GB2158126A - Crude oil recovery - Google Patents

Abstract

Crude oil is recovered from a reservoir by a method which comprises injecting an aqueous surfactant composition comprising a branched chain polyolefin sulphonate having a molecular weight in the range 200 to 1,000 into the reservoir through an injection well and recovering crude oil from the reservoir from a production well. The aqueous component of the mixture may be sea water.

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 surfactant 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 4,159,037 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 an/or unstable in saline environments such as sea water and those reservoir connate waters 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.

We have now discovered that branched chain polyolefin sulphonates are particularly suitable for use in this context.

Branched chain polyolefin sulphonates tend to be less strongly absorbed on reservoir rock than conventional surfactants such as petroleum sulphonates and alkane sulphonates.

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 surfactant composition comprising a branched chain polyolefin sulphonate having a molecular weight in the range 200 to 1,000, preferably in the range 200 to 600, into the reservoir through an injection well and recovering crude oil from the reservoir from a production well.

The cation associated with the sulphonate anion is preferably an alkali metal, most preferably sodium.

The preferred branched chain polyolefin is polyisobutene.

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.

Branched chain polyolefin sulphonates tend to be more soluble in sea water than conventional surfactants.

Even so, however, it may sometimes be desirable to increase their solubility still further. This may be done by the addition of co-surfactant. Suitable co-surfactants include those previously mentioned, and, preferably, alkylphenol alkoxy alkylene sulphonates of general formula

wherein R1 is an alkyl group containing 6 to 24 carbon atoms and R2 is an alkyl group containing 0 to 12 carbon atoms, the total number of carbon atoms in the R1 and R2 groups being in the range 12 to 26; R30 is an ethoxy group containing 2 or 3 carbon atoms; R5 is an alkyl group containing 6 to 14 carbon atoms;M is an alkali or alkaline earth metal cation, preferably sodium, oran ammonium or amine cation; xis a number corresponding to the valency of M, m is a number in the range 2 to 11, preferably 5 to 9 and n is a number in the range 1 to 14, preferably 1 to 5.

A carrier is preferably provided to improve the solution behaviour of the surfactant mixture, hereinafter termed a concentrate, by maintaining it in a single phase at low temperatures. It also modifies the density and viscosity of the concentrate. Suitable carriers include alkoxy alcohols, e.g. butoxyethanol, aliphatic hydrocarbons, e.g. kerosine and water or brine.

The carrier may also include an aliphatic alcohol containing 2 to 10 carbon atoms per molecule. The alcohol enhances the solubility properties of the concentrate and can also be used to adjust the interfacial tension. Suitable alcohols include sec-butanol, iso-amyl alcohol and n-hexanol.

The carrier will generally contain two or more components.

The total concentration of active surfactants in the aqueous composition is suitably in the range 0.05 to 5%, preferably 0.4 to 3% by weight of the total composition.

The aqueous composition may be used in oil reservoir operations for various purposes, eg, (a) to increase the rate of water inflow through injections wells (injectivity improvement), by displacing pore-blocking residual oil drops away 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 injecting surfactant solution to mobilise theswaterdrops, the well is back-flushed and oil production recommenced.

The invention is illustrated with reference to the following examples of which Example 2 is provided for purpose of comparison.

Example 1 A surfactant concentrate was prepared containing the following components: % by wt (a) Polyisobutene sulphonate - No salt 22.72 (Molecular wt 372)

(b) C18t137 < O(CH2CH20)2CH2CH2S03Na 15.15 (c) C9Hl9 t3 O(CH2CH20)7CH2CH2S03Na 12.63 (d) n- Elexano 1 6.73 (e) Kerosine 42.10 (f) Glutaraldehyde 0.67 100.00 This was dissolved in sea waterto give a solution containing 0.3% by weight total active surfactant.

This solution was pumped through a stainless steel column packed with unconsolidated core material from the Forties crude oil reservoir maintained at 960C. The concentration of surfactant in the effluent solution was monitored continuously using an inline UV detector. Monitoring continued until the detector indicated that the concentration of the surfactant in the effluent equalled the concentration in the feed. At this point adsorption was deemed to be complete.

From calibration graphs of detector responses against concentration and a knowledge of the dead volume of the system the adsorption of the surfactant expressed in mg of surfactant per gram of substrate was calculated. This was found to be 0.82 + 0.2 mg/g.

Example 2 Example 1 was repeated with the difference that the polyisobutene sulphonate was replaced by the same amount of an alkane sulphonate of molecular weight 350.

The adsorption of this material was found to be 1.68+ 0.43 mg/g, approximately twice as much as the polyisobutene sulphonate.

Claims (7)

1. A method for the recovery of crude oil from a reservoir which method comprises injecting an aqueous surfactant composition comprising a branched chain polyolefin sulphonate having a molecular weight in the range 200 to 1,000 into the reservoir through an injection well and recovering crude oil from the reservoir through an injection well and recovering crude oil from the reservoir from a production well.

2. A method according to claim 1 wherein the branched chain polyolefin sulphonate has a molecular weight in the range 200 to 600.

3. A method according to either of the preceding claims wherein the branched chain polyolefin sulphonate is polyisobutene sulphonate.

4. A method acccording to any of the previous claims wherein the aqueous surfactant composition contains a co-surfactant.

5. A method according to claim 4 wherein the cosurfactant is an alkyl phenol alkoxy alkylene sulphonate of general formula:

wherein R' is an alkyl group containing 6 to 24 carbon atoms and Rz is an alkyl group containing 0 to 12 carbon atoms, the total number of carbon atoms in the R1 and R2 groups being in the range 12 to 26; R30 is an ethoxy group or a mixture of ethoxy and propoxy groups; R4 is an alkylene group containing 2 or 3 carbon atoms; R5 is an alkyl group containing 6 to 14 carbon atoms; M is an alkali or alkaline earth metal cation, or an ammonium or amine cation; x is a number corresponding to the valency of M, m is a number in the range 2 to 11 and n is a number in the range 1 to 14.

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

7. A method according to any of the preceding claims wherein the total concentration of active surfactants in the aqueous composition is in the range 0.05 to 5% by weight of the total composition.