LOW PH CROSSLINKING OF POLYMERS

Abstract

The invention is directed to polymers that self-crosslink at acidic pH or can be crosslinked by phenolic agents in brine. Such polymers have lower viscosity and can be pumped deep into reservoirs, where they will cross link in situ, thus increasing their viscosity and/or form a gel and blocking thief zones. Methods of making and using such polymers are also provided.

Claims

1) A composition for improving sweep efficiency of a flood in a reservoir, said composition being a solution comprising a polymer plus a phenolic crosslinker plus a fluid; a) wherein said polymer comprises one or more acrylamide-based monomer(s) at 0.2-20 mole percent; b) wherein at least one of said acrylamide-based monomer(s) has pendant hydroxyl groups ranging from 0.1-99 mole percent; c) wherein said composition has a first viscosity at pH 8 and forms a gel when aged for a period of time at pH less than 5, and d) wherein said phenolic crosslinker is at 50-5000 ppm (w/w) in said solution.

2) The composition of claim 1, wherein the concentration of the polymer in said solution is 0.4-10 mole percent.

3) The composition of claim 1, wherein the phenolic crosslinker is 100-1000 ppm (w/w) in said solution.

4) The composition of claim 1, wherein said at least one of said acrylamide-based monomer(s) having a pendant hydroxyl group is chosen from a group comprising N-hydroxyethylacrylamide, N-(2-Hydroxypropyl)methacrylamide, and N-hydroxymethylacrylamide.

5) The composition of claim 1, wherein said period of time is 30 days.

6) The composition of claim 1, wherein said period of time is 60 days.

7) The composition of claim 5, wherein said phenolic crosslinker is chosen from a group consisting of phenol, catechin, resorcinol, m-cresol, phenyl acetate, salicyl alcohol, aspirin, phloroglucinol, quercetin, gallocatechin, naringenin, eriodictyol, afzelechin, and epimers thereof.

8) The composition of claim 6, wherein the said phenolic crosslinker is chosen from a group consisting of phenol, catechin, resorcinol, m-cresol, phenyl acetate, salicyl alcohol, aspirin, phloroglucinol, quercetin, gallocatechin, naringenin, eriodictyol, afzelechin and epimers thereof.

9) The composition of claim 1, wherein said at least one of said acrylamide-based monomer(s) having a pendant hydroxyl group is chosen from a group consisting of N-hydroxyethylacrylamide, N-(2-Hydroxypropyl)methacrylamide and N-hydroxymethylacrylamide, and said phenolic crosslinker is chosen from a group consisting of phenol, catechin, resorcinol, m-cresol, phenyl acetate, salcyl alcohol, aspirin, phloroglucinol, quercetin, gallocatechin, naringenin, eriodictyol, afzelechin and epimers thereof.

10) A method of increasing the recovery of oil from a reservoir, said method comprising: a) injecting the composition of claim 1 into a reservoir containing oil; b) aging said composition until it gels; c) injecting a gas into said reservoir to sweep oil towards a production well; and d) producing said oil from said production well.

11) The method of claim 10, wherein the composition is aged for at least 30 days.

12) The method of claim 10, wherein the composition is aged for at least 60 days.

13) The method of claim 10, wherein said gas is CO.sub.2.

14) The method of claim 13, wherein CO.sub.2 injections alternate with water injections to sweep oil towards said production well.

15) The method of claim 10, wherein said gas is CO.sub.2 and said CO.sub.2 is injected after said composition gels.

16) The method of claim 10, wherein said gas is CO.sub.2 and said CO.sub.2 is injected after said composition gels and CO.sub.2 injection is alternated with water injection to sweep oil towards said production well.

17) A method of increasing the recovery of oil from a reservoir, said method comprising: a) injecting the composition of claim 9 into a reservoir containing oil; b) aging said composition until it gels; c) injecting a gas into said reservoir to sweep oil towards a production well; and d) producing said oil from said production well.

18) The method of claim 17, wherein the composition is aged for at least 30 days.

19) The method of claim 17, wherein said gas is CO.sub.2 and said CO.sub.2 is injected after said composition gels.

20) The method of claim 17, wherein said gas is CO.sub.2 and said CO.sub.2 is injected after said composition gels and CO.sub.2 injection is alternated with water injection to sweep oil towards said production well.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] FIG. 1. Carbon dioxide pressure-temperature phase diagram.

[0051] FIG. 2. CO.sub.2 flooding process.

[0052] FIG. 3. Viscosity of 4 polymers over a range of pH's at 3 rpm.

[0053] FIG. 4 depicts the synthesis of exemplary polymers that can self-crosslink or crosslink with organic crosslinkers at low pH and the reactions of the polymers self-crosslinking or crosslinking with organic crosslinkers at low pH.

[0054] FIG. 5. Crosslinking of 0.5% poly(AM-NaAMPS-NHMA) with 500 ppm catechin in Synthetic Brine A at 65° C.

[0055] FIG. 6. Crosslinking of 0.5% poly(AM-NaAMPS-NHMA) and poly(AM-NHMA) with 500 ppm catechin in Synthetic Brine C at 65° C., respectively.

[0056] FIG. 7. Self-crosslinking of 0.7% poly(AM-NHMA) and crosslinking of 0.7% poly(AM-NHMA) with 200 ppm resorcinol in Synthetic Brine G at 65° C.

[0057] FIG. 8. Crosslinking of 0.5% poly(AM-NaAMPS-NHMA) with 500 ppm catechin in Synthetic Brine C, G and P at 65° C.

[0058] FIG. 9. Self-crosslinking of 1% poly(AM-NaAMPS-NHMA) and crosslinking of 1% poly(AM-NaAMPS-NHMA) with 1000 ppm catechin in pH 3.2 buffer P at 40° C.

[0059] FIG. 10. Self-crosslinking of 1% poly(AM-NaAMPS-NHMA) in Synthetic Brine E at 40° C.

[0060] FIG. 11. Self-crosslinking of 1% poly(AM-NHMA) in Synthetic Brine E at 40° C.

[0061] FIG. 12 Self Crosslinking of 0.5% poly(AM-NHMA) and 0.5% poly(AM-NaAMPS-NHMA) in synthetic Brine D under 1000 psi CO.sub.2 at 108° F.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0062] The invention provides novel polymers that gel under the acidic conditions typical of CO.sub.2 floods in situ and have particular utility in blocking thief zones of reservoirs, but other uses are possible. For example, such materials can be used for oil and gas well stimulation/clean-up treatments with acid. In addition, such materials can also be used in mining, soil remediation, agriculture, and other environments where low pH gelation would be beneficial.

Preparation of Co- and Terpolymers

[0063] Synthesis of Poly(AM-NHMA): A representative copolymer poly(acrylamide-N-hydroxymethylacrylamide), hereinafter referred to as poly(AM-NHMA), was prepared containing 5 mol % N-hydroxymethylacrylamide (NHMA) and 95 mol % acrylamide (AM) using inverse-emulsion polymerization. FIG. 4A-C shows the polymerization of this composition and related polymers and related external crosslinkers.

[0064] An aqueous phase containing 18.0 g of acrylamide (AM) and 2.9 g of 48% NHMA in 19.10 g of RO water, and an oil phase of 20 g of kerosene, 2.3 g of Span 83 and 2.7 g of polyoxyethylene sorbitol hexaoleate (PSH) were used in the synthesis.

[0065] The inverse-emulsion was prepared by mixing the aqueous phase and the oil phase, followed by rapid homogenization using a homogenizer. After transferring the inverse-emulsion into a 250 ml flask, 38 mg VAZO® 52 was added as an initiator to this solution. This inverse-emulsion was then purged with nitrogen for 20 minutes and the polymerization was carried out in a 50-55° C. oil bath for 3-4 hours.

[0066] VAZO® 52 (2,2′-Azobis(2,4-dimethylvaleronitrile), available from DuPont) is a low-temperature, oil-soluble polymerization initiator, whose rate of decomposition is first-order and is unaffected by contaminants such as metal ions.

[0067] Other oil-soluble initiators that could be used include organic peroxides (e.g., benzoyl or lauroyl), azos (e.g. 2,2′-azobisisobutyrilonitrile (AIBN)), and the like.

[0068] Synthesis of Poly(AM-NaAMPS-NHMA): Poly[acrylamide-(sodium 2-acrylamido-2-methylpropane sulfonate)-N-hydroxymethylacrylamide] terpolymer [aka poly(AM-NaAMPS-NHMA)] was synthesized by introducing sodium 2-acrylamido-2-methylpropane sulfonate (sodium AMPS, NaAMPS) as an anionic monomer into a mixture of acrylamide (AM) and N-hydroxymethylacrylamide (NHMA) similar to the synthesis described above. The synthesis and repeating unit for poly(AM-NaAMPS-NHMA) are also shown in FIG. 4A.

[0069] A representative terpolymer of poly(AM-NaAMPS-NHMA) containing 5 mol % NHMA, 5 mol % NaAMPS and 90 mol % acrylamide (AM) was prepared using inverse-emulsion polymerization. An aqueous phase containing 15.41 g of AM, 5.60 g of 50% NaAMPS, and 2.54 g of 48% NHMA in 16.51 g of RO water, and an oil mixture containing 20 g of kerosene, 2.3 g of Span 83 and 2.7 g of PSH was used in the synthesis.

[0070] The inverse-emulsion was produced by mixing the aqueous phase and the oil phase, followed by rapid homogenization using a homogenizer. After transferring the inverse-emulsion into a 250 ml flask, 38 mg VAZO® 52 was added to this inverse-emulsion as an initiator. The resulting mixture was then purged with nitrogen for 20 minutes. The polymerization was continued at 50-55° C. in an oil bath for 3-4 hours.

Preparation of Brine

[0071] The brine composition used in the polymer gelation experiments is given below.

TABLE-US-00002 TABLE 1 Synthetic Brine Compositions Ingredient Moles Brine A NaCl 3.93 × 10.sup.−1 pH = 5.5 KCl 2.03 × 10.sup.−3 CaCl.sub.2•2H.sub.2O 1.72 × 10.sup.−3 MgCl.sub.2•6H.sub.2O 5.27 × 10.sup.−3 Na.sub.2SO.sub.4 1.02 × 10.sup.−3 RO water Volume adjusted to 1 L Brine C Na.sub.2HPO.sub.4 2.46 × 10.sup.−2 pH = 3.2 Citric Acid 3.77 × 10.sup.−2 Brine A Volume adjusted to 1 L Brine D NaCl 1.71 pH = 5.9 KCl 1.14 × 10.sup.−2 CaCl.sub.2•2H.sub.2O 7.06 × 10.sup.−2 MgCl.sub.2•6H.sub.2O 3.18 × 10.sup.−2 Na.sub.2SO.sub.4 1.51 × 10.sup.−2 RO water Volume adjusted to 1 L Brine E NaCl 1.70 pH = 3.2 KCl 1.13 × 10.sup.−2 CaCl.sub.2•2H.sub.2O 7.01 × 10.sup.−2 MgCl.sub.2•6H.sub.2O 3.15 × 10.sup.−2 Na.sub.2SO.sub.4 1.50 × 10.sup.−2 Citric acid 5.00 × 10.sup.−2 NaOH 7.03 × 10.sup.−2 RO water Volume adjusted to 1 kg Brine G Glycine  5.0 × 10.sup.−2 pH = 3.2 HCl  4.1 × 10.sup.−3 Brine A Volume adjusted to 1 L Brine P Na.sub.2HPO.sub.4  5.0 × 10.sup.−2 pH = 3.2 H.sub.3PO.sub.4  5.8 × 10.sup.−2 Brine A Volume adjusted to 1 L Buffer P Na2HPO4 2.46 × 10.sup.−2 pH = 3.2 Citric Acid 3.77 × 10.sup.−2 RO water Volume adjusted to 1 L

[0072] For Synthetic Brine C, a pH of 3.2 was obtained with a citric acid and sodium phosphate buffering solution. Here, 123 ml of 0.2 M Na.sub.2HPO.sub.4 and 377 ml of 0.1 M citric acid was added to Synthetic Brine A without NaHCO.sub.3.

[0073] For Synthetic Brine G, pH 3.2 was obtained with a glycine and HCl buffering solution. Here, 250 ml of 0.2 M glycine and 41 ml of 0.1 M HCl was added to Synthetic Brine A without NaHCO.sub.3. Then, the solution was diluted to 1000 ml.

[0074] For Synthetic Brine P, pH 3.2 was obtained with a phosphate and H.sub.3PO.sub.4 buffering solution. Here, 7.10 g Na.sub.2HPO.sub.4 and 6.68 g of 85% H.sub.3PO.sub.4 were added to Synthetic Brine A without NaHCO.sub.3. Then, the solution was diluted to 1000 ml.

[0075] For Buffer P, pH 3.2 was obtained with a citric acid and sodium phosphate buffering solution. Here, 123 ml of 0.2 M Na.sub.2HPO.sub.4 and 377 ml of 0.1 M citric acid were added to RO water. Then, the solution was diluted to 1000 ml.

Viscosity Measurements

[0076] The specific viscosity of the initial mixtures in the crosslinking and self-crosslinking experiments described below was determined by using a Brookfield Viscometer. The mixtures were then incubated at a pre-determined temperature for some pre-determined amount of time, after which, the viscosity of the mixtures were again measured. Viscosity increase above 1000 cP is presumed to coincide with gel formation, but this is not certain.

Crosslinking Experiments

[0077] Crosslinking of 0.5% poly(AM-NaAMPS-NHMA) with Catechin: 0.83 g of 30% inverting surfactant was added into 97.45 g of deoxygenated Synthetic Brine C in a beaker while stirring in an oxygen-free glove box. Then 1.67 g of 30% poly(AM-NaAMPS-NHMA) (5 mole % NaAMPS, 5 mole % NHMA) and 0.05 g catechin (500 ppm) were added into the above solution under stirring, respectively. FIG. 4C shows this crosslinking reaction for poly(AM-NsAMPS-NHMA) and related polymers.

[0078] The initial viscosity of the solution was determined before dividing the rest of the solution into 6-ml vials. The vials were incubated at 65° C. for various lengths of time before removing a vial and measuring the viscosity of its content. The results for crosslinking of 0.5% poly(AM-NaAMPS-NHMA) with 500 ppm catechin in Synthetic Brine C are shown in FIG. 6. As this graph shows the viscosity of 0.5% poly(AM-NaAMPS-NHMA) solution containing 500 ppm catechin in Brine C increased with aging time, reaching a maximum of 1000 cP within 68 days of aging at 65° C.

[0079] Control experiment of 0.5% poly(AM-NaAMPS-NHMA): The initial crosslinking experiment was carried out in Synthetic Brine C, which contains citric acid. In order to verify the crosslinking not caused by citric acid, other low pH synthetic brines (Brine A) was used for additional crosslinking tests.

[0080] 0.83 g of 30% inverting surfactant was added to 97.45 g of Synthetic Brine A in a beaker while stirring in an oxygen-free glove box; and then 1.67 g of 30% poly(AM-NaAMPS-NHMA) and 0.05 g catechin were added into the above solution under stirring, respectively. The initial viscosity of the solution was determined before dividing the rest of the solution into 6-ml vials. The vials were incubated at 65° C. for various lengths of time before removing the content of a vial and measuring the viscosity of the solution.

[0081] The results for incubation of 0.5% poly(AM-NaAMPS-NHMA) with 500 ppm catechin in Synthetic Brine A are shown in FIG. 5. No significant change in viscosity of this solution was observed in 120 days of aging at 65° C. As will become obvious in the following examples, this shows that a lower pH is necessary to promote self-crosslinking, or crosslinking can be achieved with a phenolic compound.

[0082] Crosslinking of 0.5% poly(AM-NHMA) (5 mole % NHMA) with Catechin: 0.83 g of 30% inverting surfactant was added into 97.45 g deoxygenated Synthetic Brine C in a beaker while stirring in an oxygen-free glove box; and then 1.67 g of 30% poly(AM-NHMA) and 0.05 g catechin were added into the above solution under stirring, respectively. The initial viscosity of the solution was determined before dividing the rest of the solution into 6-ml vials. The vials were incubated at 65° C. for various lengths of time before measuring the viscosity of their content. The results for crosslinking of 0.5% poly(AM-NHMA) with 500 ppm catechin in Synthetic Brine C are also shown in FIG. 6. The viscosity of this solution increased with aging time and reached a maximum of 1000 cP in 70 days of aging at 65° C.

[0083] Self-crosslinking of 0.7% poly(AM-NHMA): 0.58 g of 30% inverting surfactant was added to 48.25 g of deoxygenated Synthetic Brine G in a beaker with stirring in an oxygen-free glove box. We then added 1.17 g of 30% poly(AM-NHMA) to this solution under stirring. The initial viscosity of the resulting polymer solution was determined before dividing the rest of the solution into 6-ml vials. The vials were incubated at 65° C. for various lengths of time before removing one vial at a time, transferring its content and measuring viscosity of the solution. FIG. 4B shows the self-crosslinking reaction for poly(AM-NHMA) and related polymers.

[0084] The results for self-crosslinking of poly(AM-NHMA) in Synthetic Brine G are also shown in FIG. 7. The viscosity of the solution increased with aging time at 65° C., reaching 180 cP in 66 days of aging at 65° C. Although low, the viscosity of CO.sub.2 is substantially lower than 1, thus a viscosity of 180 would still function to divert CO.sub.2.

[0085] Crosslinking of 0.7% poly(AM-NHMA) with Resorcinol: 0.58 g of 30% inverting surfactant was added into 48.25 g of deoxygenated Synthetic Brine G in a beaker while stirring in an oxygen-free glove box. Then 1.17 g of 30% poly(AM-NHMA) and 0.010 g (˜200 ppm) resorcinol were added into the above solution under stirring, respectively. The initial viscosity of the solution was determined before dividing the rest of the solution into 6-ml vials. The vials were incubated at 65° C. for various lengths of time before measuring the viscosity of their content. The results for crosslinking of 0.7% poly(AM-NHMA) with 200 ppm resorcinol in Synthetic Brine G are shown in FIG. 7. As can be seen, the viscosity of the mixture in brine G increased with aging time, reaching a maximum of about 1000 cP in 66 days of aging at 65° C.

[0086] Crosslinking of 0.7% poly(AM-NaAMPS-NHMA) with Catechin: 0.58 g of 30% inverting surfactant was added into 48.25 g of deoxygenated Synthetic Brine C in a beaker with stirring in an oxygen-free glove box; and then 1.17 g of 30% poly(AM-NaAMPS-NHMA) and 0.025 g (500 ppm) catechin were added into the above solution under stirring, respectively. The initial viscosity of the solution was determined before dividing the rest of the solution into 6-ml vials. The vials were incubated at 65° C. for various lengths of time before measuring the viscosity of their content. The results for crosslinking of 0.7% poly(AM-NaAMPS-NHMA) with 500 ppm catechin in Synthetic Brine C are shown in FIG. 8.

[0087] Crosslinking experiments of 0.7% poly(AM-NaAMPS-NHMA) with 500 ppm Catechin in Synthetic Brine G and P at 65° C. were also carried out using the above procedure. The results for crosslinking of 0.7% poly(AM-NaAMPS-NHMA) with 500 ppm catechin in Synthetic Brine G and P are also shown in FIG. 8.

[0088] As this graph indicates, the viscosity of the mixture in brine C began to rise in 4 days, reaching a maximum of about 1000 cP in 28 days of aging at 65° C. A similar result was observed for the solution aged in brine G at 65° C. The viscosity of the solution prepared in Brine P increased with aging time, reaching a maximum of about 1000 cP in 30 days of aging at 65° C. Therefore, crosslinking of poly(AM-NaAMPS-NHMA) with catechin produces gels that could be used to divert CO.sub.2 to lower permeability zones. Brine P may need longer time to reach the same viscosity, because the buffering system is made from an acid and its salt.

[0089] Self-crosslinking of 1% poly(AM-NaAMPS-NHMA): 1.67 g of 30% inverting surfactant was added to 95.00 g of deoxygenated buffer P (pH=3.2) in a beaker while stirring in an oxygen-free glove box. We then added 3.33 g of 30% poly(AM-NaAMPS-NHMA) to this solution under stirring. Then, the rest of the solution was divided into 6-ml vials. The vials were incubated at 40° C. for various lengths of time before removing one vial at a time, transferring its content and measuring viscosity of the solution from the vial.

[0090] The results for self-crosslinking of poly(AM-NaAMPS-NHMA) in pH 3.2 buffer P are shown in FIG. 9. As this graph shows, the viscosity of the solution increased with aging time at 40° C., reaching a maximum of about 1000 cP after 170 days.

[0091] Crosslinking of 1% poly(AM-NaAMPS-NHMA) with catechin: 1.67 g of 30% inverting surfactant was added to 94.90 g of deoxygenated pH 3.2 buffer P in a beaker while stirring in an oxygen-free glove box; and then 3.33 g of 30% poly(AM-NaAMPS-NHMA) and 0.10 g catechin (1000 ppm) were added into the above solution under stirring, respectively. The initial viscosity of the solution was determined before dividing the rest of the solution into 6-ml vials. The vials were incubated at 40° C. for various lengths of time before removing a vial and measuring the viscosity of its content.

[0092] The results for crosslinking of 1% poly(AM-NaAMPS-NHMA) with 1000 ppm catechin in pH 3.2 buffer P are also shown in FIG. 9. As with the self-crosslinking experiment, the viscosity increased with aging time, reaching a value of about 1000 cP after 80 days of aging at 40° C.

[0093] Self-crosslinking of 1% poly(AM-NaAMPS-NHMA) in Synthetic Buffer E: 1.67 g of 30% inverting surfactant was added to 95.00 g of deoxygenated Synthetic Brine E (pH=3.2) in a beaker while stiffing in an oxygen-free glove box. Then, 3.33 g of 30% poly(AM-NaAMPS-NHMA) was added to this solution under stirring. The rest of the solution was divided into 6-mi vials. The vials were incubated at 40° C. for various lengths of time before removing one vial at a time, transferring its content and measuring viscosity of the solution from the vial.

[0094] The results for self-crosslinking of poly(AM-NaAMPS-NHMA) in pH 3.2 Synthetic Brine E are shown in FIG. 10. As this graph shows, the viscosity of the solution increased with aging time at 40° C., reaching about 600 cP after around 60 days.

[0095] Self-crosslinking of 1% poly(AM-NHMA) in Synthetic Brine E: 1.67 g of 30% inverting surfactant was added to 95.00 g of deoxygenated Synthetic Brine E (pH=3.2) in a beaker while stirring in an oxygen-free glove box. Then, 3.33 g of 30% poly(AM-NaAMPS-NHMA) was added to this solution under stirring. The rest of the solution was divided into 6-ml vials. The vials were incubated at 40° C. for various lengths of time before removing one vial at a time, transferring its content and measuring viscosity of the solution from the vial.

[0096] The results for self-crosslinking of poly(AM-NHMA) in Synthetic brine E are shown in FIG. 11. As this graph shows, the viscosity of the solution increased with aging time at 40° C., reaching 1000 cP after around 60 days.

[0097] Self Crosslinking of 0.5% Poly(AM-NaAMPS-NHMA) and Poly(AM-NHMA) under CO.sub.2: The low pH polymer was inverted and dispersed in the synthetic Brine D in an anaerobic chamber. The polymer dispersion was then loaded into several pressure vessels. The vessels were capped and removed from the anaerobic chamber. CO.sub.2, at about 850 psi, was charged into the pressure vessels at room temperature using a gas booster pump. The vessels were then placed in a 42° C. oven. The pressure inside the vessels was monitored to maintain ˜1000 psi, wherein the CO.sub.2 was released if pressure needed to be reduced. Viscosity measurements of the polymer were taken periodically. Here, a pressure vessel was removed from the oven, the gas was released, and the polymer was taken out of the vessel for viscosity measurement. FIG. 12 shows the viscosity versus time of aging under CO.sub.2 for 0.5% poly(AM-NaAMPS-NHMA) and poly(AM-NHMA) in Brine D measured at 25° C. The viscosity of both polymers increased with aging under CO.sub.2, indicating self-crosslinking. The test is continuing at this time.

[0098] Thus, poly(AM-NaAMPS-NHMA) and poly(AM-NHMA) can both self-crosslink and can also cross link with organic crosslinkers (such as catechin) at low pH, resulting in an increase viscosity of polymer solution and/or gel formation. These polymers are good candidates for profile control and water shutoff application in reservoirs, particularly under CO.sub.2 flooding which results in acidic conditions as the CO.sub.2 partitions into the connate water.

[0099] Each of the following references are incorporated herein in their entirety for all purposes. [0100] Martin, F. D., and Kovarik, F. S., “Chemical Gels for Diverting CO.sub.2: Baseline Experiments,” Paper SPE 16728, presented at the 62nd Ann. Tech. Conf. and Exhib., Dallas, Tex., Sep. 27-30, 1987. [0101] Martin, F. D., and Kovarik, F. S., “Gels for CO.sub.2 Profile Modificatio”, Paper SPE 17330, presented at SPE/DOE Enhanced Oil Recovery Symposium, Tulsa, Okla., Apr. 17-20, 1988. [0102] Senol, N. N., “Laboratory Studies on Polymer Gels for CO.sub.2 Mobility Control at Bat Raman Heavy Oilfield, Turkey,” Paper SPE 50798, Presented at 1999 SPE International Symposium on Oilfield Chemistry, Houston, Tex., Feb. 16-18, 1999. [0103] Hild, G. P. and Wackowski, R. K., “Reservoir Polymer Gel Treatments to Improve Miscible CO.sub.2 Flood,” SPE Reservoir Eval. & Eng. 2(2), April 1999, pp 196-204. [0104] Asghari, K., and Taabbodi, L., “Laboratory Investigation of Indepth Gel Placement for Carbon Dioxide Flooding in Carbonate Porous Media,” Paper SPE 90633, Presented at the 2004 SPE International Petroleum Conference, Mexico, Nov. 8-9, 2004. [0105] Taabbodi, L. and Asghari, K., “Application of In-Depth Gel Placement for Water and Carbon Dioxide Conformance Control in Carbonate Porous Media,” Paper 2004-168 Presented at the 5th Canadian International Petroleum Conference, Calgary, Jun. 8-10, 2004. [0106] Taabbodi, L. and Asghari, K., “Application of In-Depth Gel Placement for Water and Carbon Dioxide Conformance Control in Carbonate Porous Media,” JCPT, February 2006, Vol. 45, No 2, pp 33-40. [0107] Freidmann, F., Hughes, T. L., Smith M. E., Hild, G. P., Wilson, A. and Davies, S. N., “Development and Testing of a Foam-Gel Technology to Improve Conformance of Rangely CO.sub.2 Flood,” SPE Reservoir Eval. & Eng. 2(2), February 1999, pp 4-13. [0108] Wassmuth, F. R., Green, K. and Hodgins, L., “Conformance Control for Miscible CO.sub.2 Floods in Fractured Carbonates,” Paper 2005-243, presented at Canadian International Petroleum Conference, Calgary, Jun. 7-9, 2005. [0109] Huh, C., Choi, S. K. and Sharma M. M., “A Rheological Model for pH-Sensitive ionic Polymer Solutions for Optimal Mobility Control Applications,” Paper SPE 96914 presented at 2005 Ann. Tech. Conf. and Exhibition, Dallas, Tex., Oct. 9-12, 2005. [0110] Sharma, M. M., Bryant S. and Huh, C., “pH Sensitive Polymers for Improving Reservoir Sweep and Conformance,” U.S. Department of Energy, National Energy Technology Laboratory, Semiannual Progress Report May 1, 2006 to Sep. 30, 2006. [0111] Raje, M., Asghari, K., Vossoughi S., Green, D. W. and Willhite, G. P “Gel Systems for Controlling CO.sub.2 Mobility in Carbon Dioxide Miscible Flooding,” SPE Reservoir Eval. & Eng. 2 (2), April. 1999, pp 205-210. [0112] Raje, M., Asghari, K., Vossoughi S. Green, D. W. and Willhite, G. P., “Gel Systems for Controlling CO.sub.2 Mobility in Carbon Dioxide Miscible Flooding,” Paper SPE/DOE 35379, presented at the 1988 SPE/DOE Tenth Symposium on Improved Oil Recovery, Tulsa, Okla. Apr. 21-24, 1988.