US20140008271A1

US20140008271A1 - Hydrocarbons recovery

Info

Publication number: US20140008271A1
Application number: US13/821,628
Authority: US
Inventors: Robert Moene; Wilhelmus Petrus Mul; Arian Nijmeijer
Original assignee: Individual
Current assignee: Shell USA Inc
Priority date: 2010-09-10
Filing date: 2011-09-09
Publication date: 2014-01-09
Also published as: EP2613863A1; BR112013004602A2; WO2012032161A1; CN103096989A; EA201390365A1; CN103096989B; MX2013002580A; CA2812977A1

Abstract

Methods and systems of separating a hydrocarbon phase from a mixture comprising an emulsion of water and hydrocarbons in the presence of a surfactant, comprising adjusting the salinity of the mixture to release hydrocarbons and water from the emulsion into a hydrocarbon phase and a salt-containing aqueous phase respectively; and separating at least a part of the hydrocarbon phase from the salt-containing aqueous phase wherein at least a part of the salt-containing aqueous phase is recovered for further use.

Description

FIELD OF THE INVENTION

This invention relates to the recovery of hydrocarbons. In particular, though not exclusively, the invention relates to improved processes and apparatus for separating mixtures comprising an emulsion of hydrocarbons and water.

BACKGROUND OF THE INVENTION

Hydrocarbons are a large class of organic compounds composed of hydrogen and carbon. For example, crude oils, natural gas, kerogen, bitumen, pyrobitumen and asphaltenes are all mixtures of various hydrocarbons. Though hydrocarbons are generally defined as molecules formed primarily of carbon and hydrogen atoms, they may also include other elements, such as, but not limited to, halogens, metallic elements, nitrogen, oxygen and/or sulfur.
Hydrocarbons are typically recovered or produced from subterranean formations by a variety of methods. “Primary recovery” techniques refer to those techniques that utilise energy from the formation itself to recover the hydrocarbons in the subterranean formation. However primary recovery techniques are only capable of producing a small fraction of the oil in place in the reservoir. Consequently secondary techniques, such as waterflooding, and numerous tertiary techniques (commonly referred to as “enhanced oil recovery” (EOR) techniques), have been developed, which have as their primary purpose the recovery of additional quantities of hydrocarbons known to be present in the reservoir.
One of the most common secondary techniques is waterflooding. However, even after a typical waterflood the reservoir may retain a great portion of its hydrocarbons in place. It is well known that much of the retained hydrocarbons in the reservoir after a typical waterflood are in the form of globules or droplets that are trapped in the pore spaces of the reservoir. The high normal interfacial tension between the reservoir water and hydrocarbons prevents these discrete droplets from deforming to pass through narrow constrictions in pore channels.
In certain tertiary techniques, referred to as “chemical enhanced oil recovery” (cEOR) techniques, surface-active agents or surfactants have been added to flood water solutions to lower the interfacial tension between the water and hydrocarbons and thereby allow hydrocarbon droplets to deform and flow with injected floodwater. It is generally thought that the interfacial tension between the oil and water must be reduced from the normal reservoir interfacial tension, that is on the order of about 20 dyne/cm, to less than 0.1 dyne/cm for effective recovery.
Many patents describe cEOR utilizing chemical surfactants and polymers, including among others U.S. Pat. Nos. 3,508,611, 3,823,777, 3,981,361, 4,058,467, 4,203,491, 4,232,738, 4,362,212, 4,411,816, 4,458,755, 4,493,371, 4,501,675, 4,502,541, 4,799,547, 5,031,698, 5,068,043, 6,022,834, 6,613,720, 6,989,355 and Canadian Pat. No. 1,169,759. Fundamentally, as indicated in SPE Paper No. 7053 presented at a symposium on improved methods for oil recovery in April 1978, two essential criteria must generally be met for successful recovery of residual oil by chemical flooding: (1) very low interfacial tension between the chemical bank and the residual oil and between the chemical bank and the drive fluid, and (2) small surfactant retention losses to the reservoir rock.
During cEOR, low interfacial tension in the chemical bank typically results in the formation of microemulsions (see for example U.S. Pat. No. 3,254,714, Gogarty et al, issued June 1966, and U.S. Pat. No. 3,981,361 Healy, issued Sep. 21, 1976). Microemulsions, which may be injected or formed within a hydrocarbons formation, are miscellar mixtures of oil, water and a surfactant, frequently in combination with a co-surfactant, co-solvent or other chemicals. There is some scientific debate as to the exact structure of microemulsions; in this specification, the term “microemulsion” refers broadly to emulsions which are thermodynamically stable.
Formulation of injection mixtures in enhanced hydrocarbons recovery is often based on the identification of state variables that lead to a so-called “middle-phase” (or Winsor type-III) microemulsion in equilibrium with both excess hydrocarbons and excess water. “Optimal salinity” is usually thought of as the specific salt concentration that produces the lowest interfacial tension between oil and water in a given mixture and thus results in a middle phase microemulsion.
In those reservoirs that have been subjected to a cEOR flood, the recovered liquid from a producing well is usually in the form of a mixture (emulsion) comprising hydrocarbons, water and a surfactant. These mixtures are relatively stable because of reduced interfacial tension and typically comprise, as one component, a microemulsion of water and hydrocarbons.
The petroleum industry has long sought economical and efficient methods for breaking mixtures produced from surfactant-based floods, in particular their microemulsion component, and for recovering valuable chemicals contained in such mixtures for reuse in cEOR.
For example, GB 2 146 040 describes a method for breaking oil-water-surfactant emulsions that have as one component a microemulsion. This method separates oil, brine, and surfactant, to produce pipeline quality oil and an injectable brine/surfactant phase by carefully controlling temperature and salinity within certain operable ranges.
US2009/0281003 relates to a method in which chemicals in a cEOR stream are forced into either an aqueous or an organic phase and are then concentrated and re-injected into an oil-bearing formation.
However, there remains a need for sustainable and cost-effective and methods and systems for separating hydrocarbons from cEOR mixtures. Accordingly, the present invention seeks to solve the technical problem of providing a more sustainable and/or cost-effective hydrocarbon separation method and system than the prior art.

SUMMARY OF INVENTION

From a first aspect, the invention resides broadly in a method of separating a hydrocarbon phase from an input mixture comprising an emulsion of water and hydrocarbons in the presence of a surfactant (surface-active agent), the method comprising: adjusting the salinity of the mixture to release hydrocarbons and water from the emulsion into a hydrocarbon phase and a salt-containing aqueous phase respectively; and separating at least a part of the hydrocarbon phase from the salt-containing aqueous phase, wherein at least a part of the salt-containing aqueous phase is recovered (or retained) for further use.
As the input mixture comprises hydrocarbons, water, and a surfactant, it typically comprises, at least as one component, a microemulsion. The interfacial tension between hydrocarbons and water in the input mixture may for example be below 1 dyne/cm.
The input mixture may preferably be a mixture produced by cEOR and may therefore further include a co-surfactant, a co-solvent or other chemicals, as is known in the art. Additionally, cEOR mixtures have a salinity consistent with operational requirements for hydrocarbons recovery. As lowering interfacial tension is a key objective, some cEOR mixtures may be at or close to “optimal salinity”. Thus the input mixture may advantageously have a salinity at or close to its “optimal salinity”. However, the salinity of the input mixture may vary, e.g. based on operational requirements for hydrocarbons recovery, or as a result of treatment, dilution or concentration carried out before the method of the invention.
Salinity is known to have an impact on interfacial tension in water-hydrocarbons-surfactant mixtures, and is therefore a state variable that is used in the present invention to control, particularly reduce, the degree of emulsification in such mixtures, with the aim of separating a hydrocarbon phase. Other state variables, such as temperature and pressure, may advantageously be kept substantially constant.
The method of the invention enables particularly effective and sustainable use of salinity as a state variable in the separation of hydrocarbons from emulsions, especially microemulsions, such as those obtained from cEOR. Specifically, by including the recovery of at least a part of the salt-containing aqueous phase, the method of the invention enables reuse of salt and/or water to adjust the salinity of the input mixture. Thus, in the method of the invention, salt and/or water need not be sacrificed during the hydrocarbon separation process. This is a significant advantage in the context of large-scale operations, where the costs of obtaining water, and especially salt, as well as associated transportation costs, may be significant commercial factors.
Accordingly, in a preferred embodiment of the invention, the method may comprise reusing at least a part of the salt-containing aqueous phase, for example salt and/or water contained therein, to adjust the salinity of the mixture as described above.
To provide maximum opportunity to reuse the salt-containing aqueous phase, the method of the invention may be operated continuously, in the sense that more than one batch of input mixture may be subjected (successively) to the method of the invention. Most advantageously, the method may be carried out in a continuous flow.
Alternatively the method may be carried out in a plurality of batches. Batch operation of the method is typically less effective than continuous flow operation but may be of particular benefit where the method is to be used intermittently, or where individual steps of the method are to be carried out in geographically distinct locations, as is optionally contemplated within the scope of the invention.
The release of hydrocarbons and water from an emulsion into a hydrocarbon phase and a salt-containing aqueous phase may be accomplished by either an increase or a decrease in salinity. Thus adjusting the salinity of the input mixture may comprise either increasing or decreasing the salinity of the mixture. An increase in salinity may be achieved by adding salt to the input mixture and/or by removing water from the input mixture, whereas a decrease in salinity may be achieved by removing salt from the input mixture and/or adding water to the input mixture.
The choice between increasing and decreasing the salinity of the input mixture is particularly apparent in the context of input mixtures, for example derived from cEOR, that are at or close to “optimal salinity”. In such mixtures, a relatively small adjustment of salinity, be it an increase or a decrease, can result in the release of hydrocarbons and water from the emulsion in the input mixture, i.e. the breaking of the emulsion.
In mixtures having a salinity above “optimal salinity”, an increase in salinity may be more economical, whilst in mixtures having a salinity below “optimal salinity” a decrease may be more economical. However, operational considerations play a dominant role in choosing whether to adjust the salinity of the input mixture by increasing or decreasing salinity. It is hence preferred in the present invention to adjust the salinity of the input mixture by increasing the salinity of the input mixture, as this provides particular opportunities for the reuse of the recovered salt-containing aqueous phase, as will be explained.
To enable particularly effective reuse of the salt-containing aqueous phase, the method may further comprise desalinating the salt-containing aqueous phase to provide a first stream having a relatively high salinity and a second stream having a relatively low salinity. Desalination may be carried out in any suitable desalination station or unit, for example by nanofiltration and/or reverse osmosis.
The first, high salinity stream may advantageously be used (or recycled) to increase the salinity of the mixture, i.e. to aid emulsion breaking as described above. To fulfil this function, the high salinity stream must have a salinity which is higher than the salinity of the input mixture. Thus the high salinity stream may advantageously have a salinity above the salinity of the input mixture.
The surfactant in the input mixture is generally distributed in both the water and the hydrocarbons. Following salinity adjustment of the mixture, there occurs a shift of surfactant (and any co-solvent that may be present) into either the hydrocarbon phase (if the salinity is increased) or the aqueous phase (if the salinity is decreased). However, this shift is not complete and therefore there always remains at least some surfactant (and co-solvent if present) in the hydrocarbon phase separated from the mixture, often along with a significant concentration of salt. This concentration of surfactant may be undesirable for further processing or use of the hydrocarbon phase, particularly where a salinity increase of the input mixture has driven substantial amounts of surfactant into the hydrocarbon phase. Also, surfactant that remains in the hydrocarbon phase is lost for the purpose of cEOR and therefore represents an additional cost of the cEOR process. To mitigate the loss of surfactant and purify the hydrocarbon phase, the method of the invention may advantageously further comprise washing the hydrocarbon phase to recover surfactant and optionally salt therefrom.
Conveniently, the low salinity stream produced by desalination of the salt-containing aqueous phase may be reused to wash the hydrocarbon phase to recover surfactant and optionally salt therefrom. As it is undesirable for the hydrocarbon phase to form a stable emulsion with the low salinity stream, the low salinity stream may preferably be desalinated such that its salinity is lower than the salinity of the input mixture. Thus, to cater for typical cEOR input mixtures, the low salinity stream may have a salinity below “optimal salinity”.
After washing, the hydrocarbon phase may preferably be separated to form a purified crude oil stream, leaving behind an aqueous surfactant recovery stream. The surfactant recovery stream may advantageously be re-injected into a hydrocarbons bearing formation in a cEOR process, optionally in combination with salt from the first, high salinity stream.
From a second aspect, the invention broadly resides in a hydrocarbons separation system suitable for performing the method according to the first aspect of the invention, the system comprising: an inlet for introducing a mixture comprising an emulsion of water and hydrocarbons in the presence of a surfactant; a salinity adjustment station for adjusting the salinity of the mixture; separating means for separating from the mixture a hydrocarbon phase and a salt-containing aqueous phase; and recovery means for recovering at least a part of the salt-containing aqueous phase for further use.
The recovery means may advantageously comprise a conduit for recycling at least a part of the salt-containing aqueous phase to the salinity adjustment station to adjust the salinity of the mixture.
To aid separation of the hydrocarbon phase from the aqueous phase, the separation system may comprise a membrane, for example of the ceramic type. The membrane may act as the sole separating means or may be combined with a phase separation vessel.
For effective further use of the salt-containing aqueous phase, the system may further comprise desalination means for desalinating the salt-containing aqueous phase to provide a first stream with a relatively high salinity and a second stream with a relatively low salinity. The desalination means may comprise a reverse osmosis unit, or a nanofiltration unit, or a nanofiltration, ultrafiltration or microfiltration unit upstream of a reverse osmosis unit.
Preferably, the system may comprise a conduit for channelling the first stream to the salinity adjustment station to increase the salinity of the mixture.
The system may advantageously further comprise washing means for washing the hydrocarbon phase and recovering a surfactant recycle stream, and a conduit for channelling the second, low salinity stream to the washing means. The additional advantage of a surfactant recycle in the system, is that it becomes a self-sufficient system.
The invention contemplates the further use and reuse of various components separated from a mixture comprising water, hydrocarbons and a surfactant. Thus, from a third aspect, the invention resides in a chemical enhanced oil recovery process comprising injecting surfactant recovered from a hydrocarbon phase as described anywhere herein into a hydrocarbon-bearing formation. Preferably, the surfactant may be combined with salt recovered as described anywhere herein, for example prior to injection.
Unless otherwise indicated, terms used herein are to be construed based on their standard definition in the art. Similarly, unless otherwise indicated, parameters provided herein are based on the relevant standard measuring techniques (ISO where available).
The term “emulsion” as used herein refers simply to a mixture of two or more immiscible liquids.
Those skilled in the art will appreciate that an emulsion of water and hydrocarbons in the presence of a surfactant can lead to the formation of at least an amount of a “microemulsion”, i.e. a thermodynamically stable emulsion.
The term “interfacial tension” as used herein refers to the strength of the film separating two immiscible fluids (hydrocarbons and water) measured in dynes per centimetre, according to ASTM D971.
The term “salt” as used herein refers to all salts soluble in water. Sodium chloride is a preferred salt.
The term “salinity” as used herein refers to the amount of dissolved salt in water. Salinity referred to herein may be determined according to the Practical Salinity Scale 1978 (PSS78), originally developed for seawater, which involves a conductivity comparison to a solution of 32.4356 g/kg KCl at 15° C.
The term “optimal salinity” as used herein refers to the salt concentration that produces the lowest interfacial tension between oil and hydrocarbons in a given mixture of hydrocarbons, water and a surfactant. It may be measured by standard interfacial tension measurements or be derived from other methods like phase behaviour tests that are known to persons skilled in the art.
The term “surfactant” or “surface active agent” as used herein refers to any chemical agent capable of reducing the interfacial tension between hydrocarbons and water.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a hydrocarbons separation system for separating hydrocarbons from a hydrocarbons recovery mixture according to a first embodiment of the invention;

FIG. 2 is a schematic diagram of a hydrocarbons separation system for separating hydrocarbons from a hydrocarbons recovery mixture according to a second embodiment of the invention; and

FIG. 3 is a schematic diagram of a hydrocarbons separation system for separating hydrocarbons from a hydrocarbons recovery mixture according to a third embodiment of the invention.

With reference to FIGS. 1 to 3, the hydrocarbons separation systems and methods (or processes) according to three exemplary embodiments of the invention described below each serve to extract or separate a hydrocarbon phase from an input mixture. The input mixture comprises an emulsion of water and hydrocarbons in the presence of a surfactant (surface-active agent). Whilst the input mixture could stem from any suitable source, in practice, such mixtures are most commonly obtained from chemical enhanced oil recovery (cEOR) streams.
The hydrocarbons separation systems and methods according to the three exemplary embodiments of the invention comprise adjusting, specifically increasing, the salinity of the input mixture to release hydrocarbons and water from the emulsion into a hydrocarbon phase and a salt-containing aqueous phase respectively. Thereafter, at least a part of the hydrocarbon phase is separated for further use, and at least a part of the salt-containing aqueous phase is recovered for reuse.
To maximise efficiency, the methods according to the exemplary embodiments of the invention are each carried out in a continuous flow manner, in a single system or facility. However, it will be appreciated that the individual stages of the methods could also be performed in batches, either in a single facility, or in several geographically distinct locations.
Referring firstly to FIG. 1, in a first embodiment of the invention, a conventional emulsified cEOR mixture 1 comprising hydrocarbons (in the form of crude oil), water and a surfactant is input into a hydrocarbons recovery system.
The surfactant in the cEOR mixture 1 may be of any known type suitable for cEOR. In addition to surfactants, the cEOR mixture may comprise other additives as known in the art.
The cEOR mixture 1 has an original salinity that supports effective cEOR, at which the surfactant provides a lowered interfacial tension between the oil and the water, e.g. in the order of less than 1 dyne/cm. Usually, the cEOR mixture 1 is at or close to optimum salinity.
To reduce the degree of emulsion in the cEOR mixture 1, i.e. to release oil and water into separate oil and aqueous phases, the mixture is initially channelled to a salinity adjustment station S. Salinity has a strong impact on the ability of surfactants to lower surface tension in cEOR mixtures. For example, high salinities, in excess of the “optimum salinity” of a cEOR mixture, cause microemulsions to be broken, leading to the formation of increased oil (or hydrocarbons) and aqueous (or water) phases. The salt content of the mixture tends to dissolve mainly in the aqueous phase, although some salt will also be present in the oil phase, whilst an increased proportion of surfactant is pushed into the hydrocarbons oil with increasing salinity.
In the salinity adjustment station S, which may for example simply comprise a suitable conduit (as in FIGS. 1-3) or vessel, a high-salinity input 11 is mixed with the cEOR mixture 1 to increase the salinity of the mixture to a salinity in excess of the “optimum salinity” of the cEOR mixture. The high-salinity input 11 has a greater salinity than the cEOR mixture 1 and comprises recycled salt, as will be described later.
The ratio at which the cEOR mixture 1 and the high-salinity input 11 are mixed depends on a number of factors, such as the composition (including salinity) of the cEOR mixture 1 and the salinity of the high salinity input 11. The skilled person may choose to determine the requisite salinity increase in the mixture 2 simply by carrying out periodic visual inspections to determine whether the degree of emulsion in the cEOR mixture 2 is reduced.
The salinity-increased EOR mixture 2, with its increased oil and aqueous phases, is channelled into a phase separation (or demulsifier) vessel A. Phase separation vessels are well known in the art and make use of differences in density to separate oil and aqueous phases. The phase separation vessel A, and indeed all phase separation vessels mentioned herein, may take any suitable form and may for example be of the type described in Perry's Chemical Engineer's Handbook, 6^thedition, page 21-64 and further.
The phase separation vessel A separates the salinity-increased EOR mixture 2 into an oil phase 3 and an aqueous phase 6. The oil phase 3 is purified to provide crude oil, as will be described, whilst the aqueous phase 6 is recovered for further use, specifically for reuse in salinity adjustment.
Referring still to FIG. 1, the aqueous phase 6 is channelled via a membrane M to a desalination station B. The membrane M serves to remove any remaining oil from the aqueous phase 6. As the skilled reader is aware, the process of phase separation typically only provides a certain degree of purity, which in the case of the aqueous phase 6 is supplemented by the use of the membrane M. The membrane M may be any membrane suitable for removing hydrocarbons from water, such as for example a ceramic membrane. Suitable ceramic membranes comprise TiO₂, ZrO₂, Al₂O₃or SiC. The pore size of a suitable ceramic membrane is suitably smaller than 100 nm, preferably smaller than 50 nm, more preferably smaller than 30 nm and most preferably smaller than 10 nm. A hydrophobic membrane may be used, the use of which results in an efficient removal of the oil phase from the water phase. Suitable hydrophobic membranes include grafted ceramic membranes, for example a grafted ZrO₂-containing membrane, and polymeric membranes, for example poly(dimethylsiloxane) (PDMS) or poly-imide based membranes.
Following deep oil removal by the membrane M, the aqueous phase 6 enters the desalination station B, where it is processed into a low salinity stream 7 and a high salinity stream 8. The desalination station B may employ conventional reverse osmosis or nanofiltration technology, such as that disclosed in “Reverse Osmosis—A Practical Guide for Industrial Users”, W. Byrne, Tall Oaks Publishing Inc., March 1995.
The high salinity stream 8 produced by the desalination station B may be used wholly or partly as the high salinity input 11 added to the cEOR mixture in the salinity adjustment station S. Where only partial use of the high salinity stream is desired in the salinity adjustment station S, for example to prevent an excessive build-up of salinity in the system, the high salinity stream 8 may be split at a salinity outlet 10. The salinity outlet 10 may remove salt from the system, for example for use in cEOR reinjection 12.
The aqueous low salinity stream 7 produced by the desalination station B is used to wash the oil phase 3 resulting from phase separation of the cEOR mixture 2. Referring still to FIG. 1, the oil phase 3 typically contains a substantial concentration of surfactant as a result of the salinity increase in the cEOR mixture, together with some salt. Therefore, the low salinity stream 7, is mixed with the oil phase 3 to form a mixture with a salinity below “optimum salinity” in which surfactant and salt are washed out of the oil by the low salinity stream. Despite the presence of surfactant, the mixture shows only minimal emulsification because of the low level of salinity. Furthermore, surfactant and salt are washed from the oil phase 3 into the low salinity stream 7, turning the low salinity stream into an aqueous surfactant recycle stream 9 and the oil phase into a washed crude oil stream 5.
The surfactant recycle stream 9 is separated from the crude oil stream 5 in a second phase separation (demulsifer) vessel C. The crude oil stream 5 may be refined and processed further as desired, whilst the surfactant recycle stream 9, containing substantial concentrations of surfactant and salt may be used for cEOR reinjection 12.
Notably, the crude oil stream 5 is particularly suitable for further refining because it has already been desalinated, saving on desalination operations at the refinery.
In summary, the hydrocarbons separation system and method according to the first exemplary embodiment of the invention envisage increasing the salinity of a cEOR mixture to break emulsions (particularly microemulsions) therein, separating a hydrocarbon phase from the cEOR mixture for further use, reusing or recycling salt from the remaining aqueous phase within the separation system, and reusing water and optionally remaining salt from the aqueous phase for cEOR reinjection.
Referring now to FIG. 2, a hydrocarbons separation system and method according to a second embodiment of the invention is identical to the system and method according to the first embodiment of the invention, with like reference numerals being used for like parts, save for the working of the desalination step in station B.
The desalination station of the system according to the second embodiment of the invention comprises a nanofiltration unit B1 arranged in series with a reverse osmosis unit B2. Alternatively, nanofiltration unit B1 may be an ultrafiltration unit or microfiltration unit (not shown in FIG. 2). The advantage of this arrangement is that it enables the removal of divalent cations from the system. Specifically, the nanofiltration unit B1 removes and discards divalent cations. Thereafter, the reverse osmosis unit B2 processes the remaining salt and water in the salt-containing aqueous phase 6 in the manner of the desalination station B of the first embodiment of the invention, to form the high salinity stream 8 and the low salinity stream 7.
Referring now to FIG. 3, a hydrocarbons separation system and method according to a third embodiment of the invention is identical to the system and method according to the first embodiment of the invention, with like reference numerals being used for like parts, save that the membrane M acts as a phase separator, thereby eliminating the need for a distinct phase separation vessel.
Phase separation in the third embodiment of the invention occurs at the membrane M, which permits only the passage of aqueous phase, but not of the hydrocarbon phase. The passage of aqueous phase through the membrane M, as well as the addition of salt by the high salinity input 11, cause an increase in salinity and consequential breaking of the emulsion within the input mixture 2, i.e. the release of hydrocarbons and water from emulsion into the hydrocarbon phase and aqueous phase respectively. The hydrocarbon phase can then be channelled and separated for further processing using gravimetric principles and techniques known in the art, whilst the aqueous phase continues, via the membrane M, to desalination station B.
Notably, the use of membrane M as a phase separator enables an increase in the salinity of the input mixture by the removal of water rather than the addition of salt. Accordingly, in a variant of the third embodiment of the invention, input of salt at the salinity adjustment station S is not necessary and hence omitted, meaning that salt in the high salinity stream 8 may be reused elsewhere for example in cEOR reinjection 12, via salt outlet 10.
It will be appreciated that certain aspects of the method according to the first, second and third embodiments of the invention are non-essential and may be omitted, modified or replaced without departing from the scope of the invention. Notably, it is within the scope of the invention to modify the embodiments so that water instead of salt is recycled to the salinity adjustment station, in which case the salinity of the input mixture is decreased to break the emulsion, rather than increased.

Claims

1. A method of separating a hydrocarbon phase from a mixture comprising an emulsion of water and hydrocarbons in the presence of a surfactant, the method comprising:

adjusting the salinity of the mixture to release hydrocarbons and water from the emulsion into a hydrocarbon phase and a salt-containing aqueous phase respectively; and

separating at least a part of the hydrocarbon phase from the salt-containing aqueous phase,

wherein at least a part of the salt-containing aqueous phase is recovered for further use.

2. A method according to claim 1, further comprising reusing at least a part of the salt-containing aqueous phase to adjust the salinity of the mixture.

3. A method according to claim 1, wherein the salinity of the mixture is adjusted by increasing the salinity of the mixture.

4. A method according to claim 1 further comprising desalinating the salt-containing aqueous phase to provide a first stream having a relatively high salinity and a second stream having a relatively low salinity.

5. A method according to claim 4 further comprising reusing the high salinity stream to increase the salinity of the mixture.

6. A method according to claim 4 further comprising reusing the low salinity stream to wash the hydrocarbon phase to recover surfactant.

7. A hydrocarbons separation system comprising:

an inlet for introducing a mixture comprising an emulsion of water and hydrocarbons in the presence of a surfactant;

a salinity adjustment station for adjusting the salinity of the mixture;

separating means for separating from the mixture a hydrocarbon phase and a salt-containing aqueous phase; and

recovery means for recovering at least a part of the salt-containing aqueous phase for further use.

8. A system according to claim 7 wherein the recovery means comprises a conduit for recycling at least a part of the salt-containing aqueous phase to the salinity adjustment station to adjust the salinity of the mixture.

9. A system according to claim 7 comprising a membrane for separating the hydrocarbon phase from the aqueous phase.

10. A system according to claim 9, wherein the membrane acts as the separating means, either alone or in combination with a phase separation vessel.

11. A system according to any one of claim 7, further comprising desalination means for desalinating the salt-containing aqueous phase to provide a first stream with a relatively high salinity and a second stream with a relatively low salinity.

12. A system according to claim 11, wherein the desalination means comprises a reverse osmosis unit, a nanofiltration unit, or a nanofiltration unit upstream of a reverse osmosis unit.

13. A system according to claim 11 further comprising washing means for washing the hydrocarbon phase and recovering a surfactant recycle stream, and a conduit for channelling the second, low salinity stream to the washing means.

14. A chemical enhanced oil recovery process comprising injecting surfactant recovered by the method of claim 5 into a hydrocarbon-bearing formation.

15. A chemical oil recovery process according to claim 14 wherein the surfactant is combined with salt from the high salinity stream.