METHOD FOR TERTIARY PETROLEUM RECOVERY BY MEANS OF A HYDROPHOBICALLY ASSOCIATING POLYMER

Abstract

A method of tertiary production of mineral oil from underground deposits having a deposit temperature of 70 C., in which a copolymer comprising (meth)acrylamide or derivatives thereof, monoethylenically unsaturated carboxylic acids, especially acrylic acid, and an associative monomer is used, wherein the amount of the associative monomer is 0.1% to 0.9% by weight. A water-soluble copolymer comprising (meth)acrylamide or derivatives thereof, monoethylenically unsaturated carboxylic acids, especially acrylic acid, and 0.1% to 0.9% by weight of an associative monomer.

Claims

1.-19. (canceled)

20. A method of producing mineral oil from underground mineral oil deposits comprising mineral oil and saline deposit water, in which an aqueous formulation comprising injecting at least one thickening water-soluble copolymer (P) into the mineral oil deposit through at least one injection well and withdrawing mineral oil from the deposit through at least one production well, wherein the water-soluble copolymer (P) comprises at least 65% to 85% by weight of at least one monomer (A) selected from the group consisting of (meth)acrylamide, N-methyl(meth)acrylamide, N,N-dimethyl(melh)acrylamide and N-methylol(meth)acrylamide, and 14.9% to 34.9% by weight of at least one monomer (B) selected from the group consisting of acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid and salts thereof, wherein the water-soluble copolymer (P) further comprises 0.1% to 0.9% by weight of at least one monoethylenically unsaturated monomer (C) selected from the group consisting of
H.sub.2CC(R.sup.1)O(CH.sub.2CH(R.sup.5)O).sub.kR.sup.6 (I),
H.sub.2CC(R.sup.1)(CO)O(CH.sub.2CH(R.sup.5)O).sub.kR.sup.6 (II),
H.sub.2CC(R.sup.1)R.sup.7O(CH.sub.2CH(R.sup.8)O).sub.x(CH.sub.2CH(R.sup.9)O).sub.y(CH.sub.2CH.sub.2O).sub.zR.sup.10 (III),
H.sub.2CC(R.sup.1)C(O)OR.sup.11N.sup.+(R.sup.12)(R.sup.13)(R.sup.14)X.sup.(IV) or
H2CC(R1)-C(O)N(R15)-R11-N+(R12)(R13)(R14)X(V), where the radicals and indices are defined as follows: R.sup.1: H or methyl; R.sup.5: independently H, methyl or ethyl, with the proviso that at least 70 mol % of the R.sup.5 radicals are H, R.sup.6: aliphatic and/or aromatic, linear or branched hydrocarbyl radicals having 8 to 40 carbon atoms, R.sup.7: a single bond or a divalent linking group selected from the group consisting of (C.sub.nH.sub.2n), O(C.sub.nH.sub.2n) and C(O)O(C.sub.nH.sub.2n), where n is a natural number from 1 to 6, and n and n are a natural number from 2 to 6, R.sup.8: independently H, methyl or ethyl, with the proviso that at least 70 mol % of the R.sup.8 radicals are H, R.sup.9: independently hydrocarbyl radicals of at least 2 carbon atoms, R.sup.10: H or a hydrocarbyl radical having 1 to 30 carbon atoms, R.sup.11: an alkylene radical having 1 to 8 carbon atoms, R.sup.12, R.sup.13, R.sup.14: independently H or an alkyl group having 1 to 4 carbon atoms, R.sup.15: aliphatic and/or aromatic, linear or branched hydrocarbyl radicals having 8 to 30 carbon atoms, X.sup. a negatively charged counterion, k a number from 10 to 80, a number from 10 to 50, a number from 5 to 30, and z a number from 0 to 10, the deposit temperature is 70 C., the permeability of the deposit is 100 mD, and the deposit water comprises not more than 10 g/L of divalent ions.

21. The method according to claim 20, wherein the amount of the monomer (C) is 0.2% to 0.8% by weight.

22. The method according to claim 20, wherein the amount of the monomer (C) is 0.4% to 0.6% by weight.

23. The method according to claim 20, wherein the monomer (C) is at least one monomer of the general formula (III).

24. The method according to claim 23, wherein the monomers (C) are a mixture comprising at least the following monomers:
H.sub.2CC(R.sup.1)R.sup.7O(CH.sub.2CH(R.sup.8)O).sub.x(CH.sub.2CH(R.sup.9)O).sub.yH (IIIa) and
H.sub.2CC(R.sup.1)R.sup.7O(CH.sub.2CH(R.sup.8)O).sub.x(CH.sub.2CH(R.sup.9)O).sub.y(CH.sub.2CH.sub.2O).sub.zH (IIIb), where the radicals and indices have the definition outlined above, with the proviso that, in the formula (IIIb), z is a number >0 to 10.

25. The method according to claim 24, wherein, in the formulae (IIIa) and (IIIb), R.sup.1 is H, R.sup.7 is a O(C.sub.nH.sub.2n) group, R.sup.8 is H, R.sup.9 is ethyl, x is 20 to 30, y is 12 to 25, and z is 1 to 6.

26. The method according to claim 24, wherein, in the formulae (IIIa) and (IIIb), R.sup.1 is H, R.sup.7 is OCH.sub.2CH.sub.2CH.sub.2CH.sub.2, R.sup.8 is H, R.sup.9 is ethyl, x is 23 to 26, y is 14 to 18, and z is 3 to 5.

27. The method according to claim 20, wherein the deposit temperature is 30 C. to 70 C.

28. The method according to claim 20, wherein the permeability of the deposit is 200 mD to 2 D.

29. The method according to claim 20, wherein the deposit water comprises 0.1 to 10 g/L of divalent ions.

30. The method according to claim 20, wherein the amount of monomers (A), (B) and (C) together is 100% by weight.

31. A water-soluble copolymer (P) comprising at least 65% to 85% by weight of at least one monomer (A) selected from the group consisting of (meth)acrylamide, N-methyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide and N-methylol(meth)acrylamide, and 14.9% to 34.9% by weight of at least one monomer (B) selected from the group consisting of acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid and salts thereof, wherein the water-soluble copolymer (P) further comprises 0.1% to 0.9% by weight of at least one monoethylenically unsaturated monomer (C) selected from the group consisting of
H.sub.2CC(R.sup.1)O(CH.sub.2CH(R.sup.5)O).sub.kR.sup.6 (I),
H.sub.2CC(R.sup.1)(CO)O(CH.sub.2CH(R.sup.5)O).sub.kR.sup.6 (II),
H.sub.2CC(R.sup.1)R.sup.7O(CH.sub.2CH(R.sup.8)O).sub.x(CH.sub.2CH(R.sup.9)O).sub.y(CH.sub.2CH.sub.2O).sub.zR.sup.10 (III),
H.sub.2CC(R.sup.1)C(O)OR.sup.11N.sup.+(R.sup.12)(R.sup.13)(R.sup.14)X.sup.(IV) and
H.sub.2CC(R.sup.1)C(O)N(R.sup.15)R.sup.11N.sup.+(R.sup.12)(R.sup.13)(R.sup.14)X.sup.(V), where the radicals and indices are defined as follows: R.sup.1: H or methyl; R.sup.5: independently H, methyl or ethyl, with the proviso that at least 70 mol % of the R.sup.5 radicals are H, R.sup.6: aliphatic and/or aromatic, linear or branched hydrocarbyl radicals having 8 to 40 carbon atoms, R.sup.7: a single bond or a divalent linking group selected from the group consisting of (C.sub.nH.sub.2n), O(C.sub.nH.sub.2n) and C(O)O(C.sub.nH.sub.2n), where n is a natural number from 1 to 6, and n and n are a natural number from 2 to 6, R.sup.8: independently H, methyl or ethyl, with the proviso that at least 70 mol % of the R.sup.8 radicals are H, R.sup.9: independently hydrocarbyl radicals of at least 2 carbon atoms, R.sup.10: H or a hydrocarbyl radical having 1 to 30 carbon atoms, R.sup.11: an alkylene radical having 1 to 8 carbon atoms, R.sup.12, R.sup.13, R.sup.14: independently H or an alkyl group having 1 to 4 carbon atoms, R.sup.15: aliphatic and/or aromatic, linear or branched hydrocarbyl radicals having 8 to 30 carbon atoms, X.sup. a negatively charged counterion, Tc a number from 10 to 80, a number from 10 to 50, y a number from 5 to 30, and z a number from 0 to 10.

32. The water-soluble copolymer (P) according to claim 31, wherein the amount of the monomer (C) is 0.2% to 0.8% by weight.

33. The water-soluble copolymer (P) according to claim 31, wherein the amount of the monomer (C) is 0.4% to 0.6% by weight.

34. The water-soluble copolymer (P) according to claim 31, wherein the monomer (C) is at least one monomer of the general formula (III).

35. The water-soluble copolymer (P) according to claim 34, wherein the monomers (C) are a mixture comprising at least the following monomers:
H.sub.2CC(R.sup.1)R.sup.7O(CH.sub.2CH(R.sup.8)O).sub.x(CH.sub.2CH(R.sup.9)O).sub.yH (IIIa) and
H.sub.2CC(R.sup.1)R.sup.7O(CH.sub.2CH(R.sup.8)O).sub.x(CH.sub.2CH(R.sup.9)O).sub.y(CH.sub.2CH.sub.2O).sub.zH (IIIb), where the radicals and indices have the definition outlined above, with the proviso that, in the formula (IIIb), z is a number >0 to 10.

36. The water-soluble copolymer (P) according to claim 35, wherein, in the formulae (IIIa) and (IIIb), R.sup.1 is H, R.sup.7 is a O(C.sub.nH.sub.2n) group, R.sup.8 is H, R.sup.9 is ethyl, x is 20 to 30, y is 12 to 25, and z is 1 to 6.

37. The water-soluble copolymer (P) according to claim 35, wherein, in the formulae (IIIa) and (IIIb), R.sup.1 is H, R.sup.7 is OCH.sub.2CH.sub.2CH.sub.2CH.sub.2, R.sup.8 is H, R.sup.9 is ethyl, x is 23 to 26, y is 14 to 18, and z is 3 to 5.

38. The water-soluble copolymer (P) according to claim 31, wherein the amount of monomers (A), (B) and (C) together is 100% by weight.

Description

ADVANTAGES OF THE METHOD OF THE INVENTION

[0187] In the case of polymers according to prior art having a comparatively high content of associative monomers, there is the risk that the copolymers can block the formation. This reduces the oil yield. This is avoided through the use of the inventive polymers having a comparatively low content of associative monomers.

[0188] It has additionally been found that, surprisingly, the water-soluble copolymers (P) described have temperature-switchable characteristics in core flooding tests. The copolymers (P) lead to comparatively low resistance factors (RF; as defined in the experimental) at low temperature in the core flooding test, which promotes the injectivity of these polymers into the porous medium of the underground rock formation. In the formation, the polymer solution warms up gradually until the corresponding reservoir temperature of, for example, 60 C. has been attained. With the increase in temperature, there is also a rise in the resistance factor (RF), and this leads to balancing of the heterogeneity in the rock channels. This in turn improves the sweep efficiency and hence the oil production.

[0189] The examples which follow are to illustrate the invention in detail:

TABLE-US-00001 TABLE 1 Polymers examined Intrinsic viscosity Polymer name Composition [dL/g] Polymer A 70% by weight of acrylamide about 24 (comparative) 30% by weight of sodium acrylate Polymer B 50% by weight of acrylamide about 16 (comparative) 48% by weight of sodium 2-acrylamido-2- methylpropanesulfonate 2% by weight of HBVE - 24.5 EO - 16 BuO - 3.5 EO Polymer C 69% by weight of acrylamide about 24 (comparative) 30% by weight of sodium acrylate 1% by weight of HBVE - 24.5 EO - 16 BuO - 3.5 EO Polymer D 69.5% by weight of acrylamide about 24 (inventive) 30% by weight of sodium acrylate 0.5% by weight of HBVE - 24.5 EO - 16 BuO - 3.5 EO

[0190] Preparation of the Macromonomer HBVE-24.5 EO-16 BuO-3.5 EO

[0191] First Stage

[0192] HBVE-24.5 EO

[0193] A 2 L pressure autoclave with anchor stirrer was initially charged with 135.3 g (1.16 mol) of hydroxybutyl vinyl ether (HBVE) (stabilized with 100 ppm of potassium hydroxide (KOH)) and the stirrer was switched on. 1.06 g of potassium methoxide (KOMe) solution (32% KOMe in methanol (MeOH), corresponding to 0.0048 mol of potassium) were fed in and the stirred vessel was evacuated to a pressure less than 10 mbar, heated to 80 C. and operated at 80 C. and a pressure of less than 10 mbar for 70 min. MeOH was distilled off.

[0194] In an alternative procedure, the potassium methoxide (KOMe) solution (32% KOMe in methanol (MeOH)) was fed in and the stirred vessel was evacuated to a pressure of 10-20 mbar, heated to 65 C. and operated at 65 C. and a pressure of 10-20 mbar for 70 min. MeOH was distilled off.

[0195] The mixture was purged three times with N.sub.2 (nitrogen). Thereafter, the vessel was checked for pressure retention, 0.5 bar gauge (1.5 bar absolute) was set and the mixture was heated to 120 C. The mixture was decompressed to 1 bar absolute and 1126 g (25.6 mol) of ethylene oxide (EO) were metered in until p.sub.max was 3.9 bar absolute and T.sub.max was 150 C. After 300 g of EO had been metered in, the metered addition was stopped (about 3 h after commencement) for a wait period of 30 min and the mixture was decompressed to 1.3 bar absolute. Thereafter, the rest of the EO was metered in. The metered addition of EO including the decompression took a total of 10 h.

[0196] Stirring was continued to constant pressure at approx. 145-150 C. (1 h), and the mixture was cooled to 100 C. and freed of low boilers at a pressure of less than 10 mbar for 1 h. The material was transferred at 80 C. under N.sub.2.

[0197] Second Stage

[0198] HBVE-24.5 EO-16 BuO-3.5 EO

[0199] The starting material used was HBVE-24.5 EO as described above.

[0200] A 2 L pressure autoclave with anchor stirrer was initially charged with 568.6 g (0.525 mol) of HBVE-22 EO and the stirrer was switched on. Thereafter, 2.31 g of 50% NaOH solution (0.029 mol of NaOH, 1.16 g of NaOH) were added, a reduced pressure of <10 mbar was applied, and the mixture was heated to 100 C. and kept there for 80 min, in order to distill off the water.

[0201] The mixture was purged three times with N.sub.2. Thereafter, the vessel was tested for pressure retention, 0.5 bar gauge (1.5 bar absolute) was set, the mixture was heated to 127 C. and then the pressure was set to 3 bar absolute. 57.7 g (1.311 mol) of EO were metered in at 127 C.; p.sub.max was 6 bar absolute. After waiting for 30 min for establishment of constant pressure, the mixture was decompressed to 4.0 bar absolute. 604.2 g (8.392 mol) of BuO were metered in at 127 C.; p.sub.max was 6 bar absolute. One intermediate decompression was necessary owing to increasing fill level. The BuO metering was stopped, and the mixture was left to react for 1 h until pressure was constant and decompressed to 4.0 bar absolute. Thereafter, the metered addition of BuO was continued. P.sub.max was still 6 bar (first decompression after 505 g of BuO, total BuO metering time 11 h incl. decompression break). After metered addition of BuO had ended, reaction was allowed to continue at 127 C. for 6 h. The autoclave was decompressed to 4 bar absolute.

[0202] Thereafter, 80.8 g (1.836 mol) of EO were metered in at 127 C.; p.sub.max was 6 bar absolute. After metered addition of EO had ended, reaction was allowed to continue for 4 h. The mixture was cooled to 100 C., and residual oxide was drawn off until the pressure was below 10 mbar for at least 10 min. About 1400 ppm of volatile components were removed. Then 0.5% water was added at 120 C. and volatiles were subsequently drawn off until the pressure was below 10 mbar for at least 10 min. The vacuum was broken with N.sub.2 and 100 ppm of BHT were added. The transfer was effected at 80 C. under N.sub.2.

[0203] Preparation of Polymer A:

[0204] A plastic bucket with a magnetic stirrer, pH meter and thermometer was initially charged with 102.3 g of a 35% aqueous solution of sodium acrylate and then the following were added in succession: 115.7 g of distilled water, 0.4 g of a commercial silicone-based defoamer (Dow Corning Antifoam Emulsion RD), 168.8 g of acrylamide (50% solution in water), 1.2 g of a 5% aqueous solution of diethylenetriaminepentaacetic acid pentasodium salt, and 4 g of a 4% solution (dissolved in 5% sodium hydroxide solution) of the azo initiator 4,4-azobis(4-cyanovaleric acid).

[0205] After adjustment to pH 6.75 by means of 10% sulfuric acid, the rest of the water was added to attain the target monomer concentration of 30% (total amount of water minus the amount of water already added, minus the amount of acid required), and the monomer solution was adjusted to the initiation temperature of 4 C. The solution was transferred to a thermos flask, the temperature sensor for the temperature recording was attached, the mixture was purged with nitrogen for 45 minutes and the polymerization was initiated with 4 g of a 4% methanolic solution of the azo initiator azobis(isobutyronitrile), 0.16 mL of a 1% t-BHPO solution and 0.16 mL of a 1% sodium bisulfite solution. With the onset of the polymerization, the temperature rose to 80-90 C. within about 25-30 min. On attainment of the temperature maximum, the polymer was stored at 80 C. for 2 hours. After cooling to about 50 C., the gel block was comminuted with the aid of a meat grinder, and the gel granules obtained were dried in a fluidized bed drier at 55 C. for two hours. Hard white granules were obtained, which were converted to a pulverulent state by means of a centrifugal mill.

[0206] Preparation of Polymer B:

[0207] A plastic bucket with a magnetic stirrer, pH meter and thermometer was initially charged with 146.5 g of a 50% aqueous solution of sodium ATBS and then the following were added in succession: 105 g of distilled water, 0.4 g of a commercial silicone-based defoamer (Dow Corning Antifoam Emulsion RD), 2.8 g of macromonomers, 138.2 g of acrylamide (50% solution in water), 1.2 g of a 5% aqueous solution of diethylenetriaminepentaacetic acid pentasodium salt, and 3.0 g of the nonionic surfactant iC.sub.13-(EO).sub.15H.

[0208] After adjustment to pH 6 by means of 20% sodium hydroxide solution and addition of the rest of the water to attain the target monomer concentration of 37% (total amount of water minus the amount of water already added, minus the amount of acid required), the monomer solution was adjusted to the initiation temperature of 4 C. The solution was transferred to a thermos flask, the temperature sensor for the temperature recording was attached, the mixture was purged with nitrogen for 45 minutes and the polymerization was initiated with 1.6 mL of a 10% aqueous solution of the aqueous azo initiator 2,2-azobis(2-methylpropionamidine) dihydrochloride (Wako V-50), 0.12 mL of a 1% t-BHPO solution and 0.24 mL of a 1% sodium sulfite solution. With the onset of the polymerization, the temperature rose to 80-90 C. within about 25 min. A solid polymer gel was obtained.

[0209] After cooling to about 50 C., the gel block was comminuted with the aid of a meat grinder, and the gel granules obtained were dried in a fluidized bed drier at 55 C. for two hours. Hard white granules were obtained, which were converted to a pulverulent state by means of a centrifugal mill.

[0210] Preparation of Polymer C:

[0211] A plastic bucket with a magnetic stirrer, pH meter and thermometer was initially charged with 102.3 g of a 35% aqueous solution of sodium acrylate and then the following were added in succession: 115.7 g of distilled water, 0.4 g of a commercial silicone-based defoamer (Dow Corning Antifoam Emulsion RD), 166.4 g of acrylamide (50% solution in water), 1.2 g of a 5% aqueous solution of diethylenetriaminepentaacetic acid pentasodium salt, and 4 g of a 4% solution (dissolved in 5% sodium hydroxide solution) of the azo initiator 4,4-azobis(4-cyanovaleric acid).

[0212] After adjustment to pH 6.75 by means of 10% sulfuric acid, 1.2 g of macromonomer and 1.2 g of the nonionic surfactant iC.sub.13-(EO).sub.15H were added and the pH was checked again and adjusted to pH 6.75. Subsequently, the rest of the water was added to attain the target monomer concentration of 30% (total amount of water minus the amount of water already added, minus the amount of acid required), and the monomer solution was adjusted to the initiation temperature of 4 C. The solution was transferred to a thermos flask, the temperature sensor for the temperature recording was attached, the mixture was purged with nitrogen for 45 minutes and the polymerization was initiated with 4 g of a 4% methanolic solution of the azo initiator azobis(isobutyronitrile), 0.16 mL of a 1% t-BHPO solution and 0.16 mL of a 1% sodium bisulfite solution. With the onset of the polymerization, the temperature rose to 80-90 C. within about 25-30 min. On attainment of the temperature maximum, the polymer was stored at 80 C. for 2 hours. After cooling to about 50 C., the gel block was comminuted with the aid of a meat grinder, and the gel granules obtained were dried in a fluidized bed drier at 55 C. for two hours. Hard white granules were obtained, which were converted to a pulverulent state by means of a centrifugal mill.

[0213] Preparation of Polymer D:

[0214] A plastic bucket with a magnetic stirrer, pH meter and thermometer was initially charged with 102.3 g of a 35% aqueous solution of sodium acrylate and then the following were added in succession: 115.7 g of distilled water, 0.4 g of a commercial silicone-based defoamer (Dow Corning Antifoam Emulsion RD), 167.6 g of acrylamide (50% solution in water), 1.2 g of a 5% aqueous solution of diethylenetriaminepentaacetic acid pentasodium salt, and 4 g of a 4% solution (dissolved in 5% sodium hydroxide solution) of the azo initiator 4,4-azobis(4-cyanovaleric acid).

[0215] After adjustment to pH 6.75 by means of 10% sulfuric acid, 0.6 g of macromonomer and 0.6 g of the nonionic surfactant iC.sub.13-(EO).sub.15H were added and the pH was checked again and adjusted to pH 6.75. Subsequently, the rest of the water was added to attain the target monomer concentration of 30% (total amount of water minus the amount of water already added, minus the amount of acid required), and the monomer solution was adjusted to the initiation temperature of 4 C. The solution was transferred to a thermos flask, the temperature sensor for the temperature recording was attached, the mixture was purged with nitrogen for 45 minutes and the polymerization was initiated with 4 g of a 4% methanolic solution of the azo initiator azobis(isobutyronitrile), 0.16 mL of a 1% t-BHPO solution and 0.16 mL of a 1% sodium bisulfite solution. With the onset of the polymerization, the temperature rose to 80-90 C. within about 25-30 min. On attainment of the temperature maximum, the polymer was stored at 80 C. for 2 hours. After cooling to about 50 C., the gel block was comminuted with the aid of a meat grinder, and the gel granules obtained were dried in a fluidized bed drier at 55 C. for two hours. Hard white granules were obtained, which were converted to a pulverulent state by means of a centrifugal mill.

[0216] Performance Tests:

[0217] Determination of Intrinsic Viscosity

[0218] To determine the intrinsic viscosity, the flow times of the solvent and the polymer solutions at various concentrations were determined by means of an Ubbelohde capillary viscometer. The ratio of the flow times of the polymer solution and the pure solvent was used to calculate the relative viscosities. Thereafter, the specific viscosities were formed from the difference between the relative viscosity and 1. Finally, the reduced viscosity was formed from the quotient of the specific viscosity and the polymer concentration. This was plotted against the polymer concentration and the intrinsic viscosity was obtained from extrapolation to c=0. The results are reported in table 1 above.

[0219] Brookfield Viscosity

[0220] The viscosity of polymers C and D was measured as a function of temperature with a Brookfield LV viscometer with a UL adapter (1000 ppm in 1% NaCl solution at 7 s.sup.1).

[0221] The results are shown in FIG. 1.

[0222] Core Flooding TestsOil Yield

[0223] The core flooding tests were conducted with a test setup according to API RP 63, chapter 3.7 (see FIG. 2). The apparatus was equipped with pressure sensors at regular intervals along the core, such that pressure differentials were measured over the entire core and also over subsections of the core.

[0224] In each case about one pore volume of an aqueous polymer solution of concentration about 1000 ppm was injected into a Bentheim sandstone core (length of the core: 30.3 cm, diameter: 5.06 cm, pore volume: 139.17 mL, porosity: 22.8%, water permeability: 2890 mD) at a flow rate of 0.3048 m/day. The core had previously been saturated with crude oil. During the injection of the polymer solutions, the pressure differential was measured in individual sections of the sandstone, in order to observe the propagation of the polymer solution through the core.

[0225] The results of the experiments are compiled in FIGS. 3 to 6 and in table 2.

[0226] FIG. 3 shows, for comparative purposes, the results with polymer A, i.e. a polymer without associative monomer. The pressure differential in the individual segments of the core is comparably high in each case. This result means that the polymer A flows homogeneously through the core.

[0227] FIGS. 4 and 5 show the results of the comparative experiments with polymers B (2% by weight of associative monomer) and C (1% by weight of associative monomer). In these comparative experiments, the pressure rise in the first core segment (dP1) is significantly higher than in the subsequent segments. Another observation in some cases is no stabilization at all of the pressure level. This means that a majority of the polymer is retained in the foremost portion of the core.

[0228] This in turn has adverse effects on oil production.

[0229] FIG. 6 shows the results of the inventive experiments with polymer D (only 0.5% by weight of associative monomer). This polymer has homogeneous propagation through the core, similarly to polymer A.

[0230] The results of the core flooding tests are summarized in table 2. The terms used here have the following meanings:

TABLE-US-00002 TABLE 2 Summary of the results of the core flooding tests Oil yield after polymer flooding Volume of oil produced during the [mL] polymer injection Residual oil saturation S.sub.or Oil saturation after water injection, but before polymer injection Initial oil saturation S.sub.oi Oil saturation at the start of the experiment, i.e. prior to the injection of the water Peak polymer oil cut Maximum proportion of oil in the [% by vol.] total amount of fluid produced (oil + water) Total oil yield Oil saturation at the end of the experiment after injections of all fluids Example No. C1 C2 C3 1 Polymer A B C D Amount of associative monomer 0% 2% 1% 0.5% Oil yield after polymer flooding [mL] 10.60 6.2 9.86 13.34 Oil yield after polymer flooding, based on 17.7 9.8 16.1 22.4 residual oil saturation S.sub.or [%] Oil yield after polymer flooding, based on 8.2 5.2 7.7 10.8 initial oil saturation S.sub.oi [%] Peak polymer oil cut [% by vol.] 31.1 2.2 12.1 41.4 Total oil yield, based on S.sub.oi [%] 61.9 51.6 59.9 62.5

[0231] An essential factor for the efficacy of the polymer flooding is firstly the total oil yield, which can be determined by means of the core flooding test. Another important factor is also the question of how quickly the oil can be produced. An indicator of this is the peak polymer oil cut. On injection of the polymer solution into the core, a mixture of (polymer-comprising) water and oil is typically produced. The peak polymer oil cut is the highest concentration of oil, based on water and oil, which is produced in the course of the experiment. A high value means that the greatest amount of oil is produced relatively quickly in a high concentration. A low value means that the oil production is spread over a wide range.

[0232] As can be seen in table 2, the best oil yield is achieved in experiment 1 (polymer D). In addition, the peak polymer oil cut is at its greatest at 41.4% by volume.

[0233] Core Flooding TestsTemperature Dependence

[0234] For the experiments, a solution of 1000 ppm of polymer D in synthetic seawater was used. The synthetic seawater had the following composition:

TABLE-US-00003 Salt Concentration [g/L] Na.sub.2SO.sub.4 3.408 NaHCO.sub.3 0.168 KCl 0.746 NaCl 23.5 MgCl.sub.2 6 H.sub.2O 9.149 CaCl.sub.2 2 H.sub.2O 1.911

[0235] The concentration of the divalent ions (Mg.sup.2+ and Ca.sup.2+) is 1.6 g/L. The solution was injected into Bentheim sandstone at various flow rates and temperatures, in the sequence specified in table 3 (step 1 to step 8). The pressure differential across the core was measured in each case. For comparative purposes, synthetic seawater without polymer was injected into the core under the same conditions and the pressure differential was likewise measured in each case. The quotient of the pressure differentials was used to calculate the resistance factor (RF) (RF=pressure differential of polymer in seawater/pressure differential with pure seawater). The RF is a measure of the apparent viscosity of the solution in the porous medium.

[0236] The results of the experiments are compiled in table 3.

TABLE-US-00004 TABLE 3 Determination of the resistance factor (RF) Steps Flow rate [mL/min] T [ C.] Resistance factor 1 0.5 20 14.9 2 0.5 45 93.8 3 0.5 60 152 4 0.2 60 323 5 0.1 60 562 6 0.1 20 15.9 7 0.2 20 17.0 8 0.5 20 21.4

[0237] FIG. 1 shows the Brookfield viscosity of polymers C (comparative) and D (inventive). The viscosity of C rises with increasing temperature, whereas that of polymer D decreases slightly with increasing temperature. A rise in viscosity with temperature is indeed desirable: typically, the polymer solution is at about room temperature prior to injection. After injection into the deposit, the solution heats up under the influence of the deposit, with increasing viscosity. In this respect, the person skilled in the art would consider polymer D to be of low suitability.

[0238] Surprisingly, the core flooding test with polymer D also shows that polymer D leads to better deoiling with rising temperature. As can be seen in table 2, there is a distinct rise in the RF with temperature (step 1.fwdarw.3). A high RF means a distinct reduction in the mobility of the aqueous polymer solution compared to the solution without polymer. Lower mobility leads to more homogeneous propagation of the solution through the formation, such that oil is mobilized even in regions having relatively low permeability. This behavior is remarkable because the viscosity of the polymer in synthetic seawater decreases with rising temperature. The person skilled in the art would therefore expect worsened deoiling on the basis of the viscosity measurements. The behavior is reversible. At the end of the test sequence (step 8), measurement was again effected at 0.5 mL/min and 20 C., and the RF is about the same.