GB2160242A - Crude oil recovery - Google Patents

Abstract

A surfactant concentrate comprises a branched chain polyolefin sulphonate and an alkyl aromatic alkoxy sulphate. The concentrate is suitable for dissolving in sea water to provide solutions for use in injectively improvement and enhanced oil recovery.

Description

SPECIFICATION Crude oil recovery This invention relates to a surfactant composition and to its use in the recovery of crude oil.

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 association 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 water flooding.

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 168-182 (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. The aforesaid USP 4,1 5g,037 discloses that alkylaryl polyalkoxy alkylene sulphonates may also 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 these conditions, and particularly when both are present, as in certain reservoirs under the North Sea, petroleum sulphonates are usually unsatisfactory. Treatment losses tend to be 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.

We have now discovered that a particular combination of an anionic sulphate and sulphonate is particularly effective in this context.

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 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, preferably 5 to 12; and M is an alkali or alkaline earth metal cation, preferably sodium, or an ammonium or amine cation, and x is a number corresponding to the valency of M; and, optionally, (c) a carrier.

The preferred branched chain polyolefin sulphonate is polyisobutene sulphonate.

Components (a) and (b) are preferably present in amounts 10 to 50, and 5 to 50% by weight of the composition. These percentage figures refer to the pure surfactants. Usually, however, the surfactants are supplied commercially as diluted materials.

Suitable carriers include alkoxy alcohols, eg butoxyethanol, and aliphatic alcohols, eg, butanol and pentanol. The alcohol enhances the solubility of the concentrate and is also beneficial in reducing the interfacial tension.

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 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, (a) 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, (b) to aid oil recovery from.oil production wells by displacing residual or non-residual crude oil from a large volume of the reservoir, and (c) 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 herein before 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 Example.

A surfactant concentrate was prepared by mixing the following components.

Z by wt (a) Polyisobutene sulphonate (sodium salt) 27.4 (molecular weight 371) (b)

(c) iso-amyl alcohol 7.2 (d) unreacted precursors of (a) 57.0 biocide, salt, water 100.00 The concentrate was a clear, single phase liquid.

The concentrate was readily diluted with sea water to give a solution containing 11 ,500 ppm Component (a), 3,500 ppm Component (b) and 3,000 ppm Component (c).

A composite core from the Magnus reservoir in the North Sea having a length of 30.5 cm, a diameter of 3.8 cm and an absolute permeability to brine of 71 mD was restored to a condition representing the terminal, water flooded state. This was achieved by successive flooding with connate brine, "live" Magnus crude oil and sea water at the appropriate temperature (11 6inc) and pressure (435 atmospheres).

After completion of waterflooding, initially at 11 63C and then at 105"C, it was found that the presence of residual oil restricted the permeability to sea water to 10 mD.

The surfactant solution was passed through the core at a flow rate of 0.8 ml/min (corresponding to the waterflood flow rate). This reduced the residual oil saturation from its post water flood value of 29% of the available pore volume down to 5% within 2 hours. The permeability to subsequent sea water flood rose from 10 mD to 79 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.

Claims (7)

1. A surfactant concentrate comprising (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 R' is an alkyl group containing 6-22 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 an amine cation, and x is a number corresponding to the valency of M; and, optionally, (c) a carrier.

2. A concentrate according to claim 1 wherein the branched chain polyolefin sulphonate is polyisobutene sulphonate.

3. A concentrate according to either of the preceding claims wherein R' in the formula of Component (b) represents a C9 group.

4. A concentrate according to any of the preceding claims wherein the carrier is an alkoxy alcohol or an aliphatic alcohol.

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

6. A method for the recovery of crude oil from a reservoir which method comprises injecting an aqueous surfactant composition according to claim 5 into the reservoir through an injection well and recovering crude oil from a production well.

7. A concentrate as hereinbefore described with reference to the Example.