WO2018114801A1

WO2018114801A1 - Enhanced oil recovery

Info

Publication number: WO2018114801A1
Application number: PCT/EP2017/083308
Authority: WO
Inventors: Gerardus GLASBERGEN; Carolus Hendricus Theodorus VAN RIJN; Ramez Masoud Azmy NASRALLA
Original assignee: Shell Internationale Research Maatschappij BV; Shell Oil Co
Current assignee: Shell Internationale Research Maatschappij BV; Shell USA Inc
Priority date: 2016-12-19
Filing date: 2017-12-18
Publication date: 2018-06-28
Anticipated expiration: 2019-06-19

Abstract

A process for recovering oil from an oil-bearing formation by injecting an oil recovery formulation comprising expandable polymer into the formation which process comprises: (a) injecting into the formation a pre-treating composition comprising an adsorption reducing agent, (b) subsequently injecting into the formation the enhanced oil recovery formulation, and (c) recovering oil from the oil- bearing formation.

Description

ENHANCED OIL RECOVERY

FIELD OF THE INVENTION

The present invention is directed to a process for recovering oil from an oil- bearing formation.

BACKGROUND TO THE INVENTION

In the recovery of oil from a subterranean formation, only a portion of the oil in the formation generally is recovered using primary recovery methods utilizing the natural formation pressure to produce the oil. A portion of the oil that cannot be produced from the formation using primary recovery methods may be produced by chemical enhanced oil recovery, also referred to as improved oil recovery or EOR.

One enhanced oil recovery method utilizes aqueous polymer mixtures to flood an oil-bearing formation to increase the amount of oil recovered from the formation. Aqueous dispersion of a polymer is injected into an oil-bearing formation to increase recovery of oil from the formation, either after primary recovery or after a secondary recovery water flood. Such processes are described in GB2220687A,

WO2016100103 and US20120067579. Without wishing to be bound by any theory, it is thought that the polymer increases the viscosity of the enhanced oil recovery oil recovery formulation, preferably to the same order of magnitude as the oil in the formation in order to force the mobilized oil through the formation for production by the polymer containing flood. Other benifical effects which may take place are changes in pressure profiles due to the injection of the polymer and crossflow of oil from low into higher permeability parts of the reservoir.

A disadvantage of flooding a formation with polymer mixture is that the polymer containing mixtures to be injected tend to have a relatively high viscosity which makes it difficult to inject these into the formation. It has been investigated whether polymers can be made to increase in viscosity after having been injected into the formation. Such polymers are known in the art and have been described in documents such as US20140209304, US8389446 and US7300973. Expansion is generally induced by an increase in temperature in the targeted part of the formation. Polymers of which the viscosity increases after having been injected into a formation, are hereinafter referred to as expandable polymers. A secondary benefit of these expandable polymers is that they are much more resistant to mechanical shear forces. When these shear forces are applied to conventional (hydrogenated) polyacrylamide polymer, the polymer chains are scissored resulting in much lower viscosity of the polymer containing solution. These mechanical forces are especially pronounced in offshore applications where sea water often is used as the make up brine.

Ceasing or decreasing injection of expandable polymer can lead to injectivity loss or fluid fingering through higher permeability layers of the formation thereby deteriorating the sweep efficiency of the enhanced oil recovery formulation. Without wishing to be bound to any theory, it is thought that this is caused by an increase of the temperature near the well. As in all injection operations, the temperature in the reservoir near the well tends increase at the moment injection decreases or ceases because heat from the overburden and the deep reservoir remains available while the influx of cold water is reduced or stops. Due to the higher temperature, expandable polymer can expand in an area near to the wellbore. Moreover, expandable polymer which adheres to the formation can be activated due to the higher temperature. When re- starting the injection, adhered but now expanded polymer tends to desorb especially when it comes into contact with sweep fluid of increased viscosity. This is a self-propagating effect of continuously increasing polymer concentration and viscosity thereby desorbing further polymer.

SUMMARY OF THE INVENTION

It is an aim to provide an adapabtale process for enhanced oil recovery comprising injecting expandable polymer. It is a further aim to provide a process for enhanced oil recovery in which it is possible to fluctuate the rate of injection of expandable polymer containing enhanced oil recovery formulation. It is a further aim to provide a process for enhanced oil recovery in which it is possible to reduce or stop injection of enhanced oil recovery formulation comprising expandable polymer.

In one aspect, the present invention is directed to a process for recovering oil from an oil-bearing formation by injecting an enhanced oil recovery formulation comprising expandable polymer into the formation which process comprises:

(a) injecting into the formation a pre-treating composition comprising an adsorption reducing agent,

(b) subsequently injecting the oil recovery formulation into the formation, and

(c) recovering oil from the oil-bearing formation. DET AILED DESCRIPTION OF THE INVENTION

The process for oil production comprises injecting into an oil-bearing formation an enhanced oil recovery formulation comprising an adsorption reducing agent. Without wishing to be bound by any theory, it is thought that expandable polymer tends to adhere less to a formation if adsorption reducing agent was previously injected into the formation. Reduction of adsorption can be due to change in the adsorption capacity of the formation such as in the case of alkali or can be due to other compounds such as sacrifycing agents already having been adsorbed.

The process involves the injection of a pre-treating composition comprising the adsorption reducing agent. Pre-treating involves injecting a total amount of adsorption reducing agent composition which is smaller than the total amount of enhanhanced oil recovery formulation injected into the same part of the formation. A suitable amount of pre-treating composition would be of from 0.0001 to 0.2 pore volume of the formation, more specifically at least 0.0005, more specifically at least 0.001, more specifically at least 0.005 pore volume. Preferably, the pre-treating involves a limited amount of adsorption reducing agent composition such as at most 0.15 pore volume, more specifically at most 0.10 pore volume, more specifically less than 0.10 pore volume. Another reason that a limited amount of adsorption reducing agent composition suffices is that injectivity problems due to expandable polymer are only expected for cooler zones of the formation where expandable polymer has been adsorbed but not yet expanded. Normally, expandable polymer is expected to expand in the formation and thereafter is thought to behave as conventional polymer. This makes that only part of a formation has to be treated.

The pore volume is the volume of the relevant part of the formation in so far as the volume is available for the storage of liquid before start of the enhanced oil recovery. The pore volume can be determined in any way known to the person skilled in the art.

The process also involves injecting the oil recovery formulation. A suitable amount of oil recovery formulation can be of from 0.1 to 1 pore volume of the formation, more specifically at least 0.11, more specifically at least 0.15, more specifically at least 0.20 pore volume of the formation. Preferably, the amount of oil recovery formulation is limited to the amount required and preferably is less than 1 pore volume, more preferably less than 0.9 pore volume, more preferably less than 0.8 pore volume of the formation.

A wide variety of adsorption reducing agents can be considered. Preferably, the adsorption reducing agent is chosen from the group consisting of alkali, polymers including but not limited to expanded expandable polymers, surfactants and scale inhibitors. The polymer preferably is an enhanced oil recovery polymer. The surfactant preferably is an enhanced oil recovery surfactant. More preferably, the adsorption reducing agent is a sacrifycing agent selected from the group consisting of surfactants and polymers including but not limited to expanded expandable polymers. More preferably, the adsorption reducing agent is a polymer, more specifically an enhanced oil recovery polymer, including but not limited to expanded expandable polymer.

Preferably, the adsorption reducing agent is an organic compound having a weight average molecular weight of at least 100 dalton, more preferably at least 150 dalton, more preferably at least 200 dalton, more preferably at least 300 dalton, more preferably at least 400 dalton, more preferably at least 500 dalton. An organic compound consists of carbon and hydrogen and optionally oxygen and/or sulphur. The deprotonated form of an acid is considered for determining whether either the acid per se or the deprotonated acid is considered organic.

Surfactant for use as adsorption reducing agent can be any surfactant known to be suitable by somebody skilled in the art. The surfactant can be cationic, nonionic or anionic. Generally, the surfactant will be a nonionic surfactant or an anionic surfactant or a mixture of these. Examples of nonionic surfactants are alcohol ethoxylates such as NEODOL 91-8 and NEODOL 2512. NEODOL is a trademark of the Shell group of companies. Preferably, the anionic surfactant is chosen from the group consisting of hydrocarbon sulphates and hydrocarbon sulphonates wherein the hydrocarbon consists of hydrogen, carbon and optionally oxygen. More preferably, the surfactant is selected from the group consisting of an alpha olefin sulfonate compound, an internal olefin sulfonate compound, a secondary alkyl sulfonate compound, a branched alkyl benzene sulfonate compound, a propylene oxide sulfate compound, an ethylene oxide sulfate compound, a propylene oxide-ethylene oxide sulfate compound, or a blend thereof. The surfactant preferably contains of from 10 to 30 carbon atoms. Specific surfactants which can be used are ENORDET surfactant J771, ENORDET surfactant Jill 11, ENORDET surfactant J13131, ENORDET surfactant 0352, ENORDET surfactant 0242, ENORDET surfactant 0342, ENORDET surfactant 0332, ENORDET surfactant A771, PETROSTEP surfactant Al, PETROSTEP surfactant A6, PETROSTEP surfactant S 1 , PETROSTEP surfactant

S2, PETROSTEP surfactant S-8, PETROSTEP surfactant S-9, PETROSTEP surfactant S-10, PETROSTEP surfactant S-13, STEOL surfactant CS330,

PETROSTEP surfactant C3 and PETROSTEP surfactant C4. ENORDET is a trademark of the Shell group of companies. PETROSTEP and STEOL are trademarks of Stepan Company.

Scale inhibitors which can be used as adsorption reducing agent preferably are chosen from the group consisting of phosphates, phosphate esters, phosphoric acids, phosphonates and phosphonic acids.

Alkali which can be used as adsorption reducing agent preferably is selected from the group consisting of ammonia, lithium hydroxide, sodium hydroxide, potassium hydroxide, lithium carbonate, sodium carbonate, potassium carbonate, lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, lithium silicate, sodium silicate, potassium silicate, lithium phosphate, sodium phosphate, potassium phosphate, and mixtures thereof. Most preferably, the alkali is sodium hydroxide.

More preferably, the adsorption reducing agent is polymer, more preferably enhanced oil recovery polymer, including but not limited to expanded expandable polymer. The enhanced oil recovery polymer for use as adsorption reducing agent preferably is polyacrylamide, more specifically an at least partially hydrolyzed polyacrylamide. More preferably, the adsorption reducing agent is selected from the group consisting of polyacrylamide, partially hydrolyzed polyacrylamide, hydrolyzed polyacrylamide and expanded expandable polymer. Expansion of the expandable polymer can be attained in any way known to somebody skilled in the art. More preferably, the adsorption reducting agent is selected from the group consisting of polyacrylamide, partially hydrolyzed polyacrylamide and hydrolyzed

polyacrylamide.

The weight average molecular weight of the adsorption reducing agent preferably is at least the weight average molecular weight of the expandable polymer. The adsorption reducing agent preferably has an anionc charge density which is at least the anionic charge density of the expandable polymer, preferably higher than the anionic charge density of the expandable polymer. The anionic charge density is also sometimes referred to as anionicity and can be determined by

determining the total organic carbon content and assaying the ionic charge of the tested compound in a dilute solution (in distilled water at natural pH). If the adsorption reducing agent or expandable polymer consists of compounds having different anionic charge density, the latter is to be averaged based on the weight of the compounds.

Each the adsorption reducing agent and the expandable polymer generally will be injected into the formation as a mixture of water and adsorption reducing agent and expandable polymer, respectively.

The use of pure water can be preferred but pure water is not always available in sufficient quantity. Pure water is considered to be water having a total dissolved solids content (TDS, measured according to ASTM D5907) of at most 4000 ppm, more specifically at most 2000 ppm, more specifically at most 1000 ppm, most specifically at most 500 ppm. The expression "ppm" indicates parts per million by weight on total weight amount present.

In case pure water is not readily available, an alternative preferred embodiment is to apply a combination of pure water and water having a relatively high TDS or use water from other sources such as sea water, brackish water, aquifer water, formation water and brine. Water which can be used with the adsorption reducing agent and/or with the expandable polymer generally has a TDS of more than 1,000 ppm, more specifically at least 2,000 ppm, more specifically at least 4,000 ppm, more specifically at least 5,000 ppm. Preferably, the water has a TDS of less than 20,000 ppm, more specifically less than 15,000 ppm, more specifically is at most

10,000 ppm, most specifically at most 8,000 ppm. Most preferably, the water used for preparing the aqueous mixture has a reduced ionic strength namely of 0.15 M or less. The water preferably has an ionic strength of at most 0.1 M or at most 0.05 M, or at most 0.01 M, and may have an ionic strength of from 0.01 M to 0.15 M, or from 0.02 M to 0.125 M, or from 0.0 3M to 0.1 M. Ionic strength, as used herein, is defined by the equation

I = ½*∑i=i ⁿ c ; z ; ² where I is the ionic strength, c is the molar concentration of ion i, z is the valency of ion i, and n is the number of ions in the measured mixture. Such water including its preparation is described in WO20140041856.

It is especially advantageous if the water used for preparing the aqueous mixture contains a limited amount of divalent ions such as less than 4000 ppm, more specifically less than 2000 ppm, more specifically less than 1000 ppm, more specifically less than 500 ppm, more specifically less than 100 ppm, most specifically less than 20 ppm of divalent ions based on total amount of water. More specifically, these amounts relate to the calcium and/or magnesium containing salts.

Water of appropriate TDS content can be obtained by reverse osmosis of saline water using a membrane having a first surface and a second surface and (i) feeding the saline source water to the first surface of the membrane, and (ii) removing treated water of reduced salinity from the second surface of the membrane.

The expandable polymer is injected subsequent to injection of the adsorption reducing agent. It is preferred that most, and more preferably substantially all, of the formation has been in contact with the adsorption reducing agent before being brough into contact with the expandable polymer. Adsorption reducing agent can still be in a certain part of the formation when injection of the expandable polymer starts into another part of the formation.

The expandable polymer generally is intended to provide the oil recovery formulation with a viscosity of the same order of magnitude as the viscosity of oil in the formation under formation temperature conditions to allow the oil recovery formulation to drive mobilized oil across the formation for production from the formation with a minimum of fingering of the oil through the oil recovery formulation and/or fingering of the oil recovery formulation through the oil. The expandable polymer can be a single compound or can be a mixture of compounds. Preferably, the polymer is selected from the group consisting of polyacrylamide; partially hydrolyzed polyacrylamide; polyacrylate; ethylenic co-polymer;

carboxymethylcelloluse; polyvinyl alcohol; polystyrene sulfonate;

polyvinylpyrrolidone; biopolymers; 2-acrylamide-methyl propane sulfonate (AMPS); styrene-acrylate copolymer; co-polymers of acrylamide, acrylic acid, AMPS and n- vinylpyrrolidone in any ratio; and combinations thereof. Examples of ethylenic co-polymers include co-polymers of acrylic acid and acrylamide, acrylic acid and lauryl acrylate, and lauryl acrylate and acrylamide. Examples of biopolymers include xanthan gum, guar gum, schizophyllan and scleroglucan.

Most preferably, the expandable polymer is polyacrylamide or hydrolyzed polyacrylamide or partly hydrolyzed polyacrylamide.

Preferably, expandable polymers have a particle size after expansion of at 2 least times the particle size before expansion, more specifically at least 5 times, more especially at least 10 times, more specifically at least 20 times, more specifically at least 50 times, most specifically at least 100 times. The particle size is the longest distance across the particle such as its length in case of a fiber or rod shaped particle. Especially suitable expandable polymers are expandable crosslinked polymeric microparticles as described in US20140209304. More specifically, these polymers have an unexpanded volume average particle size diameter of from 0.05 to 10 microns. Expansion of the polymer is caused by differences in conditions between the environment in the formation and outside the formation before injection. The expansion generally will be triggered by an increase in temperature and/or a change in pH.

The process of the present invention is especially suitable for use offshore in which case the oil containing formation is located under a layer of water more specifically under the seabed.

The concentration of the polymer in the oil recovery formulation to be injected into the formation preferably is sufficient to provide the oil recovery formulation with a dynamic viscosity in the formation of at least 0.3 mPa s (0.3 cP), more specifically at least 1 mPa s (1 cP), or at least 10 mPa s (10 cP), or at least 100 mPa s

(100 cP), or at least 1000 mPa s (1000 cP) at 25°C or at a temperature within a formation temperature range. The concentration of polymer in the oil recovery formulation preferably is from 250 ppm to 10000 ppm, or from 500 ppm to 5000 ppm, or from 1000 to 2000 ppm.

The molecular weight number average of the polymer in the oil recovery formulation preferably is at least 10000 dalton, or at least 50000 dalton, or at least 100000 dalton. The polymer preferably has a molecular weight number average of from 10000 to 30000000 dalton, or from 100000 to 15000000 dalton. The oil recovery formulation may also comprise co- solvent with water, where the co-solvent may be a low molecular weight alcohol including, but not limited to, methanol, ethanol, and iso-propanol, isobutyl alcohol, secondary butyl alcohol, n- butyl alcohol, t-butyl alcohol, or a glycol including, but not limited to, ethylene glycol, 1,3-propanediol, 1 ,2-propanediol, diethylene glycol butyl ether, triethylene glycol butyl ether, or a sulfosuccinate including, but not limited to, sodium dihexyl sulfosuccinate. The co-solvent may be utilized for assisting in prevention of formation of a viscous emulsion. If present, the co-solvent preferably is present in an amount of from 100 ppm to 50000 ppm, or from 500 ppm to 5000 ppm of the total oil recovery formulation. A co-solvent may be absent from the oil recovery formulation.

The oil recovery formulation may additionally contain paraffin inhibitor to inhibit the formation of a viscous paraffin-containing emulsion in the mobilized oil by inhibiting the agglomeration of paraffins in the oil. The mobilized oil, therefore, may flow more freely through the formation for production relative to mobilized oil in which paraffins enhance the formation of viscous emulsions. The paraffin inhibitor of the oil recovery formulation may be a compound effective to inhibit or suppress formation of a paraffin-containing emulsion. The paraffin inhibitor may be a compound effective to inhibit or suppress agglomeration of paraffins to inhibit or suppress paraffinic wax crystal growth in the oil of the formation upon contact of the oil recovery formulation with the oil in the formation. The paraffin inhibitor may be any commercially available conventional crude oil pour point depressant or flow improver that is dispersible, and preferably soluble, in the fluid of the oil recovery formulation in the presence of the other components of the oil recovery formulation, and that is effective to inhibit or suppress formation of a paraffin-nucleated emulsion in the oil of the formation. The paraffin inhibitor may be selected from the group consisting of alkyl acrylate copolymers, alkyl methacrylate copolymers, alkyl acrylate vinylpyridine copolymers, ethylene vinylacetate copolymers, maleic anhydride ester copolymers, styrene anhydride ester copolymers, branched poly ethylenes, and combinations thereof.

Commercially available paraffin inhibitors that may be used in the oil recovery formulation include HiTEC 5714, HiTEC 5788, and HiTEC 672 available from Afton Chemical Corp; FLOTRON D1330 available from Champion Technologies; and INFINEUM V300 series available from Infineum International.

The paraffin inhibitor is present in the oil recovery formulation in an amount effective to inhibit or suppress formation of a viscous paraffin-containing emulsion when the oil recovery formulation is introduced into an oil-bearing formation and contacted with oil in the formation to mobilize the oil, and the mobilized oil is produced from the formation. The paraffin inhibitor may be present in the oil recovery formulation in an amount of from 5 ppm to 5000 ppm, or from 10 ppm to 1000 ppm, or from 15 ppm to 500 ppm, or from 20 ppm to 300 ppm based on total amount of formulation.

In a preferred method of the present invention, the oil recovery formulation is introduced into an oil-bearing formation containing oil having a dynamic viscosity under formation conditions (in particular, at temperatures within the temperature range of the formation) of at least 0.1 mPa s (0.1 cP), or at least 0.5 mPa s (0.5 cP), or at least 1 mPa s (1 cP). Most preferably, the oil contained in the oil-bearing formation may have a dynamic viscosity under formation temperature conditions of at most 10,000 mPa s (10,000 cP), more specifically at most 1,000 mPa s (1,000 cP), more specifically at most 500 mPa s (500 cP), more specifically at most 100 mPa s (100 cP), more specifically at most 50 mPa s (50 cP), more specifically at most 20 mPa s (20 cP), most specifically at most 10 mPa s (10 cP).

It was observed that after a formation or well bore has been subjected to a clean up or stimulation treatment such as by treating with hydrochloric acid or an oxidizer, the adsorption reduction treatment seems to be no longer effective.

Therefore, the present process further comprises a process in which a clean up or stimulation treatment precedes the adsorption reduction and enhanced oil recovery process steps (a), (b) and (c).

The present disclosure is not limited to the embodiments as described above and the appended claims. Many modifications are conceivable and features of respective embodiments may be combined.

The following examples of certain aspects of some embodiments are given to facilitate a better understanding of the present invention. In no way should these examples be read to limit, or define, the scope of the invention.

Example

The following solutions were used in the experiments. A solution containing 1,000 parts per million by weight (ppm) of expandable polymer was prepared by diluting concentrated expandable polymer solution with synthetic North Sea water having a total dissolved solids contents (TDS) of 35,878 mg/1.

The tracer solution contained 20 ppm KI in water.

Example 1

A first test was performed with a Boise outcrop core of 30 cm. Seawater was injected to saturate the core. After injection of 2 pore volumes of tracer solution, the expandable polymer solution was injected in an amount of 5 pore volumes at 30 cm/day and 25 °C. The pressure drop was stable from which as deduced that there are no issues with plugging or injectivity at this temperature and with this concentration of polymer. Comparison of the profile of polymer production to tracer showed a significant delay of produced expandable polymer namely more than 1.5 pore volume. It is estimated that the retention of the expandable polymer is about 285 x 10 ⁶ g expandable polymer/g core.

Example 2

A further test was performed by injecting several pore volumes of tracer solution followed by several pore volumes of a solution containing 1 ,000 ppm of hydrolyzed polyacrylamide polymer followed by brine injection. Thereafter, 2 pore volume of tracer solution was injected to check the production profile after the hydrolyzed polyacrylamide polymer injection. Finally, 1,000 ppm expandable polymer solution was injected at a rate of 30 cm/day at 25 °C.

Comparing the hydrolyzed polyacrylamide polymer to tracer production profile, it shows a 0.1 pore volume delay. The adsorption was estimated to be 16 x 10 ⁶ g hydrolysed polyacrylamide polymer/g core. The second tracer solution injected after the hydrolysed polyacrylamide showed an early breakthrough compared to the first tracer. The expandable polymer solution also showed an early breakthrough compared with the first tracer solution. Without wishing to be bound to any theory, the latter is attributed to hydrodynamic acceleration.

Example 3

A further test was performed by injecting several pore volumes of tracer solution followed by 0.15 pore volume of a solution containing 1,000 ppm of hydrolyzed polyacrylamide polymer followed by 2 pore volume of 1,000 ppm expandable polymer solution at a rate of 30 cm/day at 25 °C.

Comparison of the expandable polymer to tracer production profile showed a 0.2 pore volume delay of the expandable polymer solution.

Claims

C L A I M S

1. A process for recovering oil from an oil-bearing formation by injecting an enhanced oil recovery formulation comprising expandable polymer into the formation which process comprises:

(b) subsequently injecting the oil recovery formulation into the formation, and

(c) recovering oil from the oil-bearing formation.

2. A process according to claim 1 in which the polymers have an unexpanded volume average particle size diameter of from 0.05 to 10 microns.

3. A process according to claim 1 in which the expandable polymer is partially hydrolyzed polyacrylamide.

4. A process according to claim 1 in which the adsorption reducing agent is an organic compound having a weight average molecular of at least 200 dalton.

5. A process according to claim 1 in which the adsorption reducing agent has a weight average molecular weight which is at least the weight average molecular weight of the expandable polymer.

6. A process according to claim 1 in which the adsorption reducing agent has a an anionic charge density which is at least the anionic charge density of the expandable polymer

7. A process according to claim 1 in which the adsorption reducing agent is a polymer selected from the group consisting of polyacrylamide; partially hydrolyzed polyacrylamide; polyacrylate; polyethylene glycol; ethylenic copolymer; carboxymethylcelloluse; polyvinyl alcohol; polystyrene sulfonate; polyvinylpyrrolidone; biopolymers; 2-acrylamide-methyl propane sulfonate (AMPS); styrene-acrylate copolymer; co-polymers of acrylamide, acrylic acid, AMPS and n-vinylpyrrolidone in any ratio; and combinations thereof.

8. A process according to claim 1 in which the adsorption reducing agent is

expanded expandable polymer.

9. A process according to claim 1 in which the adsorption reducing agent is a scale inhibitor chosen from the group consisting of phosphates, phosphate esters, phosphoric acids, phosphonates and phosphonic acids.

10. A process according to claim 1 in which the pre-treating composition consists of water, at most 2000 parts per million by weight (ppmw) of surfactant and of from 1000 to 5000 ppmw of polymer.

11. A process according to claim 1 in which the enhanced oil recovery formulation consists of water, of from 500 to 5000 ppmw of surfactant and of from 2000 to 10,000 ppmw of polymer.