GB2148982A - Crude oil recovery - Google Patents

Abstract

Crude oil is recovered from a reservoir by injecting an aqueous composition containing a surfactant into the reservoir, the surfactant being a substituted polysaccharide comprising monomer units of formula: Saccharide [O(R)N<+>R3<1>X<->]m wherein R represents an alkylene group containing 1 to 5 carbon atoms, R<1> represents an alkyl group containing 1 to 5 carbon atoms, m is a number in the range 0.3 to 3. The surfactant has improved wetting properties.

Description

SPECIFICATION Crude oil recovery This invention relates to the use of a surfactant composition 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 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.

Sometimes, however, water injection is rendered difficult by the presence of pore blocking oil droplets in the vicinity of the injection well.

We have now discovered a surfactant composition which has improved wetting properties and which displaces the oil droplets, thereby permitting an increased volume of water to be injected.

Thus, according to the present invention, there is provided a method for the recovery of crude oil from a reservoir which method comprises injecting an aqueous composition containing a surfactant compound into the reservoir through an injection well and recovering crude oil from the reservoir through a production well wherein the surfactant compound is a substituted polysaccharide comprising monomer units of formula:saccharide [O(R)N + R'3X ]m wherein R represents an alkylene group containing 1 to 5 carbon atoms, optionally containing a hydroxyl group, R' represents an alkyl group containing 1 to 5 carbon atoms, X represents an anion, preferably a halide and m is a number in the range 0.3 to 3.

"m", the number of substituent groups per monomer unit of the polysaccharide, is also known as the degree of substitution (D.S.).

The molecular weight of the polysaccharide can lie in the range 10,000 to 5,000,000.

Quaternary ammonium ethers of polysaccharides are known. 4-(trimethylammonium chloride)2-butenyl ethers of guar gum and locust bean gum are disclosed by R.N. DeMartino and A.B.

Conciatori (Celanese Corp., NY, US Patent 4,031,307) and the reaction of glycidyltrimethylammonium chloride with starch is described by J.E. Dlouhy, E.W. Kaiser and S.J. O'Flaherty (Corn Products Co, French Patent 1,375,126).

Scleroglucan. a neutral glucan produced by aerobic fermentation of D-glucose by selected species of Sclerotium, and guar gum, a galactomannan obtained from the seed of the guar plant are suitable for use in the present application, but other polysaccharides such as cellulose, modified celluloses and starch can also be employed. Further details concerning these and other polysaccharides are to be found in "Industrial Gums", R.L. Whistler, ed, Academic Press. NY, 1 9 7 3, 2nd edition.

The aqueous component of the dispersion is suitably sea water where this is readily available.

The pH of the dispersion is preferably less than 7. Since sea water is usually slightly alkaline, it should be acidified when used as the aqueous component.

The concentration of surfactant in the dispersion is suitably in the range 0.01 to 1.0% by weight.

The invention is illustrated with reference to the following Examples.

Example 1 Scleroglucan (10g) was slurred with aqueous isopropanol (60ml, 50%) and the mixture was purged with nitrogen for 1 h at 40"C. Aqueous sodium hydroxide (3g, 50%) was added and the mixture was stirred at 40'C for 0.25h. Glycidyltrimethylammonium chloride (10g) was added as a powder and the mixture was stirred at 40-60"C for 4h. After cooling, glacial acetic acid was added to bring the mixture to pH6.A grey solid was filtered off, washed with aqueous isopropanol (100my, 50%) and isopropanol (50ml). and then dried to give a grey spongy solid (10g). The resulting product, Compound A, was of formula:

OH Scleroglucan-OCH2CH CH2N+ (CH3)3C1 to D.S. 0.85 Compound A was dissolved in acidified sea water (pH 6.5-adjusted with HCI) to a concentration of 0.2% by weight.

Forties separator sand particles (500 micron in size) which has previously been extracted with methanol and toluene were mixed with excess surfactant at 96'C for a period of 18 hours. The sand was then filtered with hot (96"C) distilled water several times until no foam remained and then dried in an oven.

Wetting Test Several grains of sand were placed in a bottle and the sea water solution of surfactant was added. The sand was contacted with a drop of Forties Oil injected from a micrometer syringe and the oil wetting behaviour of the sand was recorded.

Results were ranged according to the scale: 1 = Grains not attached to the oil drop 2 = Grains weakly attached-removed by light shaking 3 = Grains moderately attached removed by moderate shaking 4 = Grains strongly attached-removed by strong shaking 5 = Grains very strongly attached and not removed by shaking.

Example 2 Example 1 was repeated with the sclerogiucan replaced by guar gum. The resulting product, Compound B, was of formula:

OH Guar-OCH2CH CH2N+(CH3)3CI to D.S. 0.48 Example 3 For comparison the wetting test was repeated but in the absence of surfactant, employing the sea water, sand and oil only.

The results were as follows: Example Wetting by Oil 1 1 2 1 3 5 These results demonstrate that in the absence of the surfactant the sand grains which are normally water wettable quickly take on an oil wet character, but in the presence of the surfactant the surface of the sand can retain its water wettability in the presence of the crude oil.

Claims (8)

1. A method for the recovery of crude oil from a reservoir which method comprises injecting an aqueous composition containing a sufactant compound into the reservoir through an injection well and recovering crude oil from the reservoir from a production well wherein the surfactant compound is a substituted polysaccharide comprising monomer units of formula: saccharide [O(R) N+R3aX-]m wherein R represents an alkylene group containing 1 to 5 carbon atoms, R' represents an alkyl group containing 1 to 5 carbon atoms, X represents an anion, and m is a number in the range 0.3 to 3.

2. A method according to claim 1 wherein X represents a halide anion.

3. A method according to either of the preceding claims wherein the alkylene group R additionally contains a hydroxyl group.

4. A method according to any of the preceding claims wherein the molecular weight of the polysaccharide is in the range 10,000 to 5,000,000.

5. A method according to any of the preceding claims wherein the polysaccharide is guar gum, scleroglucan, cellulose or starch.

6. A method according to any of the preceding claims wherein the aqueous component of the composition is sea water.

7. A method according to any of the preceding claims wherein the concentration of the surfactant in the composition is in the range 0.01 to 1.0% by weight.

8. A method as hereinbefore described with reference to Examples 1 and 2.