GB2184763A - Surfactant composition - Google Patents

Abstract

A surfactant composition comprises: (a) a branched chain polyolefin sulphonate having a molecular weight in the range 200 to 1,000, (b) an alkyl aromatic alkoxy sulphate of general formula <IMAGE> wherein R<1> is an alkyl group containing 6 to 22 carbon atoms; R<2> is an ethoxy or propoxy group, n is a number in the range 1 to 18; and M is an alkali or alkaline earth metal cation, or an ammonium or amine cation, and x is a number corresponding to the valency of M; and, a carrier comprising, (c) an oil-soluble alcohol, The composition is suitable for use in injectivity improvement.

Description

SPECIFICATION Surfactant composition This invention relates to a surfactant composition and to its use in the recovery of crude oil. Whilst the composition is suitable for use with light and medium crude oils, it is particularly effective with heavy crude oils which are generally more difficult to treat.

A petroleum reservoir is formed by a suitably shaped porous stratum of rock sealed with an impervious rock. The nature of the reservoir rock is extremely important as the oil is present in the small spaces or pores which separate the individual rock grains.

Crude oil is generally found in a reservoir in associated with water, which is often saline, and gas.

Dependent upon the characteristics of the crude, the temperature and the pressure, the gas may exist in solution in the oil or as a separate phase in the form of a gas cap. The oil and gas occupy the upper part of the reservoir and below there may be a considerable volume of water, known as the aquifer, which extends throughout the lower levels of the rock.

For oil to move through the pores of the reservoir rock and into a well, the pressure under which the oil exists in the reservoir must be greater than the pressure at the well.

The water contained in the aquifer is under pressure and is one source of drive. The dissolved gas associated with the oil is another and so is the free gas in the gas cap when this is present.

A time will come in the life of an oilfield when the natural pressure of the reservoir declines to such an extent that it is no longer sufficient to force the oil out of the pores of the rock into the well. This stage is often reached before the greater part of the oil is recovered from the reservoir.

Formerly it was the practice to rely on natural drive for as long as possible, only resorting to artificial production methods when the natural pressure dropped too low to sustain a reasonable flow. However, it has now been established that the eventual recovery of oil from a reservoir can be much greater if the pressure is not allowed to drop significantly in the early stages of production. Similarly, by utilising artificial means of maintaining pressure early in the life of a reservoir, production offtake rates may often be adjusted to economic advantage.

Thus in order to maintain pressure, or to accelerate the natural drive, or to initiate a drive where none occurs naturally, it is frequently necessary to employ the technique known as secondary recovery. The simplest method of forcing the oil out of the reservoir rock is by direct displacement with another fluid.

When water is used, the secondary recovery process is called waterflooding.

However, water flooding alone leaves a considerable proportion of the oil unrecovered. Water does not displace crude oil with high efficiency because water and oil are immiscible and the interfacial tension between water and oil is high. Increasingly, the water tends to by-pass the remaining oil and eventually, unless action is taken, only water will be produced, leaving behind unrecovered oil, known as the residual oil saturation to waterflood (SORW). This limitation of water flooding has been recognised and further techniques have been proposed. One such technique involves the use of surfactants for decreasing the interfacial tension between oil and water.

Numerous attempts have been made to develop formulations for enhanced oil recovery and the patent literature is replete with descriptions of surfactant compositions, see for example, USP's 4,424,135, 4,159,037,4,110,228, 4,066,124 and 4,018,278.

A useful summary of the art is given in Kirk-Othmer's Encyclopedia of Chemical Technology, Third Edition, Volume 17, pages 16 & 82 (1978). This indicates that most compositions contain (a) a main surfactant which is either a petroleum or a synthetic hydrocarbyl sulphonate and (b) co-surfactants which include simple alcohols, ethoxylated alcohols and sulphated ethoxylated alcohols. It has also been disclosed that alkyl and alkylaryl polyalkoxy alkylene sulphonates may be used as co-surfactants.

However, many problems exist with this technique. Some surfactants are active only at high concentrations, some have unfavourable solubility characteristics, some are thermally unstable and some are ineffective and/or unstable in saline environments such as sea water and reservoir connate water which contain a high proportion of divalent ions.

In particular, petroleum sulphonates are frequently recommended because of their activity and relatively low cost. Unfortunately, they are particularly sensitive to the presence of divalent cations and to high temperatures. Under either of these conditions, and particularly when both are present, petroleum sulphonates tend to be unsatisfactory in operation. Treatment losses are often high because of adsorption on to the reservoir and because of precipitation. The latter can lead to pore blocking, usually with adverse consequences.

Offshore reservoirs are becoming increasingly important and it is clearly advantageous to make use of the readily available sea water for injection. This, of course, requires a surfactant composition which is tolerant of salts.

When a reservoir contains heavy crude oil, then the problem becomes more complex because of conflicting solubility requirements. Surfactant formulations designed to treat heavy crude oil must be more oil-soluble than those proposed for lighter crudes, and yet they must retain their solubility in brine. It is, however, well known that increasing the oil-solubility of a surfactant decreases its water-solubility and still more its solubility in brine. Coventional surfactant formulations for heavy crude oils tend to be inherently insoluble in a saline environment.

We have now discovered that the combination of an alkoxy sulphate, a polyolefin sulphonate and a blend of solubilising ágents is particularly effective in this context and is capable of mobilising oils of equivalent alkane carbon number in the range 2 to 14.

Thus according to the present invention there is provided a surfactant composition, hereinafter termed a concentrate, comprising (a) a branched chain polyolefin sulphonate having a molecular weight in the range 200 to 1,000, preferably in the range 200 to 600, (b) an alkyl aromatic alkoxy sulphate of general formula

wherein R' is an alkyl group containing 6 to 22 carbon atoms; R2 is an ethoxy or propoxy group, n is a number in the range 1 to 18, preferably 3 to 12; and M is an alkali or alkaline earth metal cation, preferably sodium, or an ammonium or amine cation, and xis a number corresponding to the valency of M; and a carrier comprising, (c) an oil-soluble alcohol preferably containing from 5 to 7 carbon atoms per molecule.

The carrier may additionally comprise: (d) an alcohol which is at least partially water-soluble containing 2 to 4 carbon atoms per molecule, and (e) a hydrocarbon liquid boiling in the kerosine and/or middle distillate range.

The preferred branch chain polyolefin sulphonate is polyisobutene sulphonate Components (a) and (b) are preferably present in amounts 10 to 40%, and 5 to 40% by weight of the composition. These percentage figures refer to the pure surfactants. Usually, however, the surfactants are supplied commercially as diluted materials.

The carrier is preferably present in amount 10 to 85% by weight of the total composition.

Components (c), (d) and (e) are preferably present in approximate weight ratios of 2:3:2 with respect to each other.

Component (a) is the main surfactant and is chiefly responsible for reducing the interfacial tension between the oil and water. Component (b) is a co-surfactantwhich is provided to adjust the HLB of the formulation.

The carrier improves the solution behaviour of the concentrate by maintaining it in a single phase even at low temperatures. It also modifies the density and viscosity of the concentrate. Within the carrier itself, the oil-soluble alcohol (c) is primarily responsible for modifying phase behaviour and the lower alcohol (d) for breaking down any liquid crystals which tend to form. The hydrocarbon crystals which tend to form. The hydrocarbon liquid (e) improves the rheological properties of the concentrate and permits storage and transportation down to temperatures as low as -20 C.

The concentrate will generally be in the form of a single phase liquid or a microemulsion and is suitable for dispersion in water for use in oil recovery.

The concentrate is particularly suitable for dispersion in sea water despite the fact that sea water is frequently an unsatisfactory dispersant for other formulations since it causes the precipitation of many surfactants.

Thus according to another aspect of the present invention there is provided an aqueous composition comprising a dispersion of a surfactant concentrate as hereinbefore described in water, suitably sea water.

The total concentration of active surfactants in the aqueous composition is suitably in the range 0.05 to 5%, preferably 0.4 to 2% by weight.

The aqueous composition may be used in oil reservoir operations for various purposes, eg, (i) to increase the rate of water inflow through injection wells (injectivity improvement), by displacing pore-blocking residual oil drops outwards from the vicinity of the injection well, (ii) to aid oil recovery from oil production wells by displacing residual or non-residual crude oil from a large volume of the reservoir, and (iii) to remove pore-blocking water drops from the oil-bearing porous reservoir in the vicinity of the production well. After removal of the waterdrops, the well is back-flushed and oil production recommenced.

Thus according to a further aspect of the invention there is provided a method for the recovery of crude oil from a reservoir which method comprises injecting an aqueous surfactant composition as hereinbefore described into the reservoir through an injection well and recovering crude oil from the reservoir from a production well.

The invention is illustrated by the following Examples.

EXAMPLE 1 A surfactant concentrate was prepared by mixing the following components.

%bywt (a) Polyisobutene sulphonate (sodium salt) (molecular weight 368) 26.5

10.4 (c) n-hexanol 4.8 (d) sec-butanol 12.1 (f) unreacted precursors of (a) and (b) biocide, salt, and water 46.2 100.00 The concentrate was a clear, single phase liquid which was readily diluted with sea water to give a solution containing 11,000 ppm Component (a), 4,000 ppm Component (b), 2,000 ppm Component (c) and 5,000 ppm Component (d).

This solution was submitted to a laboratory core test to determine its injectivity improvement properties.

A composite reservoir core having a length of 24.9 cm, a diameter of 3.8 cm and an absolute permeability to brine of 163 mD was restored to a condition representing the terminal water flooded state.

This was achieved by successive flooding with connate brine, "live" Prudhoe Bay crude and sea water at the appropriate temperature (107"C) and pressure (175 atmospheres).

This was waterflood to completely degas the crude thus stimulating near wellbore reservoir conditions.

After completion ofwaterflooding, initially at 1 070C and then at 49"C, it was found that the presence of residual oil restricted the permeability to sea water to 31 mD.

The surfactant solution was passed through the core at a flow rate of 0.7 ml/min (corresponding to the waterflood flow rate). This reduced the residual oil saturation from its post water flood value of 30.4% of the available pore volume down to 5.5% within 2 hour. The permeability to subsequent sea water flood rose from 31 mD to 115 mD in consequence.

Within the limits of experimental error, the treatment resulted in the restoration of the permeability of the core to its absolute value. The recovery of residual oil was also nearly complete.

Injectivity improvement on this linear coreflood test was 270%.

EXAMPLE 2 A surfactant concentrate was prepared by mixing the following components.

(a) Polyisobutene sulphonate (sodium salt) (molecular weight 368) 21.4

9.2 (c) iso amyl alcohol 9.2 (eel) kerosine 2.3 (e2) light paraffin oil 2.3 (f) unreacted precursors of (a) and (b) biocide, salt, and water 55.6 100.00 The concentrate was a clear, single phase liquid which was readily diluted with sea water to give a solution containing 14,000 ppm Component (a), 6,000 ppm Component (b) 6,000 ppm Component (c), 1,500 ppm Component (e1) and 1,500 ppm Component (e2).

The solution was submitted to a core test as in Example 1. This resulted in an injectivity improvement of 290%.

EXAMPLE 3 A surfactant concentrate was prepared by mixing the following components.

% bywt (a) Polyisobutene sulphonate (sodium salt) (molecular weight 386) 13.0

15.2 (c) iso amyl alcohol 8.1 (e1) kerosine 2.3 (e2) light paraffin oil 4.7 (f) unreacted precursors of (a) and (b) biocide, salt and water 56.7 100 The concentrate was a clear, single phase liquid which was readily diluted with water to give a solution containing 9,250 ppm Component (a), 10,750 ppm Component (b) 5,000 ppm Component 1,667 ppm Component (e1) and 3,333 ppm Component (e2).

This solution was submitted to a field trial in the Welton oil field. The radial injectivity improvement was found to be 40%.

Claims (9)

1. Asurfactantcomposition: (a) a branched chain polyolefin sulphonate having a molecular weight in the range 200 to 1,000, (b) an alkyl aromatic alkoxy sulphate of general formula

wherein R1 is an alkyl group containing 6 to 22 carbon atoms; R2 is an ethoxy or propoxy group, n is a number in the range 1 to 18; and M is an alkali or alkaline earth metal cation, or an ammonium or amine cation and x is a number corresponding to the valency of M; and, a carrier comprising, (c) an ol-soluble alcohol,

2. A surfactant composition according to claim 1 wherein the carrier also comprises (d) an alcohol which is at least partially water-soluble containing 2 to 4 carbon atoms per molecule.

3. A surfactant composition according to either of the preceding claims wherein the carrier also comprises (e) a hydrocarbon liquid boiling in the kerosine and/or middle distillate range.

4. A surfactant composition according to any of the preceding claims wherein the branched chain polyolefin sulphonate has a molecular weight in the range 200 to 600.

5. A surfactant composition according to any of the preceding claims wherein the branched chain polyolefin sulphonate is polyisobutene sulphonate.

6. A surfactant composition according to any of the preceding claims wherein the oil soluble alcohol contains from 5 to 7 carbon atoms per molecule.

7. An aqueous composition comprising a dispersion of a surfactant concentrate according to any of the preceding claims in sea water.

8. A method for the recovery of crude oil from a reservoir which method comprises injecting an aqueous composition according to claim 7 into the reservoir through an injection well and recovering crude oil from the reservoir from a production well.

9. A surfactantcomposiition as hereinbefore described with reference to the Examples.