv as input data.

c. Mole fraction is based on the total moles of the hydrated [ZnCl₄._n]^n"2 species and hydrated Zn²⁺ ions. Examples 3 and 4

Coreflood tests were earned out under reduced conditions at a pore pressure of 20 barg, a pH of 5.5 and at temperatures of 55 and 72°C for Reservoir Cores 5 and 6 respectively, using the test method described above. Reservoir Cores 5 and 6 were taken from the same sandstone reservoir and were matched as closely as possible in physical and chemical properties. The cores had quartz contents ranging from 78% to 90% by weight and total clay contents ranging from 5 to 9% by weight, as measured by X-ray diffraction. For each coreflood, a secondary waterflood using Fluid 1 (synthetic seawater of Table 1) was followed by a tertiary waterflood using Fluid 1 containing 1000 ppmv of zinc chloride. The results are shown in Tables 6 and 7 below. It can be seen that incremental oil was observed for each coreflood test. Table 6 - Coreflood test for Reservoir Core 5 (at a temperature of 55°C)

Experimental N_a value derived from EXAFS data performed on Fluid 1 containing 1000 ppmv ZnCl₂ at a temperature of 50°C.

Nci value derived by running the geochemical model at a temperature of 50°C with the brine composition of Fluid 1 and a ZnCl₂ concentration of 1000 ppmv as input data.

Mole fraction is based on the total moles of the hydrated [ZnCl₄._n]^n~2 species and hydrated Zn²⁺ ions.

Table 7 - Coreflood test for Reservoir Core 6 (at a temperature of 72°C)

a. Experimental Ν_α value derived from EXAFS data performed on Fluid 1 containing 1000 ppmv ZnCl₂ at a temperature of 70°C.

b. Nci value derived by running the geochemical model at a temperature of 70°C with the brine composition of Fluid 1 and a ZnCl₂ concentration of 1000 ppmv as input data.

c. Mole fraction is based on the total moles of the hydrated [ZnCl₄._n]^n"2 species and hydrated Zn²⁺ions. Example 5 - Liquid Chromatography Experiments

The degree of loss of zinc to a sandstone reservoir rock was studied using solutions of zinc chloride in brines having differing concentrations of dissolved chloride using an Inverse Liquid Chromatography apparatus. Five grams of disaggregated rock were packed into a chromatography column. The rock was a sandstone rock having a quartz content of 86% by weight and a total clay content of 8.1% by weight, as measured by X-ray diffraction. The disaggregated rock was initially flooded with a brine that did not contain any zinc chloride for 16 hours at a flow rate of 1.2 ml/hour. The injected fluid was then changed to a 1000 ppmv solution of zinc chloride in the same brine at a flow rate of 1.2 ml/hour. The zinc concentration in the effluent removed from the column was measured by ICP-MS (Inductively coupled plasma mass spectrometry) and the amount of zinc lost to the rock was determined from the difference between the known zinc concentration of the injected fluid and the zinc concentration of the effluent. The pH of the brines and the zinc chloride solutions employed in the experiments was 5.5. The experiments were carried out at a temperature of 50°C .

In Test 1, the pack of disaggregated rock was first flooded with Fluid 1 (the synthetic seawater of Table 1), followed by a 1000 ppmv solution of zinc chloride in Fluid 1.

In Test 2, the pack of disaggregated rock was first flooded with Fluid 4 (having the composition shown in Table 1), followed by a 1000 ppmv solution of zinc chloride in Fluid 4.

In Test 3, the pack of disaggregated rock was first flooded with Fluid 3 (the highly saline brine of Table 1), followed by a 1000 ppmv solution of zinc chloride in Fluid 3.

It was found that the concentration of zinc chloride in the effluent only reached the concentration in the injected fluid in Test 3. In Tests 1 and 2, loss of zinc chloride continued even after more than 22 pore volumes of the zinc solutions had been injected. Accordingly, the degree of loss of zinc chloride is given as the amount of zinc chloride lost per 5 grams of disaggregated sandstone rock after 22 pore volumes throughput. The results presented in Table 8 show that the amount of zinc chloride lost to the disaggregated sandstone rock (either through adsorption on the disaggregated sandstone rock or through precipitation of insoluble zinc species) decreases with increasing chloride concentration of the brine and hence with increasing mole fraction of the sum of the [ZnCl₄._n]^n"2 species present in the zinc solutions. Table 8 - Loss of Zinc Chloride

Geochemical Modelling

Geochemical modelling of a number of aqueous fluids was carried out using a validated geochemical model (obtained using PHREEQC software and validated using EXAFS data).

Minimum Chloride Concentrations in Absence of Additional Complexing Anions

The data inputted into the geochemical model included the compositions of various aqueous fluids having dissolved zinc concentrations in the range of 100 to 3750 ppmv and chloride concentrations from 0 to 5 M (moles/litre). The salts inputted into the model were zinc chloride and sodium chloride. The chloride concentration was therefore based on the total chloride in solution arising from both the zinc chloride salt and the chloride salt. The equilibrium concentrations of the various zinc species were modelled at a number of different temperatures and at a pH of less than 6.5. The output of the model included the molar concentrations of the individual [ZnCl₄__n]^n"2 species and of the Zn²⁺ ions at each modelled temperature.

Figure 1 shows plots of the mole fraction of the sum of the [ZnCl_4-n]^n"2 species versus chloride concentration (moles/litre) at 3,750 ppmv dissolved zinc concentration at temperatures of 20, 40, 60, 80, 100, 120 and 150°C. Similar plots can be obtained for different dissolved zinc concentrations of, for example, 100, 200, 1000, or 2000 ppmv.

Figure 2 shows plots of chloride concentration (moles/litre) versus temperature for mole fractions of the sum of the [ZnCl₄__n]^n"2 species of 0.4, 0.5, 0.75 and 0.9 (wherein "x(ZnCl)" is an abbreviation for "mole fraction of the sum of the [ZnCl_4-n]^n"2 species"). Figure 2 is based on calculations for a dissolved zinc concentration of 3,750 ppmv. Similar plots can be obtained for different dissolved zinc concentrations of, for example, 100, 500, 1000, or 2000 ppmv. By "similar plots" is meant that the plots are substantially overlying.

The data for x(ZnCl) of 0.4 (calculated for a dissolved zinc concentration of 3,750 ppmv) were found to fit a curve defined by the following equation:

[CI] = 1.9677e^{"0 035T} (Equation 3) wherein "[CI]" is the minimum molar chloride concentration (moles/L), "e" is the natural exponential function, and "T" is the temperature in degrees Centigrade (this equation corresponds to Equation 3). A statistical regression analysis gave a coefficient of determination, R , for how well the modelled data fits the curve of 0.9986.

The data for x(ZnCl) of 0.5 (calculated for a dissolved zinc concentration of 3,750 ppmv) were found to fit a curve defined by the following equation:

[CI] = 2.3392e^{" u3 U} (Equation 4) wherein "[CI]", "e" and "T" are as defined above (this equation corresponds to Equation 4). A statistical regression analysis gave a coefficient of determination, R², for how well the data fits the curve defined by this equation of 0.9974.

The data for x(ZnCl) of 0.75 (calculated for a dissolved zinc concentration of 3,750 ppmv) were found to fit a curve defined by the following equation:

[CI] = 3.2209e^"°-^022T (Equation 5) wherein "[CI]" , "e" and "T" are as defined above (this equation corresponds to Equation 5). A statistical regression analysis gave a coefficient of determination, R², for how well the data fits the curve of 0.9983.

The data for x(ZnCl) of 0.9 (calculated for a dissolved zinc concentration of 3,750 ppmv) were found to fit a curve defined by the following equation: [CI] = 3.6758 e^{"U Ui41} (Equation 6) wherein "[CI]" , "e" and "T" are as defined above (this equation corresponds to Equation 6). A statistical regression analysis gave a coefficient of determination, R , for how well the data fits the curve of 0.9995.

Tables 9 and 10 below show mole fractions of the sum of the [ZnCl_4-n]^n"2 species for aqueous displacement fluid containing 100 and 200 ppmv of dissolved zinc respectively that were calculated using the minimum chloride concentrations deteimined using

Equations 1, 4, 5 and 6 (with bromide and additional complexing anions such as sulfate, set to zero in the geochemical model). It can be seen that the calculated values of x(ZnCl) are in good agreement with the target values. Thus, the calculated x(ZnCl) for aqueous displacement fluids containing 100 ppmv dissolved zinc are within 10% of the target value, apart from three data points which are within 15% of the target value. Also, , the calculated x(ZnCl) for aqueous displacement fluids containing 200 ppmv dissolved zinc are all within 10% of the target value. Accordingly, Equations 1, 4, 5 and 6 may be used to determine the minimum chloride concentration for an aqueous displacement fluid across a range of zinc concentrations of from 100 to 3750 ppmv. Table 9 - Calculated x(ZnCl) at 100 ppmv Dissolved Zinc Concentration

Temperature x(ZnCl)^a x(ZnCl) x(ZnCl)° x(ZnCl)^d

(°C) (target=0.4) (target = 0.5) (target = 0.75) (target = 0.9^}

20 0.41 0.54 0.82 0.91

40 0.36 0.46 0.76 0.91

60 0.37 0.46 0.72 0.9

80 0.36 0.48 0.72 0.89

100 0.36 0.48 0.72 0.89

120 0.34 0.46 0.73 0.89

150 0.34 0.44 0.75 0.88 a. calculated using [Chloride] _mj_n determined using Equation 3.

b. calculated using [Chloride] _mi_n determined using Equation 4.

c. calculated using [Chloride]_mi_n determined using Equation 5.

d. calculated using [Chloride]_raj_n determined using Equation 6.

Table 10 - Calculated x(ZnCl) at 200 ppmv Dissolved Zinc Concentration

a. calculated using [Chloride] _mi_n determined using Equation 3.

b. calculated using [Chloride]_mi_n determined using Equation 4.

c. calculated using [Chloride] _mj_n determined using Equation 5.

d. calculated using [Chloride] _mi_n determined using Equation 6.

Tolerable Bromide Content

Further geochemical modelling was carried out for aqueous fluids comprising 1000 ppmv zinc chloride (500 ppmv dissolved zinc when expressed to one significant figure) in brines having total halide concentrations of 0.6 and 1.5 M (moles/litre). The modelled brines were sodium chloride brines (0 mol% bromide), sodium bromide brines (100 mol% bromide) and brines comprising varying ratios of sodium bromide and sodium chloride. Additional complexing anions such as sulfate, nitrate and nitrite were set to zero in the model.

Figures 3 and 4 show plots of the mol% of the sum of the [ZnBr_4-n]^n"2 species (based on total dissolved zinc) versus mol% bromide (based on total halide) at room temperature (20°C) and at a temperature of 70°C respectively. Figure 3 shows that, at room temperature, less than 1 mol% (less than 0.01 mole fraction) of the dissolved zinc in brines containing up to about 40 mol% of bromide is comprised of [ZnBr₄__n]^n"2 species (labelled as ZnBrx). Figure 3 also shows that a mole fraction of [ZnBr_4-n]^n~2 species of 0.4 cannot be achieved at room temperature even with a 1.5 M sodium bromide brine (100 mol% bromide). Thus, the model predicts a mol% of [ZnBr_4-n]^n~2 species of less than 4.5 (a mole fraction of [ZnBr_4-n]^n"2 species of 0.045) for a 1.5 M sodium bromide brine (100 mol% bromide) at room temperature. Figure 4 shows that higher concentrations of [ZnBr_4- n]^n"2 species are predicted to exist at the higher temperature of 70°C. It was found that the dissolved zinc in brines containing up to about 16 mol% of bromide comprised less than 1 mol% of [ZnBr_4-n]^n"2 species. This mol% of bromide is significantly higher than the amounts of bromide that exist in naturally occurring saline waters that are available as source waters for the aqueous displacement fluid of the present invention. Tolerable Sulfate Concentration

Further geochemical modelling was carried out for aqueous fluids comprising 3750 ppmv of dissolved zinc using the minimum chloride concentrations determined from Equation 3 to 6 corresponding to target xZnCl values of 0.4, 0.5, 0.75 and 0.9 respectively in the presence of 5 mol% of sulfate anions (based on the total amount of chloride anions). Other zinc-complexing inorganic anions such as bromide and zinc-complexing organic anions such as carboxylates were set to zero in the model. Figure 5 shows the percentage amount of additional chloride anions (compared with the minimum chloride concentration in the absence of sulfate anions) required to achieve mole fractions of the sum of the

[ZnCl_4-n]^n"2 species of 0.4, 0.5, 0.75 and 0.9 at temperatures of 20, 80 and 150°C. It can be seen that in the presence of 5 mol% of sulfate anions, it may be necessary to add up to about 20% of additional chloride anions.

Claims

Claims

1. A method for recovering crude oil from a reservoir comprising a porous and permeable rock having crude oil and formation water in the pore space thereof wherein the reservoir has a temperature in the range of 20 to 150°C and the formation water has a pH of less than 7.5 and a concentration of dissolved hydrogen sulfide of less than 10 mg/L, (0.290 mmol/L), the method comprising:

injecting an aqueous displacement fluid into the reservoir and recovering crude oil from the reservoir wherein the aqueous displacement fluid comprises a solution of a zinc salt in an aqueous solvent having (i) at least one chloride salt, and optionally, (ii) at least one bromide salt dissolved therein, characterized in that the aqueous displacement fluid has:

(a) a concentration of dissolved zinc in the range of 100 to 3,750 mg/L (1.52 to 57.35 mmol/L), preferably, 175 to 3,750 mg/L (2.66 to 57.35 mmol/L), wherein the dissolved zinc is in the form of hydrated Zn²⁺ ions, one or more hydrated [ZnCl_4-n]^n~2 species wherein n is an integer selected from 0, 1, 2 and 3, and, optionally, one or more hydrated [ZnBr_4-n]^n" species wherein n is an integer selected from 0, 1 , 2 and 3 ;

(b) a mol% of bromide anions of from 0 to 10 mol% based on the total molar concentration of chloride and bromide anions;

(c) a pH of less than 7.5; and

(d) a mole fraction of the sum of the hydrated [ZnCl_4-n]^n"2 species of at least 0.4 based on the total molar concentration of dissolved zinc species.

2. A method as claimed in Claim 1 wherein the aqueous displacement fluid has a mole fraction of the sum of the [ZnCl_4-n]^n"2 species of at least 0.5, preferably, at least 0.75, most preferably, at least 0.9 based on the total molar concentration of dissolved zinc species

3. A method as claimed in Claims 1 or 2 wherein the aqueous displacement fluid has a mol% of sulfate anions of less than 0.5 (based on the molar concentration of chloride anions).

4. A method as claimed in any one of the preceding claims wherein, prior to injection into the reservoir, the aqueous displacement fluid has a concentration of hydrosulfide anions of less than 2 mg/L (0.058 mmol/L), more preferably, less than 1 mg/L (0.029 mmol/L), most preferably less than 0.5 mg/L (0.0145 mmol/L).

5. A method as claimed in any one of the preceding claims wherein the aqueous displacement fluid has an equivalent concentration of one or more organic zinc- complexing anions of less than 0.1 mEq/L wherein the organic zinc-complexing anion(s) has one or more anionic zinc-complexing functional groups, and the equivalent concentration is based on the number of anionic functional groups in the organic zinc- complexing anion(s).

6. A method as claimed in any one of the preceding claims wherein the aqueous displacement fluid has an equivalent concentration of zinc-complexing ligands of less than 0.05 mEq/L wherein the zinc-complexing ligand is monodentate or polydentate, and the equivalent concentration is based on the denticity (number of donor groups) of the ligand(s).

7. A method as claimed in any one of the preceding claims wherein the aqueous displacement fluid has a mol% of chloride anions and optional bromide anions of at least 99.5 mol% based on the total molar concentration of inorganic anions.

8. A method as claimed in any one of Claims 3 to 7 wherein the aqueous displacement fluid has a molar concentration of chloride anions at or above a minimum value,

[Chloride]_mi_n, that is predicted to give a mole fraction of the sum of the [ZnCL-_n]^11"2 species of 0.4, determined using Equation 3:

[Chloridej_mi_n = 1.9677exp(-0.035 x T) Equation 3

wherein "exp" is the natural exponential function, and "T" is the reservoir temperature in degrees centigrade (°C).

9. A method as claimed in Claim 8 wherein the molar concentration of chloride anions is at or above a minimum value, [Chloride] _mi_n, that is predicted to give a mole fraction of the sum of the [ZnCl_4-n]^n"2 species of 0.5, determined using Equation 4: [Chloride]_min (moles/liter) = 2.3392exp(-0.031 x T) Equation 4 wherein "exp" is the natural exponential function, and "T" is the reservoir temperature in degrees centigrade (°C).

10. A method as claimed in Claim 9 wherein the molar concentration of chloride anions is at or above a minimum value, [Chloride]_mi_n, that is predicted to give a mole fraction of the sum of the [ZnCl₄._n]^n~2 species of 0.75, determined using Equation 5: [Chloride]_min (moles/litre) = 3.2209exp(-0.022 x T) Equation 5 wherein "exp" is the natural exponential function, and "T" is the reservoir temperature in degrees centigrade (°C).

11. A method as claimed in Claim 10 wherein the molar concentration of chloride anions is at or above a minimum value, [Chloride]_mi_n, that is predicted to give a mole fraction of the sum of the [ZnCl_4-n]^n"2 species of 0.90, determined using Equation 6:

[Chloride]_min (moles/litre) = 3.6758exp(-0.014 x T) Equation 6 wherein "exp" is the natural exponential function, and "T" is the reservoir temperature in degrees centigrade (°C).

12. A method as claimed in any one of claims 1 to 2 and 4 to 7 wherein the aqueous displacement fluid comprises a solution of a zinc salt in an aqueous solvent comprising a naturally occurring saline water selected from the group consisting of seawater, estuarine water and mixtures thereof having a sulfate concentration of in the range of 3.0 to 7.0 mol%, preferably, 4.0 to 6.5 mol%, in particular, 4.5 to 5.5 mol% (based on the molar chloride concentration) and wherein, if necessary, a chloride salt is added to the aqueous displacement fluid to increase the chloride concentration to a value at least 20% higher than the value of [Chloride]_mi_n deteraiined using Equation 3 as defined in claim 8.

13. A method as claimed in claim 12 wherein the chloride salt is added to the aqueous displacement fluid to increase the chloride concentration to a value at least 20% higher than the value of [Chloride]_mi_n determined using any one of Equations 4 to 6 as defined in claims 9 to 11 respectively.

14. A method as claimed in any one of claims 3 to 11 wherein the aqueous solvent is a naturally occurring saline water selected from a produced waters, saline aquifer water and mixtures thereof having a concentration of sulfate anions of less than 0.5 mol% (based on the molar chloride concentration).

15. A method as claimed in any one of claims 3 to 11 wherein the aqueous solvent is a sulfate reduced saline water wherein the sulfate reduced saline water has a residual sulfate concentration of less than lOOmg/L, preferably, less than 40 mg/L.

16. A method as claimed in claim 15 wherein the sulfate reduced saline water is a permeate stream formed by passing a naturally occurring saline water selected from the group consisting of seawater, estuarine water and mixtures thereof through a nanofiltration membrane.

17. A method as claimed in any one of the preceding claims wherein the aqueous displacement fluid is formed by adding a zinc salt selected from zinc hydroxide, basic zinc carbonate, zinc nitrite, zinc nitrate, zinc chloride, zinc bromide, zinc sulfate and mixtures thereof to the aqueous solvent with the proviso that when the aqueous solvent is a sulfate reduced saline water, the zinc salt is not zinc sulfate.

18. A method as claimed in claim 17 in which a protic acid selected from the group consisting of hydrochloric acid, hydrobromic acid, nitric acid, sulfuric acid and mixtures thereof is added to the aqueous solvent either before or after the addition of the zinc salt, to adjust the pH of the aqueous displacement fluid to a value of less than 7.5, preferably, less than 7.0, more preferably, less than 6.5 with the proviso that when the aqueous solvent is a sulfate reduced water, the acid is not sulfuric acid.

19. A method as claimed in claims 17 or 18 in which the zinc salt is added to the aqueous solvent in the form of a concentrate comprising an aqueous solution of a zinc salt having a concentration of zinc salt of at least 10,000mg/L, preferably, at least 15,000 mg/L.

20. A method as claimed in claim 19 in which the concentrate has a concentration of zinc salt in the range 20 to 75% by weight, preferably, 30 to 70% by weight.

21. A method as claimed in claims 19 to 20 wherein an oil-bearing zone of the reservoir is penetrated by an injection well and the zinc concentrate and aqueous solvent are injected separately into the injection well and are mixed within the injection well to form the aqueous displacement fluid at a location at or immediately above the oil-bearing zone of the reservoir.

22. A method as claimed in claim 21 wherein the injection well has: (a) an injection tubing extending from a wellhead to a location in the injection well at or immediately above the oil-bearing zone; and (b) an injection line arranged either in the annulus formed between the injection tubing and the wall of the injection well or within the injection tubing wherein the injection line extends to a region at or immediately below the lower end of the injection tubing; and wherein the aqueous solvent is injected down the injection tubing, the zinc concentrate is injected down the injection line, and the aqueous solvent and zinc concentrate are mixed in the injection well in the region at or immediately below the lower end of the injection tubing.

23. A method as claimed in any one of claims 17 to 22 in which the aqueous

displacement fluid is formed by adding a chloride salt selected from Group IA metal and Group IIA metal chlorides to the aqueous solvent.

24. A method as claimed in claim 23 wherein the chloride salt is added to the aqueous solvent in the form of a concentrate comprising an aqueous solution of the chloride salt having a concentration of chloride salt in the range 20 to 36% by weight.

25. A method as claimed in claim 24 wherein the chloride concentrate is mixed with the zinc concentrate and the mixed concentrate is injected down the injection line and is mixed with the aqueous solvent in the injection well in the region at or immediately below the lower end of the injection tubing.

26. A method as claimed in claim 24 wherein the chloride concentrate and aqueous solvent are injected separately into the injection well and are mixed within the injection well to form the aqueous displacement fluid at a location at or immediately above the oil- bearing zone of the reservoir.

27. A method as claimed in claim 26 wherein a further injection line is arranged either in the annulus formed between the injection tubing and the wall of the injection well or within the injection tubing wherein the further injection line extends to a region at or immediately below the lower end of the injection tubing; and wherein the chloride concentrate is injected down the further injection line and the aqueous solvent, zinc concentrate and chloride concentrate are mixed in the injection well in the region at or immediately below the lower end of the injection tubing.

28. A method as claimed in any one of claims 23 to 27 wherein the amount of the concentrates mixed with the aqueous solvent is controlled by adjusting the volumetric flow rate for the concentrates into the injection line(s), the volumetric flow rate for the aqueous solvent in the injection tubing or both.