GB2148356A

GB2148356A - Adsorption reducing composition

Info

Publication number: GB2148356A
Application number: GB08426339A
Authority: GB
Inventors: John Howard Clint; Phillip Kenneth Gordon Hodgson; Edward John Tinley
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1983-10-20
Filing date: 1984-10-18
Publication date: 1985-05-30
Also published as: GB8328122D0; GB8426339D0; GB2148356B

Abstract

Water soluble polyvinyl compounds act as sacrificial agents in reducing the adsorption of surfactants onto reservoir rock surfaces during enhanced oil recovery operations. Suitable polyvinyl compounds include sulphonated and unsulphonated polyvinyl alcohol and polyvinyl pyrrolidone, and sulphonated polystyrene.

Description

SPECIFICATION Adsorption reducing composition The present invention relates to the recovery of crude oil, and more particularly to the recovery of crude oil from an oil reservoir using the injection of water into the reservoir to maintain or increase the oil flow.

A petroleum reservoir consists of a suitably shaped porous stratum of rock which is 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. Sandstone and limestone are generally porous and in the main these are the most common types of reservoir rocks. Porous rocks may sometimes also contain fractures or fissures which will add to the oil storing capacity of the reservoir.

Crude oil is generally found in a reservoir in association with water, which is often saline, and gas. The oil and gas occupy the upper part of the reservoir and below there may be a considerable volume of water which extends throughout the lower levels of the rock. This water bearing section of the reservoir is known as the "aquifer". 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. Dependent upon the shape of the structure, the petrography of the reservoir rocks, the crude migration history and the geology of the areas, the aquifer may or may not be present as a recognisable entity.

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

The water contained in the aquifer is under pressure and is one source of potential energy. The dissolved gas associated with the oil is another and so is the free gas in the gas cap when this is present. The production mechanisms which owe their existance to these sources of energy are referred to as "water drive", "solution gas drive" (or "depletion drive") and "gas cap drive" respectively.

The present invention is particularly applicable to the water drive mechanism.

A time may come in the life of an oilfield when the natural pressure of the reservoir has declined to an extent where it is no longer sufficiently large to force the oil out of the pores of the rock into the bottom of the well. This stage can be reached before the greater part of the oil has been 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.

Water flooding is one of the most successful and extensively used secondary recovery methods. Water is injected under pressure into the reservoir rock via injection wells and drives the oil through the rock into nearby producing wells.

However, water does not displace crude oil with high efficiency because water and oil are immiscible, and also because the interfacial tension between water and oil is high. This weakness of water flooding has been recognised and many surfactants have been suggested for use in decreasing the interfacial tension between the injected water and the formation petroleum.

Several types of interactions which hinder oil recovery may occur between the recovery reagents and the reservoir rock or brine. The surfactant may be adsorbed from the flood onto the reservoir rocks, depleting the flood of its primary ingredient. Relatively inexpensive compounds, known as "sacrificial agents", are often added to the flood to satisfy some of the adsorption sites in the reservoir rock. Polyvalent cations (e.g.

Cay', Mg++) in the brine may decrease oil production precipitating the surfactant from the flood. Other ions in the reservoir brine can also hinder the effectiveness of the surfactant flood. A low salt, aqueous preflush may be injected ahead of the flood to provide a favourable environment for the flood.

We have now discovered that certain vinyl polymers act as sacrificial agents.

Thus according to the present invention there is provided a method for treating a formation to reduce the tendency of surfactants to adsorb thereon in the presence of crude oil and water, which method comprises contacting the formation with an effective amount of an aqueous solution of water soluble polyvinyl compound of molecular weight in the range 5,000 to 5,000.000.

Suitable polyvinyl compounds include polyvinyl alcohol and polyvinyl pyrrolidone.

The polyvinyl compound alcohol or pyrrolidone may be at least partially sulphonated in which case at least 10% of the polymer molecules should contain substituentsulphonate groups.

Preferred molecular weights of the polyvinyl alcohol or polyvinyl pyrrolidone, in the sulphonated or unsulphonated form, are in the range 10,000 to 500,000.

Another suitable water soluble polyvinyl compound is sulphonated polystyrene in which at least 50% of the polymer molecules contain substituent sulphonate groups.

The preferred molecular weight of the sulphonated polystyrene is in the range 50,000 to 1,000,000.

The aqueous component of the solution is suitably sea water where this is readily available. Since sea water is usually slightly alkaline, it should be acidified before use.

The concentration of the polyvinyl compound in the solution is preferably in the range 0.005% to 2% by weight.

The solution may also contain an anionic andlor a non-ionic surfactant of the type used in enhanced oil recovery, e.g. a petroleum sulphonate, an alkylaryl alkoxy sulphonate or an alkyl aryl alkoxylate.

Alternatively, the latter may be added subsequentlyto the formation.

The weight ratio of the concentration of the polyvinyl compound to the anionic and/or non-ionic surfactant is suitably in the range 1:25 to 1:2.

The aqueous composition may be used for various purposes, e.g., (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, (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.

The invention is illustrated by the following Examples.

Examples Examples 1 and 7 are provided for comparative purposes only and do not illustrate the invention.

The surfactants tri-isobutyl phenol-(CH2CH2O)(CH2)2SO3Na and a petroleum sulphonate of molecule weight 415-430 were dissolved in filtered acidified sea water (pH 6.5) to give a solution containing 1,200 and 800 ppm, respectively.

The adsorption of the surfactants from solution on Forties sand (50 to 150 clam fraction) was determined at 96"C by the standard batch method of adsorption measurement using ultra violet (UV) absorbence to determine surfactant concentration changes.

Examples 2-6 Example 1 was repeated with the difference that the saline solution additionally contained 500 ppm polyvinyl alcohol (PVA), sulphonated PVA or polyvinyl pyrrolidone (PVP). These polymers do not give a UV signal and therefore adsorption Figures relate only to the su rfactant.

Example 7 Dinonyl phenol-(CH2CH2O)65CH2CH2SO3Na was dissolved in sea water treated as in Example 1 to give a solution containing 3,000 ppm.

The adsorption of the surfactant from solution on Forties sand (50 to 150 Fm fraction) was determined at 20"C as in Example 1.

Example 8 Example 7 was repeated with the difference that the saline solution additionally contained 1,500 ppm polystyrene sulphonate (PSS).

Experimental results are set out in the following Table.

TABLE Example Composition SurfactantAdsorption mglg 1 Surfactantalone 0.53 2 +500ppmPVA(mwt14,000) 0.17 3 +500 ppm PVA (mwt 125,000) 0.21 4 +500 ppm PVA sulphonate 0.26 (mwt 14,000, D.S. = 18%) 5 +500 ppm PVA sulphonate 0.25 (mwt 125,000, D.S. = 18%) 6 +500 ppm PVP (mwt 44,000) 0.20 7 Surfactant alone 0.63 8 +1,500 ppm PSS 0.28 (mwt 500,000, DS = 100%) Thus the presence of PVA, sulphonated PVA, PVP or PSS results in a significant reduction in the adsorption of the surfactant.

Claims

1. A method for treating a formation to reduce the tendency of surfactants to adsorb thereon in the presence of crude oil and water, which method comprises contacting the formation with an effective amount of an aqueous solution of a water soluble polyvinyl compound of molecular weight in the range 5,000 to 5,000,000.

2. A method according to claim 1 wherein the water soluble polyvinyl compound is polyvinyl alcohol or polyvinyl pyrrolidone.

3. A method according to claim 2 wherein the molecular weight of the polyvinyl alcohol or polyvinyl pyrrolidone is in the range 10,000 to 500,000.

4. A method according to any of the preceding claims wherein at least 10% of the polymer molecules contain subsequent sulphonate groups.

5. A method according to claim 1 wherein the water soluble polyvinyl compound is sulphonated polystyrene in which at least 50% of the polymer molecules contain substituent sulphonate groups.

6. A method according to claim 6 wherein the molecular weight of the sulphonated polystyrene is in the range 50,000 to 1,000,000.

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

8. A method according to any of the preceding claims wherein the concentration of the polyvinyl compound in the solution is in the range 0.005% to 2% by weight.

9. A method according to any of the preceding claims wherein the solution also contains an anionic and/or a non-ionic surfactant.

10. A method according to claim 9 wherein the weight ratio of the concentration of the polyvinyl compound to the anionic and/or non-ionic surfactant is in the range 1:25 to 1:2.

11. A method according to claim 1 as hereinbefore described with reference to Examples 2-6 and 8.