COMPOSITION AND METHOD FOR ENHANCED OIL RECOVERY FROM

SUBTERRANEAN DEPOSIT

Field of the Invention

The present invention relates to a composition for enhanced oil recovery (EOR) from subterranean deposits and to a method of enhanced oil recovery in which the composition is used .

Background of the invention

In industrialized economies crude oil has been the most important source of energy and starting material for numerous products of chemical industry since the early decades of the 20^h century. It is commonly produced from subterranean deposits that formed over millions of years. Therefore, global stocks are limited and it is generally accepted that deposits will be depleted within the

forthcoming decades. In view of this, it is generally desired to maximize the amount of oil recovered from developed deposits. The recovery of oil from subterranean deposits can be generally divided into the phase of primary, secondary and tertiary recovery.

Primary recovery relates to the phase of oil production during which oil can be extracted from a subterranean deposit by means of the natural pressure of the deposit which pushes oil from the deposit to the surface through a drilled well. Primary recovery usually allows the

extraction of about 5 to 10 % of the oil in the deposit.

When the natural pressure of the deposit is exhausted and further extraction by means of primary recovery hence is not possible or if the pressure naturally present in the deposit is not sufficient for extraction of oil, secondary recovery is usually employed, i.e. pressurized water or gas is injected into the deposit in order to drive oil

remaining after the phase of primary recovery from the deposit. This is achieved by so-called voidage replacement, i.e. the increase and/or re-establishing of natural

pressure that was present when extraction of oil was begun, as well as by sweeping or displacing oil from the deposit such that it is pushed towards a well through which it is transported to the surface. If the material injected into the deposit is water the technique is also referred to as water-flooding. Secondary recovery usually allows the extraction of additional 20 to 30 % of the oil in the deposit.

Tertiary recovery relates, among others, to the phase of oil production during which chemicals such as polymers, alkaline agents, surfactants or combinations of such chemicals are injected into the deposit in order to improve the flow of the oil remaining after primary and secondary recovery phases. This is also referred to as chemical flooding or chemical enhanced oil recovery (commonly abbreviated as EOR) .

Tertiary recovery usually allows the extraction of

additional 10 to 20 % of the oil in the deposit.

The chemicals injected into the deposit increase the oil's flow characteristics and push the oil towards the well from which it is extracted from the deposit and transported to the surface. Injection of alkaline agents into deposits of oil containing naturally occurring organic acid groups results in the formation of soaps by means of

neutralization of the acid groups. Such soaps lower the interfacial tension between oil and water and/or the sediment of the deposit. Thus, the displacement of oil droplets through the deposit is enhanced. Injection of surfactants likewise results in a reduced interfacial tension and thus enhances the displacement of oil droplets through the deposit. Injection of surfactants can hence contribute to the effect achieved by the injection of alkaline agents. The injection of surfactants can replace the injection of alkaline agents in particular in cases where the oil does not contain acid groups in an amount sufficient for the formation of soaps by means of injection of alkaline agents. Injection of polymers increases the viscosity of the water present in the deposit and

advantageously affects the water/oil mobility ratio.

The specific chemicals or combination of chemicals suitable for achieving an optimum enhancing of the mobility of oil droplets through the deposit depends on the specific conditions of the deposit. For instance, the injected chemicals have to be stable at the temperature of the deposit which can be significantly higher than room

temperature. The activity of any type of surfactant depends on the salinity, water hardness, temperature and pH of the surrounding aqueous medium and, therefore, depending on the salinity and pH of the water present in the deposit the suitable type of surfactant has to be selected. Likewise, the chemical composition and the physical properties of the oil and the rock of the deposit affect the efficiency of a specific combination of chemicals to enhance the

displacement of the oil and have to be taken into account in order to select a suitable combination of chemicals.

Chemicals and combinations of chemicals for chemical flooding are usually formulated as aqueous solutions or aqueous dispersions in order to obtain a composition that can be conveniently handled by pumping, transported through pipes and injected into drilled wells. Compositions for chemical flooding have been described in the art .

For instance, US 4,426,303 discloses a surfactant

composition comprising (1) at least one alkylated

diaromatic sulfonate, (2) at least one petroleum sulfonate, (3) at least one condensation product of an alkanol and (4) an ethylene or propylene oxide as a solubilizing and wetting agent for said petroleum sulfonate and at least one glycol ether. The composition is described as having good compatibility with deposit water having high salinity and divalent ion concentration.

US 5,000,262 discloses a foam forming mixture which can be effectively employed with steam in a method of enhancing recovery of petroleum from oil bearing formations and comprises water, and alkyl aromatic sulfonate in which at least one of the alkyl groups comprises 16 to 40 carbon atoms and a viscosity control agent such as an -olefin sulfonate having 10 to 24 carbon atoms.

US 3,945,437 describes compositions suitable for displacing oil within a subterranean deposit in the presence of water having a high salinity and a high content of multivalent cations at temperatures below 150 °F (65 °C) . The

compositions comprise a mixture of aromatic ether

polysulfonates such as Dowfax surfactants and petroleum sulfonate surfactants. Exemplary compositions were tested using deposit water having a temperature of 95 °F (35 °C) , a total salt content of 136,000 ppm and a multivalent cations content of 3000 ppm as well as deposit water having a temperature of 170 °F (77 °C) , a total salt content of 136,000 ppm and a multivalent cations content of 2900 ppm. Tests revealed that compositions for chemical flooding known from the prior art are not suitable for achieving enhanced oil recovery to a satisfactory extent from a deposit containing, in sandstone having a low content of clay, water having a low salinity of less than 2500 ppm (2.5 g/L) , multivalent cations in a concentration of less than 20 ppm and a pH of about 8-10 and oil that is

essentially unreactive to alkaline agents at a relatively high temperature of about 80 to 90 °C. In particular, it was revealed that composition known from the prior art as being suitable for chemical flooding are not suitable for achieving enhanced oil recovery to a satisfactory extent from a deposit having the aforementioned characteristics, wherein the salts dissolved in the water of the deposit essentially consist of at least 70 % by weight of sodium hydrogen carbonate or sodium carbonate and sodium chloride. Therefore, as a first aspect, it has been desired to provide a composition suitable for enhanced recovery of oil from a deposit having such particular conditions. As a second aspect, it has been desired to provide a method of recovering oil from a deposit having such particular conditions.

Brief description of the Invention

It was surprisingly found that the first aspect of the aforementioned technical problem is solved by a composition for enhanced oil recovery comprising a) at least one alkali metal dialkylbenzene sulfonate of formula (1) 1

formula (1) wherein

M-L is an ion selected from Li ⁺ , Na ⁺ and K ⁺ ,

R1 and are the same or different and each are selected from alkyl groups having 4 to 16 carbon atoms; sulfonate of formula

formula (2) wherein

M ² and M ³ are the same or different and each are an ion selected from Li ⁺ , Na ⁺ and K ⁺ ,

R3 represents an alkyl group having 14 to 18 carbon atoms and

represents hydrogen or an alkyl group having 1 to 18 carbon atoms; c) at least one polyacrylamide selected from (cl) a

partially hydrolyzed polyacrylamide, (c2) a sulfonated polyacrylamide and (c3) a sulfonated partially hydrolyzed polyacrylamide, wherein

said partially hydrolyzed polyacrylamide (cl) consists of repeating units represented by formulae (3a) , (3b) and (3c) such that x + yl + y2 = 100 %, x > 0, yl + y2 > 0, zl = z2 = 0 and wherein (yl + y2)/ (x + yl + y2) is in the range of from 20 % to 35 %,

said sulfonated polyacrylamide (c2) consists of repeating units represented by formulae (3a) , (3d) and (3e) such that x + zl + z2 = 100 %, x > 0, zl + z2 > 0, yl = y2 = 0 and wherein (zl + z2)/ (x + zl + z2) is in the range of from 20 % to 30 %,

said sulfonated partially hydrolyzed polyacrylamide (c3) consists of repeating units represented by formulae (3a) , (3b) , (3c) , (3d) and (3e) such that x + yl + y2 + zl + z2 = 100 %, x > 0, yl + y2 > 0, zl + z2 > 0 and wherein (yl + y2)/ (x + yl + y2 + zl + z2) is in the range of from higher than 0 % to 40 % and (zl + z2)/(x + yl + y2 + zl + z2) is in the range of from higher than 0 % to 30 %,

formula (3a) formula (3b) formula (3c)

formula (3d) formula (3e) wherein x, yl, y2, zl and z2 represent the number of occurrences of the respective repeating unit in the polymer relative to the total number of occurrences of all

repeating units in the polymer,

and wherein M ⁴ in the aforementioned formulae (3c) and (3e) is selected from Li ⁺ , Na ⁺ and K ⁺ ; d) at least one alkaline agent selected from sodium

metaborate, sodium hydroxide, sodium tetraborate or sodium carbonate ; e) water; and f) optionally, one or more additive selected from biocides, oxygen scavengers, anti-scaling agents, and corrosion inhibitors ; wherein

- the weight ratio of alkali metal dialkylbenzene sulfonate (a) and alkali metal alkyl diphenyl ether disulfonate (b) (a) : (b) is in a range of 75:25 to 95:5, and the total amount of (a) and (b) is 3000 mg/1 or more, - said polyacrylamide (c) is present in an amount of at least 300 mg/1,

- said alkaline agent (d) is present in an amount of at least 2000 mg/1, and

- said water (e) is present in an amount of from 96 to 99 % by weight relative to the total weight of components (a) to (e) and, if present, (f) of the composition; and

- the total weight of components (a) to (e) and, if

present, (f) is 99.5 to 100 % by weight of the entire composition.

Likewise, it was surprisingly found that the second aspect of the aforementioned technical problem is solved by a method of enhanced oil recovery comprising

a step (II) of introducing a composition according to the aforementioned first aspect of the invention into a

subterranean oil deposit through a first well, wherein said subterranean oil deposit contains in inorganic sediment and/or sedimentary rock, crude oil and water having a salinity of less than 2500 ppm (2.5 g/1), multivalent cations in a concentration of less than 20 ppm and a pH of 8 to 10 at a temperature of 80 to 90 °C; and

a step (E) of extracting a mixture comprising oil from said subterranean deposit through a second well, wherein the extraction (E) can be carried out simultaneously or

sequentially to introduction (II).

Preferred embodiments of the present invention become apparent from the following description and the attached dependent claims.

Brief description of the drawings

Figure 1 shows a flow chart of the procedure for preparing a typical alkali-surfactant-polymer composition in

continuous mode on field level as described in example 6. Figure 2 shows a flow chart of the procedure for preparing a hydrolyzed polyacrylamide mother solution as described in example 6. Detailed description of the Invention

In the following the composition of the present invention will be described in more detail. (A) Composition

The composition according to the first aspect of the invention comprises

a) at least one alkali metal dialkylbenzene sulfonate of formula (1)

formula (1)

M-L is an ion selected from Li ⁺ , Na ⁺ and K ⁺ ,

R1 and are the same or different and each are selected from alkyl groups having 4 to 16 carbon atoms;

b) at least one alkyl diphenyl ether disulfonate of formula (2)

formula (2) wherein

and M-^ are the same or different and each are an ion selected from Li ⁺ , Na ⁺ and K ⁺ ,

R3 represents an alkyl group having 14 to 18 carbon atoms and

represents hydrogen or an alkyl group having 1 to 18 carbon atoms; c) at least one polyacrylamide selected from (cl) a

partially hydrolyzed polyacrylamide, (c2) a sulfonated polyacrylamide and (c3) a sulfonated partially hydrolyzed polyacrylamide, wherein

said partially hydrolyzed polyacrylamide (cl) consists of repeating units represented by formulae (3a) , (3b) and (3c) such that x + yl + y2 = 100 %, x > 0, yl + y2 > 0, zl = z2 = 0 and wherein (yl + y2)/ (x + yl + y2) is in the range of from 20 % to 35 %,

said sulfonated polyacrylamide (c2) consists of repeating units represented by formulae (3a) , (3d) and (3e) such that x + zl + z2 = 100 %, x > 0, zl + z2 > 0, yl = y2 = 0 and wherein (zl + z2)/ (x + zl + z2) is in the range of from 20 % to 30 %,

said sulfonated partially hydrolyzed polyacrylamide (c3) consists of repeating units represented by formulae (3a) , (3b) , (3c) , (3d) and (3e) such that x + yl + y2 + zl + z2 = 100 %, x > 0, yl + y2 > 0, zl + z2 > 0 and wherein (yl + y2)/ (x + yl + y2 + zl + z2) is in the range of from higher than 0 % to 40 % and (zl + z2)/(x + yl + y2 + zl + z2) is in the range of from higher than 0 % to 30 %,

formula (3a) formula (3b) formula (3c)

wherein x, yl, y2, zl and z2 represent the number of occurrences of the respective repeating unit in the polymer relative to the total number of occurrences of all

repeating units in the polymer,

and wherein M ⁴ in the aforementioned formulae (3c) and (3e) is selected from Li ⁺ , Na ⁺ and K ⁺ ; d) at least one an alkaline agent selected from sodium metaborate, sodium hydroxide, sodium tetraborate and sodium carbonate ; e) water; and f) optionally, one or more additive selected from biocides, oxygen scavengers, anti-scaling agents, corrosion

inhibitors and clarifiers.

The components of the composition will be described in more detail in the following.

(a) Alkali metal dialkylbenzene sulfonate of formula (1)

The composition of the present invention comprises at least one alkali metal dialkylbenzene sulfonate of formula (1) .

formula (1)

In said formula (1), M ¹ is an alkali metal ion selected from Li ⁺ , Na ⁺ and K ⁺ . and are the same or different and each are selected from alkyl groups having 4 to 16 carbon atoms . In a preferred embodiment, and are the same or different and are independently selected from linear or branched alkyl groups having 4 to 14 carbon atoms. More preferably, and R^ are the same or different, wherein is selected from linear or branched alkyl groups having 4 to 14 carbon atoms and R^ is selected from linear or branched alkyl groups having 10 to 14 carbon atoms. Most preferably, R^ and R^ are the same or different, wherein is selected from linear alkyl groups having 4 to 14 carbon atoms and R^ is selected from linear alkyl groups having 10 to 14 carbon atoms. In an embodiment, R^ is selected from linear or branched alkyl groups having 4 to 8 carbon atoms, and R^ is selected from linear or branched alkyl groups having 10 to 14 carbon atoms . In an embodiment, R^ is selected from linear alkyl groups having 4 to 8 carbon atoms, and R^ is selected from linear alkyl groups having 10 to 14 carbon atoms.

In an embodiment, R^ and R^ are the same or different and each are selected from linear or branched alkyl groups having 10 to 14 carbon atoms.

In an embodiment, R^ and R^ are the same or different and each are selected from linear alkyl groups having 10 to 14 carbon atoms.

In an embodiment, component (a) comprises more than one alkali metal dialkylbenzene sulfonates of formula (1). In a particular embodiment, component (a) comprises two alkali metal dialkylbenzene sulfonates of formula (la) and formula ( lb) : formula (la) wherein

R1 is selected from linear or branched alkyl groups having 4 to 8 carbon atoms,

is selected from linear or branched alkyl groups having 10 to 14 carbon atoms, and

M-L is an alkali metal ion selected from Li ⁺ , Na ⁺ and

K+;

1 formula (lb) wherein

R1 and are the same or different and each are selected from linear or branched alkyl groups having 10 to 14 carbon atoms, and

M-L is an alkali metal ion selected from Li ⁺ , Na ⁺ and K+.

In an embodiment, the alkyl groups in formula (la) and (lb) are linear alkyl groups.

In a particular embodiment, component (a) comprises two alkali metal dialkylbenzene sulfonates of formula (la) and formula (lb) in a weight ratio of 20:80 to 60:40,

preferably from 30:70 to 50:50, more preferably 35:65 to 45:55.

In a particular embodiment, component (a) comprises two alkali metal dialkylbenzene sulfonates of formula (la) and formula ( lb) :

formula (la) wherein

R1 is selected from linear alkyl groups having 4 to 8 carbon atoms,

is selected from linear alkyl groups having 10 to carbon atoms, and

formula (lb) wherein

R1 and are the same or different and each are selected from linear alkyl groups having 10 to 14 carbon atoms, and

M ¹ is Na+; and wherein the weight ratio between formula (la) and formula (lb) is 30:70 to 50:50, preferably 35:65 to 45:55.

In each of the embodiments described in this passage in relation to alkali metal dialkylbenzene sulfonate (a) , it is particularly preferred that is Na ⁺ .

Alkali metal dialkylbenzene sulfonate of formula (1) that can be used in the composition of the present invention are commercially available, for instance from CEPSA Quimica under the tradename Recodas 185 (CAS number: 85117-47-1) .

In the composition according to the present invention, the alkali metal dialkylbenzene sulfonate (a) is present in such an amount that the weight ratio of alkali metal dialkylbenzene sulfonate (a) and alkali metal alkyl diphenyl ether disulfonate (b) (a) : (b) is in the range of 75:25 to 95:5, and the total amount of (a) and (b) is 3000 mg/1 or more. In preferred embodiments, the weight ratio (a) : (b) is in the range of 78:22 to 90:10. In likewise preferred embodiments, the total amount of (a) and (b) is 4000 mg/1 or more and the weight ratio (a) : (b) is in the range of 75:25 to 95:5, more preferably in the range of 78:22 to 90:10. In a more preferred embodiment, the total amount of (a) and (b) is 4000 mg/1 to 6000 mg/1 and the weight ratio (a) : (b) is in the range of 75:25 to 95:5, most preferably in the range of 78:22 to 90:10.

The amount of each of components (a) and (b) and the total amount of components (a) and (b) (i.e. (a) plus (b) ) is indicated in mg/1 relative to the volume of the entire composition .

(b) Alkali metal alkyl diphenyl ether disulfonate

formula (2)

The composition of the present invention comprises at least one alkyl diphenyl ether disulfonate of formula (2) .

formula (2)

In said formula (2), and M-^ are the same or different and each are an alkali metal ion selected from Li ⁺ , Na ⁺ and K ⁺ . R3 represents an alkyl group having 14 to 18 carbon atoms and represents hydrogen or an alkyl group having 1 to 18 carbon atoms.

R3 represents a linear or branched alkyl group having 14 to 18 carbon atoms, preferably 14 to 16 carbon atoms, more preferably 16 carbon atoms. In respect of each of these meanings of R^ , it is also preferred that represents a linear alkyl group. represents hydrogen or a linear or branched alkyl group having 1 to 18 carbon atoms.

According to a more specific preferred embodiment, the alkyl diphenyl ether sulfonate is a mono-alkyl diphenyl ether sulfonate, i.e. represents hydrogen and R3

represents a linear or branched alkyl group having 14 to 18 carbon atoms. More preferably, R^ represents hydrogen and R3 represents a linear or branched alkyl group having 14 to 16 carbon atoms. Still more preferably, R^ represents hydrogen and R3 represents a linear or branched alkyl group having 16 carbon atoms.

As indicated, it is also preferred that R3 represents a linear alkyl group and, thus, according to a further more specific preferred embodiment, R^ represents hydrogen and R3 represents a linear alkyl group having 14 to 18 carbon atoms. More preferably, R^ represents hydrogen and R3 represents a linear alkyl group having 14 to 16 carbon atoms. Still more preferably, R^ represents hydrogen and R3 represents a linear alkyl group having 16 carbon atoms.

In each of the embodiments described in this passage in relation to alkyl diphenyl ether disulfonate (b) , it is particularly preferred that M^ and M3 each is Na ⁺ .

Alkali metal alkyl diphenyl ether disulfonate of formula (2) that can be used in the composition of the present invention are commercially available, for instance from Dow Chemical under the tradename Dowfax such as Dowfax 8390.

In the composition according to the present invention, the alkali metal alkyl diphenyl ether disulfonate (b) is present in such an amount that the weight ratio of alkali metal dialkylbenzene sulfonate (a) and alkali metal alkyl diphenyl ether disulfonate (b) (a) : (b) is in a range of 75:25 to 95:5, and the total amount of (a) and (b) is 3000 mg/1 or more.

Information regarding the preferred amounts of component (b) is provided hereinabove with respect to component (a) .

(c) Polyacrylamide

Polyacrylamides and, in particular partially hydrolyzed polyacrylamides, have been widely employed in compositions for EOR applications in order to increase the viscosity of the composition such that the composition is capable of efficiently displacing the oil in a deposit, thus allowing efficient EOR. However, it has likewise been known that partially or fully hydrolyzed polyacrylamides suffer from the drawback that they are not stable at elevated

temperatures such as about 80 to 90 °C. This lack of stability results in an undesired decrease of viscosity of a composition within a period of a few days after the composition was injected into an oil deposit for EOR.

Sulfonated polyacrylamides and partially hydrolyzed

sulfonated polyacrylamides which are essentially stable at temperatures of 100 °C or more have been developed in response to this drawback.

It was surprisingly found that the viscosity of the

composition according to the present invention remains essentially constant over weeks, even when the composition is exposed to a temperature of about 80 to 90 °C and when no sulfonated polyacrylamide or partially hydrolysed sulfonated polyacrylamide is present in the composition.

The composition of the present invention comprises at least one polyacrylamide, which is selected from (cl) a partially hydrolyzed polyacrylamide, (c2) a sulfonated polyacrylamide and (c3) a sulfonated partially hydrolyzed polyacrylamide. These polyacrylamides are composed of repeating units represented by the following formulae (3a) , (3b) , (3c) , (3d) and (3e) , wherein x, yl, y2, zl and z2 represent the number of occurrences of the respective repeating unit in the polymer relative to the total number of occurrences of all repeating units in the polymer.

(3a) (3b) (3c) (3d) (3e) In the aforementioned formulae (3c) and (3e) , is

selected from Li ⁺ , Na ⁺ and K ⁺ .

The partially hydrolyzed polyacrylamide (cl) consists of repeating units represented by formulae (3a) , (3b) and (3c) such that x + yl + y2 = 100 % , x > 0 , yl + y2 > 0 and zl = z2 = 0. The partially hydrolyzed polyacrylamide (cl) is characterized in that (yl + y2)/ (x + yl + y2) is in the range of from 20 % to 35 %, preferably 25 % to 30 %. The sulfonated polyacrylamide (c2) consists of repeating units represented by formulae (3a) , (3d) and (3e) such that x + zl + z2 = 100 %, x > 0, zl + z2 > 0 and yl = y2 = 0. The sulfonated polyacrylamide (c2) is characterized in that (zl + z2)/ (x + zl + z2) is in the range of from 20 % to 30 %, preferably 23 % to 28 %.

The sulfonated partially hydrolyzed polyacrylamide (c3) consists of repeating units represented by formulae (3a) , (3b) , (3c) , (3d) and (3e) such that x + yl + y2 + zl + z2 = 100 %, x > 0, yl + y2 > 0 and zl + z2 > 0. The sulfonated partially hydrolyzed polyacrylamide (c3) is characterized in that (yl + y2)/ (x + yl + y2 + zl + z2) is in the range of from higher than 0 % to 40 %, preferably in the range of from 5 to 30 %, and (zl + z2)/ (x + yl + y2 + zl + z2) is in the range of from higher than 0 % to 30 %, preferably in the range of from 5 % to 25 %.

In each of the embodiments described in this passage in relation to polyacrylamide (c) , it is particularly

preferred that is Na ⁺ . The molecular weight of the polyacrylamide can generally be in the range of from 2-10^ to 25-10^ g/mol, preferably in the range of from 5-10^ to 22-10^ g/mol, more preferably in the range of from 6-10^ to 21-10^ g/mol, still more

preferably in the range of from 7-10^ to 20-10^ g/mol.

The molecular weight of the polyacrylamide is determined by means of the intrinsic viscosity which is correlated to molecular weight by Mark-Houwink constants as described in API RP-63 ("Recommended Practices for the Evaluation of Polymers used in Enhanced Oil Recovery") , 1 ^st edition, June 1990, American Petroleum Institute, Washington DC, USA.

Exemplary polymers suitable for being used as the partially hydrolyzed polyacrylamide (cl), the sulfonated

polyacrylamide (c2) or the sulfonated partially hydrolyzed polyacrylamide (c3) in the present invention are

commercially available, such as from SNF Floerger, France, under the tradename Flopaam, for instance. The chemical constitution of polyacrylamides (cl), (c2) and (c3) , i.e. the amount of repeating units represented by formulae (3a) , (3b) , (3c) , (3d) and (3e) can be determined by 13Q_N] [R and/or elementary analysis as quantitative methods of determining the amount of carbonyl carbon atoms present in the acrylamide, acrylic acid, acrylate and sulfonated acrylamide. Methods suitable for this purpose are described by Zurimendi et al . in Polymer, vol. 25, (September 1984), pages 1314-1316 (Butterworth & Co.

(Publishers) Ltd.) . The synthesis of copolymers of

acrylamide and acrylamido methylpropyl sulfonic acid or a salt thereof are described by Durmaz et al . in Polymer, vol. 41, (2000), pages 3693-3704 (Elsevier Science Ltd.).

By means of the polyacrylamide, the viscosity of the composition is increased and adjusted in order to improve the capability of the composition to displace oil droplets in the deposit. From the viewpoint of an efficient EOR, in certain cases it can be desirable to adjust the viscosity of the composition to a value of 5 to 15 mPa-s (at 80 °C) .

In the composition according to the present invention, the polyacrylamide is present in an amount of 300 mg/1 or more, preferably in an amount of 500 mg/1 or more, more

preferably in an amount of 600 to 1000 mg/1.

(d) Alkaline agent

The composition of the present invention comprises at least one alkaline agent that is selected from sodium metaborate, sodium hydroxide, sodium tetraborate or sodium carbonate. In a preferred embodiment, the alkaline agent is sodium metaborate.

By means of the alkaline agent, the total salinity of the composition is adjusted such that the combination of surfactants (a) and (b) can be most effective and the interfacial tension between the composition and the crude oil in the deposit can be reduced such a value close to the minimum or, preferably, the minimum can be reached. In the composition according to the present invention, the

alkaline agent is present in an amount of 2000 mg/1 or more, preferably 4000 mg/1 or more, more preferably 5000 mg/1 or more, most preferably 6000 to 9000 mg/1. (e) Water

The composition of the present invention contains water in an amount of 96 to 99 % by weight relative to the total weight of compounds (a) , (b) , (c) , (d) , (e) and, if

present, (f) . This amount of water is expressed as the amount of chemically pure water.

Water that is used for the preparation of the composition according to present invention can be chemically pure, but it is also possible to use water that contains impurities such as dissolved salts for preparing the composition. In this case, it is important to know the amount of impurities contained in the water, because the amount of impurities- containing water to be used for the preparation of the composition has to be adapted such that the correct amount of chemically pure water is used. Thus, for example, when water containing 0.1 % of impurities is used in the

preparation of the composition, it has to be taken into account that chemically pure water represents only a portion of 99.9 % of the impurities-containing water.

Therefore, in order to provide a specific amount X of chemically pure water using water containing an amount of 0.1 % of impurities, an amount of (X / 99.9 · 100) of said impurities-containing water has to be used.

Since subterranean deposits of crude oil usually contain significant amounts of water, so-called deposit water (also referred to as formation water) is a by-product of oil production from subterranean deposits which is obtained in high amounts. From the viewpoint of sustainability, recycling of by-products of oil production from

subterranean deposits is desirable. Accordingly, it is preferred that at least a portion of the water present in the composition is water that was produced from the deposit. It is more preferred that the entirety of the water present in the composition is water that was produced from the deposit.

In any case, it has to be borne in mind that the activity of any type of surfactant depends on the salinity, water hardness, temperature and pH of the surrounding aqueous medium as set out hereinabove. Thus, in order to prevent that ions present in the water used for preparing the composition have a detrimental effect on the suitability of the composition to enhance the mobility of oil droplets through the deposit, the water used for preparing the composition should have a salinity and/or a concentration of multivalent cations exceeding the corresponding

conditions in the crude oil deposit characterized by means of these parameters. Likewise, the pH of the water used for preparing the composition should not be significantly higher or lower than the pH of the water present in the deposit . In view of the data mentioned hereinabove regarding the conditions of a specific deposit (water having a low salinity of less than 2500 ppm (2.5 g/1), concentration of multivalent cations of less than 20 ppm, pH of about 8-10), it is preferred that the water used for preparing the composition according to the present invention has a salinity not higher than 2500 ppm (2.5 g/1), a

concentration of multivalent cations not higher than 20 ppm and a pH not lower than 6 and not higher than 10. (f) Optional components

Various optional components can be present in the

composition in order to impart and/or adjust specific properties .

In particular, the following exemplary optional components can be mentioned. Biocides (fl) such as glutaraldehyde and quaternary

ammonium compounds, especially those containing long alkyl chains which can be derived from fatty acids, having antimicrobial activity. Examples of such quaternary

ammonium compounds are compounds containing at least one of benzalkonium, benzethonium, methylbenzethonium,

cetalkonium, cetylpyridinium, cetrimonium, cetrimide, dofanium, tetraethylammonium, didecyldimethylammonium and domiphen as the quaternary ammonium ions and chloride or bromide as the anion.

The biocides can be typically present in the composition in an amount of 100 mg/1 or less, such as in an amount of 25 to 100 mg/1.

Oxygen scavengers (f2) such as sodium hydrogensulfite .

The amount of oxygen scavengers added to the composition depends on the oxygen present in the composition in

dissolved form. When a hydrogensulfite compound such as sodium hydrogensulfite is used as the oxygen scavenger, it can typically be used in a molar ratio of hydrogensulfite to oxygen of about 8:1. Oxygen scavengers can be typically present in the composition in an amount 50 mg/1 or less, such as 10 to 50 mg/1.

Anti-scaling agents (f3) such as phosphonate compounds (hexamethylene tetramethylene phosphonate (HMDP) ,

diethylenetriamine penta (methylphosphonate) (DETPMP) , bis (hexamethylene) triamine pentabis (methylene phosphonate) (HMTPMP) , nitrilotris ( (methylene) tri phosphonate (NTP) , pentaethylene hexamineoctakis- (methylene phosphonate) (PEHOMP) , for instance) and/or poly-phosphino carboxylic acid (PPCA) . The amount of anti-scaling agents present in the

composition depends on the amount of calcium ions,

magnesium ions and/or sulfate ions present in the

composition. Typically, the amount of anti-scaling agents present in the composition can be 50 mg/1 or less, for instance .

Corrosion inhibitors (f4) such as quaternary ammonium compounds. Benzalkonium chloride or benzalkonium bromide can be mentioned as examples.

Typically, an amount of up to about 100 mg/1 of corrosion inhibitors can be present in the composition. Clarifiers (f5) such as tannin polymers having a low molecular weight.

Typically, an amount of 50 mg/1 or less, such as 15 to 50 mg/1, of clarifiers can be present in the composition.

Preferred embodiments of the present invention are

characterized in that in formula (1) represents Na ⁺ , and M- in formula (2) each represents Na ⁺ , and in formulae (3c) and (3e) , if present, represents Na ⁺ .

Independently preferred embodiments of the present

invention are characterized in that in formula (1) is selected from linear or branched alkyl groups having 4 to 14 carbon atoms and in formula (1) is selected from linear or branched alkyl groups having 10 to 14 carbon atoms, and R3 in formula (2) represents a linear or branched alkyl group having 14 to 16 carbon atoms and R^ in formula (2) is hydrogen. Further independently preferred embodiments of the present invention are characterized in that the weight ratio

(a) : (b) is in the range of 78:22 to 90:10. More preferred embodiments of the invention are

characterized in that in formula (1) represents Na ⁺ , and M- in formula (2) each represents Na ⁺ , and in formulae (3c) and (3e) , if present, represents Na ⁺ and, furthermore, in that in formula (1) is selected from linear or branched alkyl groups having 4 to 14 carbon atoms and in formula (1) is selected from linear or branched alkyl groups having 10 to 14 carbon atoms, and R3

formula (2) represents a linear or branched alkyl group having 14 to 16 carbon atoms and R^ in formula (2) is hydrogen .

Independently more preferred embodiments of the invention are characterized in that R^ in formula (1) is selected from linear or branched alkyl groups having 4 to 14 carbon atoms and R^ in formula (1) is selected from linear or branched alkyl groups having 10 to 14 carbon atoms, and R3 in formula (2) represents a linear or branched alkyl group having 14 to 16 carbon atoms and R^ in formula (2) is hydrogen and, furthermore, in that the weight ratio (a) : (b) is in the range of 78:22 to 90:10.

Still more preferred embodiments of the invention are characterized in that in formula (1) represents Na ⁺ , and M- in formula (2) each represents Na ⁺ , and in formulae (3c) and (3e) , if present, represents Na ⁺ , in that R1 in formula (1) is selected from linear or branched alkyl groups having 4 to 14 carbon atoms and R^ in formula (1) is selected from linear or branched alkyl groups having 10 to 14 carbon atoms, and R3 in formula (2) represents a linear or branched alkyl group having 14 to 16 carbon atoms and R^ in formula (2) is hydrogen and, furthermore, in that the weight ratio (a) : (b) is in the range of 78:22 to 90:10.

Further preferred embodiments of the invention are

characterized in that in one of the afore-described preferred embodiments, more preferred embodiments and still more preferred embodiments the total amount of (a) and (b) is in the range of 4000 mg/1 to 6000 mg/1. Other preferred embodiments of the invention are

characterized in that in one of the afore-described

preferred embodiments, more preferred embodiments and still more preferred embodiments the amount of (c) is in the range of 600 mg/1 to 1000 mg/1.

Yet other preferred embodiments of the invention are characterized in that in one of the afore-described

preferred embodiments, more preferred embodiments and still more preferred embodiments the amount of (d) is in the range of 6000 to 9000 mg/1.

Thus, more preferred embodiments of the invention are characterized in that the total amount of (a) and (b) is in the range of 4000 mg/1 to 6000 mg/1 and the amount of (c) is in the range of 600 mg/1 to 1000 mg/1.

Independently, preferred embodiments of the invention are characterized in that the total amount of (a) and (b) is in the range of 4000 mg/1 to 6000 mg/1 and the amount of (d) is in the range of 6000 to 9000 mg/1.

Still more preferred embodiments of the invention are characterized in that the total amount of (a) and (b) is in the range of 4000 mg/1 to 6000 mg/1, the amount of (c) is in the range of 600 mg/1 to 1000 mg/1 and the amount of (d) is in the range of 6000 to 9000 mg/1.

The total weight of components (a) to (e) and, if present, (f) is 99.5 to 100% by weight of the entire composition, preferably 99.7 to 100%, more preferably 99.9 to 100% by weight of the entire composition. Therefore, the composition for enhanced oil recovery of the invention may comprise up to 0.5%, for instance up to 0.3 or up to 0.1% by weight of the entire composition of other components different from (a) -(f).

In a particular embodiment, the composition of the

invention further comprises an alkali metal

monoalkylbenzene sulfonate in an amount of up to 500 mg/L, preferably up to 400 mg/L, more preferably up to 300 mg/L. In a particular embodiment, the alkali metal

monoalkylbenzene sulfonate is present in the composition of the invention in an amount of between 50 mg/L and 500 mg/L, preferably between 100 mg/L and 400 mg/L, more preferably between 150 mg/L and 300 mg/L.

In another embodiment, the composition comprises a

monoalkylbenzene sulfonate in an amount less than or equal to 6% by weight with respect to component (a) of the composition .

Preferably, the alkali metal ion in the alkali metal monoalkylbenzene sulfonate is selected from Li ⁺ , Na ⁺ and K ⁺ and the alkyl group is a linear or branched alkyl group having 4 to 14 carbon atoms, preferably 10 to 14 carbon atoms.

In a preferred embodiment, the alkali metal ion in the alkali metal monoalkylbenzene sulfonate is Na ⁺ and the alkyl group is a linear alkyl group having 10 to 14 carbon atoms.

It is to be understood that other preferred, more preferred and still more preferred embodiments of the present

invention can be derived by the skilled person from the present description by combining embodiments described as being preferred with other embodiments described as being preferred and/or more preferred. All the trade names mentioned herein refer to the products with the composition as of the date of priority of the present application (i.e. 8 April 2016).

(B) Method of preparing the composition

The composition according to the present invention can be manufactured by any method suitable for combining

components (a) to (f) such that an essentially uniform mixture is obtained. For example, appropriate amounts of components (a) , (b) , (c) , (d) and optionally (f) can be added to water (e) under agitation in a vessel suitable for this purpose. It is generally possible to add components (a) , (b) , (c) , (d) and optionally (f) to water (e)

simultaneously or sequentially.

Some of the components (a) , (b) , (c) , (d) and optionally (f) are commercially available in the form of aqueous preparations such as solutions, dispersion, emulsions etc. (instead of or in addition to solid form) and these aqueous preparations can be used in the method for preparing the composition. The amount of water present in these aqueous preparations has to be taken into account when it is calculated which amount of water has to be provided

separately during preparation of the composition of the present invention is prepared.

(C) Method of enhanced oil recovery

The composition according to the present invention can be used in enhanced recovery of oil from a porous substrate such as inorganic sediment or a sedimentary rock which can be sandstone, sand, silt or clay, for instance. The composition according to the present invention can be used in a method of enhanced oil recovery as described in the following. The method of enhanced oil recovery

represents a further aspect of the present invention.

The method of enhanced oil recovery comprises (II)

introduction of the composition according to the present invention as defined hereinabove into a subterranean oil deposit through a first well, wherein said subterranean oil deposit contains in inorganic sediment and/or sedimentary rock, crude oil and water having a salinity of less than 2500 ppm (2.5 g/1), multivalent cations in a concentration of less than 20 ppm and a pH of 8 to 10 at a temperature of 80 to 90 °C; and (E) extraction of a mixture comprising oil from said subterranean deposit through a second well, wherein the extraction (E) can be carried out

simultaneously or sequentially to introduction (II).

For the introduction (II) of said composition into the deposit pressure is usually applied.

Introduction (II) has the purpose of reducing the

interfacial tension between the displacing fluid and crude oil, such that at least a part of the crude oil present in capillaries of the sediment forming the deposit is

mobilized and made available for being moved towards the well through which the oil is extracted from the deposit such that it can be transported to the surface. Thus, droplets of crude oil are detached from the sediment. By means of the polymer in the composition an appropriate mobility ratio between the displacing fluid and the crude oil is maintained. The composition used in introduction (II) is also referred to "alkali-surfactant-polymer

composition" or "ASP composition".

According to a preferred embodiment, the method of enhanced oil recovery furthermore comprises subsequent to said introduction (II) an introduction (12) of a second

composition containing said component (c) in an amount of 600 to 1000 mg/1, said component (d) in an amount of 0 to 4000 mg/1, optionally said component (f) and water (e) in an amount of 99.2 % by weight to 99.93 % by weight relative to the weight of the entire second composition through said first well. Each of said components (c) , (d) and (f) in the composition used in introduction (12) are selected from those defined for the composition used in introduction (II). Each of said components (c) , (d) and (f) in the composition used in introduction (12) may be the same or different to those present in the composition used in introduction (II) . That is, each component (c) , (d) and (f) in the composition used in introduction (II) may be the same as or different from component (c) , (d) and (f) , respectively, in the composition used in introduction (12), but in both compositions they are selected from those defined hereinabove for the composition according to the present invention.

As mentioned, extraction (E) can be carried out

simultaneously or sequentially to introduction (II). It can be carried out simultaneously or sequentially to

introduction (12) .

Introduction (12) of a composition comprising component (c) and, optionally, component (d) is carried out subsequent to introduction (II) . For the introduction (12) of said composition into the deposit pressure using pumps is usually applied.

Introduction (12) has the purpose of reducing the amount of chemical components introduced into the deposit, while maintaining the mobility ratio and improving the efficiency of the method by maintaining a stream of liquid towards the well through which the oil is extracted from the deposit and transported to the surface while the so-called viscous fingering phenomenon is prevented. In oil production, fingering relates to the effect that the fluid being injected into a deposit undesirably does not contact the entire deposit but bypasses sections of the fluids

contained in the deposit in a finger-like manner. The composition used in introduction (12) is also referred to as "polymer post-flush composition" or "post-flush

composition" . By means of extraction (E) usually a mixture comprising oil is obtained, i.e. a mixture of crude oil and deposit water. It is possible that one or more components of the

compositions introduced into the deposit in introduction (II) or (12) is also contained in said mixture.

Examples

In the following, the invention will be illustrated by means of examples. However, these examples are not to be construed as limiting the invention.

Determination of parameters The following equipment and/or procedures were used in order to determine the parameters mentioned in the

examples .

(1) Interfacial tension

Interfacial tension between EOR compositions and crude oil are determined using a spinning-drop tensiometer Kruss Site 100. Measurements are carried out at a temperature of 80 °C. (2) Viscosity

A TA Instruments AR-1500Ex shear rheometer with concentric cylinder geometry is used to which a Peltier device is coupled in order to control temperature. Measurements are carried out at a temperature of 80 °C.

(3) Total pore volume

The total pore volume of a rock sample is determined in accordance with the procedure described in the American Petroleum Institute standard "RP-40 Recommended Practices for Core Analysis", 2 ^nd edition, February 1998, API

Publishing Services, as an indication of the total void space in the sample, i.e. the volume of connected and isolated pores.

(4) Effective porosity

Effective porosity as an indication of the volume of connected pores is determined using the liquid saturation method as described in the American Petroleum Institute standard "RP-40 Recommended Practices for Core Analysis", 2 ^nd edition, February 1998, API Publishing Services. (5) Oil recovery

Oil recovery from a rock sample by means of flooding is determined according to the following procedure. A rock sample is also referred to as a core. · Apparatus

The apparatus used in all operations comprising the introduction of liquids into a core and the extraction of liquids from a core is described in the following.

The apparatus is composed of a core holder assembly, devices for delivering streams of solvents, oil and water into the core and out of the core (which devices comprise hardware such as corrosion-resistant tubing, valves and fittings) and a pressure-control loop comprising a back ^¬ pressure regulator pump and pressure gauges at the inlet and outlet of the core holder assembly such that the pressure drop along the core can be recorded and

controlled. Any liquid effluent produced in the course of flooding experiment can be collected in a graduated vessel or conveyed to an on-line UV spectrometer cell.

Continuous-flow type system high pressure liquid

chromatography (HPLC) pumps are used for pressurizing and delivering solvents and water to the core holder assembly for injection into a core. Since crude oil may damage the pump internals, indirect pumping is used instead.

Specifically, crude oil is pumped by means of a floating piston vessel. A HPLC pumping device pumps water and by means of the pressure thus exerted on the floating piston the crude oil is injected into the core.

• Core preparation

A core was placed in a core holder, and a confining

pressure of 40 bar is applied by means of an expandable Viton® sleeve. The core was cleaned by sequential solvent flushing with toluene and methanol.

Subsequently, the core is restored by means of flooding with formation water and crude oil. As the first step, the core is saturated with formation water. To this end, degasified water is injected into the core by means of a HPLC pump at a flow rate of 4 ml/h. The water displaces the methanol remaining from the aforementioned cleaning step. The concentration of methanol in the effluent liquid is measured by means of an in-line UV spectrophotometer. Water is flushed into the core until the methanol concentration in the effluent water stream is lower than 1000 mg/1. When this value is reached, it is assumed that the core is fully saturated with water, i.e. all the network of voids in the core is filled with water. As the second step, crude oil is injected into the core in order to restore the native state of the rock core in a reservoir . Thus, oil is injected into the core under constant-pressure flow and the liquid effluent from the sample is

continuously collected in a graduated vessel. At the beginning of oil injection the liquid effluent from the core consists of water that is displaced from the core. As oil injection proceeds the amount of water in the effluent liquid decreases and the amount of oil in the effluent liquid increases. The presence of water in the liquid effluent from the core is detected by observing whether phase separation occurs such that the volume of the water phase increases. Oil injection into the core is stopped when no more water is detected in the effluent liquid.

Thus, a core saturated with oil and formation water is obtained . The volume of water collected in the course of oil

injection is in the following referred to as "volume of displaced water".

Water saturation (SW) of the core saturated with water and oil indicates the portion of total pore volume of the core that is soaked with water and is calculated according to the following formula.

SW = 100 x ( (total pore volume - volume of displaced water) / total pore volume)

Likewise, oil saturation (SO) of the core saturated with water and oil indicates the portion of the total pore volume of the core that is soaked with oil and is

calculated according to the following formula.

SO = 100 - SW After this flushing, the core saturated with formation water and oil is allowed to age by storing the core in the core-holder at 86 °C and 10 bar for 3 weeks with no

flushing at all. After this aging for 3 weeks, the core was ready for being used in the flooding experiment in order to evaluate the oil recovery that can be achieved by means of a specific flooding composition. · Flooding experiment and evaluation of oil recovery

The aged rock sample (core) is flooded with a flushing liquid by supplying the flushing liquid to the core under pressure. The liquid is stored in a vessel and pumped by a HPLC piston pump (flow rate 4 ml/h) . The flushing consist of the following sequence of steps.

(1) Injecting formation water into the core until oil production ceases, i.e. until no more oil is detected in the liquid effluent from the core

(2) Injecting recovery composition into the core in an amount of the total pore volume of the core multiplied by 0.3

(3) Injecting post-flush composition into the core in an amount of the total pore volume of the core multiplied by 0.3

(4) Injecting formation water into the core until oil production ceases, i.e. until no more oil is detected in the liquid effluent from the core

The liquid exiting the rock sample is a mixture of oil and water and is collected in samples having a volume of 1 ml. The vials are centrifuged to separate oil and water.

The water phase (bottom of the vial) is removed with a syringe with a long thin cannula. The cannula is introduced softly and slowly in order to avoid mixing the water and oil phases. Afterwards the vials are rinsed with

dichloromethane (DCM) to recover all the oil. 1 mL of DCM is added and the vial is softly stirred. Then, the DCM is allowed to settle and it is collected with a clean syringe. Finally, the collected DCM and the crude oil are poured into a glass beaker, previously weighed. The mixture is placed in a fume hood equipped with a filter for organic vapour trapping and it is kept overnight until full

evaporation of DCM is achieved. The beaker is weighed until obtaining a constant value of mass.

Thus, the mass of oil displaced from the core is

calculated. The mass of oil displaced from the core is converted to the volume of oil displaced from the core by means of the density of the oil at 15 °C (determined in accordance with ASTM D1298-12b, 1.1.1) according to the following formula.

Displaced volume of oil = displaced mass of oil / density at 15 °C

The oil recovery is calculated as percentage according to the following formula. Oil recovery = 100 x (displaced volume of oil) / (total pore volume · SO/100)

In the examples, crude oil having the following properties was used.

Density at 15 °C: 0.93445 g/cm ³

API gravity: 19.9

Sulfur content: 0.8 %

Kinematic viscosity at 40.0 °C: 82.32 mm^-s--'- Kinematic viscosity at 100.0 °C: 9.93 1 mm^-s--'- Total Acid Number: 0.07 mg KOH/g

Asphaltenes: 8.7 % (IP-143-04) Crude oil having these properties was obtained from a deposit in South America. Water having the following properties was used.

Content of Na : 343 mg/1

Mg: 1.7 mg/1

Ca: 7.4 mg/1

Ba: 0.3 mg/1

K: 19 mg/1

Fe: 0.2 mg/1

Chlorides: 377 mg/1

Hydrogen carbonates: 493 mg/1

Carbonates: < 10 mg/1

Sulfates: 96 mg/1

pH: 8.17

Conductivity: 1388 yS/cm Water having such a content of ions was obtained from the same oil deposit as the aforementioned crude oil and it is hence also referred to as "formation water".

The following components are used for the preparation of exemplary composition:

Component (a) : Recodas 185 (Sodium dialkylbenzene

sulfonate; commercially available from

CEPSA Quimica, Spain)

Component (b) : Dowfax 8390 (Alkali metal alkyl diphenyl ether disulfonate ; commercially available from Dow Chemical Company)

Component (c) : Flopaam 3530S (partially hydrolyzed

polyacrylamide, degree of hydrolysis about 25-30 mol-%, molecular weight about 16-10^

Da; commercially available from SNF

Floerger SAS, France. Example 1: Preparation of a typical alkali-surfactant- polymer composition In the following the preparation of a composition according to the present invention is illustrated which suitable for being used in introduction (II) .

A stock solution of 2000 mg/L of hydrolyzed polyacrylamide (Flopaam 3530S) is prepared according to the standard API

RP-63 Recommended Practices for Evaluation of Polymers Used in Enhanced Oil Recovery Operations . American Petroleum Institute, 06/01/1990. One liter of formation water is filtered through a 22 microns filter. Then, the water is poured into a beaker and stirred by means of a paddle stirrer. The stirring rate is fixed at 350-400 rpm in order to obtain a vortex flow. 2 grams of powder hydrolyzed polyacrylamide are weighed and added slowly into the beaker. The water is stirred at 350-400 rpm until total dispersion of the powder is achieved (30-60 seconds) and then the stirring rate is reduced down to 50-100 rpm in order to avoid air bubbles entering the bulk water. Then, the solution is stirred during 8 hours. Once the powder is fully dissolved, the air solved into the water must be removed to avoid chemical degradation of the

polyacrylamide. To this end, the solution is poured into a vacuum flask. The flask is connected to a helium vessel and to a vacuum system by a valve manifold. A pressure safety valve is also installed into the manifold. Firstly, the valve for helium is opened and the gas is added by

pressurizing the flask until the maximum pressure

acceptable for the safety valve is reached. Secondly, the helium valve is closed and the vacuum valve opened, so all the gas is removed. Finally, this cycle is repeated 3 times. Hence, the amount of air dissolved into the liquid is reduced. The solution is stored under helium inert atmosphere . 100 ml of stock solution of 5 % of surfactants into water is prepared. The solution contains both Recodas 185 and Dowfax 8390 in the desired ratio. For a 78:22 ratio, the procedure is as follows: 3.14 grams of Dowfax 8390®

(supplied as 35 % purity in water solution) are weighed into a beaker, then, 40 ml of filtered formation water are added and the mixture is stirred by a magnetic stirred device. Low stirring rates are selected (100-200 rpm) to avoid foaming phenomena. When the solution is fully

homogeneous, it is allowed to settle for 5-10 minutes.

Afterwards, 9.07 grams of the surfactant Recodas 185

(supplied as 43 % purity in water solution) are weighed into another beaker. 40 ml of warm water (50-60 °C) are poured into the beaker and the mixture is vigorously stirred in a magnetic stirrer device, although foam

formation must be avoided (50-100 rpm) . Once the product is fully dissolved into the formation water, the solution of Recodas is blended with the solution of Dowfax. The blend is stirred smoothly in a magnetic stirrer device and then it is transferred to a 100 ml volumetric flask. Formation water is added till the 100 ml mark of the flaks is

reached. The solution is allowed to settle for 15 minutes before use.

100 ml of stock solution of 5 % of sodium metaborate into water is prepared. To this end, 10.63 grams of sodium metaborate tetrahydrate ( aB02'4H20) are weighed into a beaker and, then, 80 ml of formation water are added. The mixture is stirred strongly (200-300 rpm) until full dissolution of the sodium metaborate. Finally, the

homogeneous solution is poured into a 100-ml volumetric flask and formation water is added till the 100-ml mark. Once all the stock solutions are prepared, the following procedure is applied to obtain a 100-ml alkali-surfactant- polymer solution. 50 grams of polymer stock solution are weighed into a 250-ml beaker. Then, 14 ml of the sodium metaborate solution are pipetted and poured into the polymer solution. The blend is smoothly stirred by means of a magnetic stirrer. Once the sodium metaborate and polymer are well mixed, 12 grams of the surfactant stock solution are added and the blend is kept under soft stirring. When the blend is totally homogeneous, it is transferred to a 100-ml volumetric flask and formation water is added till the 100-ml mark. The resulting composition is shown in table 1.

Table 1: Constitution of typical alkali-surfactant- polymer composition

Example 2: Preparation of post-flush composition

In the following the preparation of a composition is illustrated which suitable for being used in introduction (12) .

The same stock solutions prepared in Example 1 are used for this preparation. 26.66 grams of polymer stock solution are weighed into a beaker. Then, 6 ml of sodium metaborate stock solution are added and the blend is stirred in a magnetic stirred system. After the mixture is totally homogeneous, it is poured into a 100-ml volumetric flask and formation water is added until the 100-ml mark is reached. The solution is gently stirred by turning around the flask three times. The solution is ready for injection. Its composition is shown in table 2.

Table 2: Constitution of typical post-flush

Example 3: Tests of surfactant/co-surfactant combinations

Negatively-charged surfactants and polymers dissolved into water may suffer phase separation. According to the DVLO theory, when the double-layers of the negative-charged particles are compressed, it will be easier for the

particles to aggregate and phase separation to occur. The stability of alkali-surfactant-polymer solutions is

determined by preparing solutions as described in the same manner as described in example 1 and then observing their visual appearance during 24 hours. Unstable solutions show a characteristic phase separation into an aqueous

surfactant-rich phase and a polymer-rich phase. Stable solutions form a single, clear and homogeneous liquid phase. The specific compositions prepared are shown in table 3.

In the present example, the formulation must yield an interfacial tension below 10 ^~2 mN/m in order to be rated as being satisfactory for enhanced oil recovery. The total surfactant concentration is 6000 mg/l in each composition. All measurements and determinations of properties are carried out at 80 °C. After the composition was allowed to stand at 80 °C for 24 hours, it was checked by visual inspection whether phase separation occurred or whether the composition was stable. Viscosity is determined at a temperature of 80 °C and a shear rate of 7 s ^~ l.

For the preferred ranges of each chemical, it was found that the higher the amount of sodium metaborate the poorer stability. However, this detrimental effect may be

alleviated by increasing the concentration of the co- surfactant Dowfax 8390. As a matter of fact, the use of a single surfactant provided extremely poor result in terms of phase stability. Table 3: Evaluation of various compositions as regards phase separation and interfacial tension

Recodas Dowfax Flopaam Interfacial

NaB0 ₂

Entry 185 8390 3530S Phase tension

paration

[mg/l] [mg/l] [mg/l] [mg/l] se

[mN/m]

1 6000 0 400 5000 Yes

2 4680 1320 800 7000 No 0.00229

3 4680 1320 800 9000 No 0.00185

4 4680 1320 800 1 1000 No 0.00050

5 5160 840 400 3000 No 0.00175

6 5160 840 400 5000 No 0.00061

7 5160 840 400 7000 Yes 0.00076

8 5160 840 400 9000 Yes 0.00146

9 4920 1080 400 3000 No 0.00724

10 4920 1080 400 5000 No 0.00275

1 1 4920 1080 400 7000 No 0.00124

12 4920 1080 400 9000 Yes 0.00098

13 4680 1320 400 3000 No 0.145

14 4680 1320 400 5000 No 0.0132

15 4680 1320 400 7000 No 0.00229 16 4680 1320 400 9000 No 0.00185

Table 4: Evaluation of various compositions as regards interfacial tension (IFT)

Example 4 : Flooding of Berea sandstone cores

Flushing effectiveness of compositions was evaluated using Berea sandstone as a standard porous medium.

Core samples of an uniform Berea sandstone (length of 245 mm, diameter of 1 inch (2.54 cm)) were used in this experiment. After conditioning of the samples in accordance with the procedure described hereinabove under " (5) Oil recovery", the following flow sequence is applied, wherein the flow rate is 4 ml/h and the volume of liquid applied to the core sample is expressed as a multiple of the total pore volume.

(1) 16 total pore volumes of formation water

(2) 0.3 total pore volumes of alkali-surfactant-polymer composition as shown in table 5.

Table 5: Constitution of alkali-surfactant-polymer composition

Concentration

Ingredient

[mg/l]

(a) Recodas 185 5280

(b) Dowfax 8390 720

(c) Flopaam 3530S 400

(d) NaB0 ₂ 3000

(e) Water Balance

0.3 total pore volumes of polymer post-flushing composition as shown in table 6.

Table 6: Constitution of post-flush

(4) Formation water until oil production ceases, i.e.

until no more oil is detected in the liquid effluent from the core The oil recovery calculated for the water-flooding step (1) accounted for 53 %. After the alkali-surfactant-polymer flooding phase of steps (2) to (4), an additional oil recovery of 19 % is achieved, i.e. an oil recovery of 72 % in total was observed.

Example 5: Flooding of deposit rock cores

Flushing effectiveness of compositions was evaluated using deposit rock core samples (length of 232 mm, diameter of 1 inch (2.54 cm)) obtained from the same deposit in South America from which the deposit water and the crude oil mentioned hereinabove under " (5) Oil recovery" were

obtained as the porous medium.

After conditioning of the samples in accordance with the procedure described hereinabove under " (5) Oil recovery", the following flow sequence is applied, wherein the flow rate is 4 ml/h and the volume of liquid applied to the core sample is expressed as a multiple of the total pore volume.

(1) 10.1 total pore volumes of water

(2) 0.3 total pore volumes of alkali-surfactant-polymer composition as prepared in example 1 (cf . table 1) .

(3) 0.3 total pore volumes of polymer post-flushing composition as prepared in example 2 (cf . table 2) . (4) Formation water until oil production ceases, i.e.

until no more oil is detected in the liquid effluent from the core

The oil recovery calculated for the water-flooding step (1) accounted for 41 %. After the alkali-surfactant-polymer flooding phase of steps (2) to (4), an additional oil recovery of 28 % is achieved, i.e. an oil recovery of 69 % in total was observed.

Example 6: Preparation alkali-surfactant-polymer

composition on field level

An amount of 1600 bbl/day (1600 · 158.987 1/day = 254379 1/day) of the composition described in table 1 is prepared in continuous mode according to the procedure illustrated in the following with reference to figure 1 which shows a flow chart of the process.

In figure 1, a) to e) have the following meanings. a) : Water at a constant flow rate of 10.6 m ³ /h

b) : Sodium metaborate tetrahydrate (NaBC>2 * 4 ¾()) as a powder added at a constant rate of 155.3 kg/h

c) : Dowfax 8390 in the form of an aqueous solution

containing 35 % by weight of active matter added at a constant flow rate of 40.1 kg/h

d) : Recodas 185 in the form of an aqueous solution

containing 55 % by weight of active matter added at a constant flow rate of 90.6 kg/h

e) : Stock solution of Flopaam 3530S having a concentration of 5000 mg/1 added at a constant flow rate of 1.7 m ³ /h

The stock solution of Flopaam 3530S is prepared in

continuous mode according to the procedure illustrated in the following with reference to figure 2 which shows a flow chart of the process.

In figure 2, a) to c) have the following meanings. a) : Water at a constant flow rate of 1.7 m ³ /h

b) : Flopaam 3530S added as a powder at a constant rate of 8.5 kg/h c) : Container having an interior volume of 2 m^ in order to ensure that stock solution has a hydration time of at least 1 hour

According to the procedure shown in figures 1 and 2, a composition containing each component in a concentration shown in table 3, entry 2, can be prepared.