NANOGELS FOR DELAYED GELATION

Abstract

The instant application relates to nanogels or compositions that hold multivalent metal ions until some level of nanogel degradation has occurred, then slowly release the multivalent metal ions for gelation with carboxylate containing polymers. Compositions comprising such nanogels, together with polymers that can be crosslinked with multivalent metal ions, allow the deployment of such mixtures in various applications, and greatly increased gelation times.

Claims

1) A degradable composition comprising a polymer having releasable carboxylate groups bound to with a multivalent metal ion, said degradable composition lasting at least 5 days at 85° C. in a brine solution having 23 g/l NaCl, and thereafter degrading and releasing said multivalent metal ion, wherein said composition is in the form of a nanogel.

2) The composition of claim 1, wherein said nanogel has an average particle size of less than one micron.

3) The composition of claim 1, wherein said nanogel has an average particle size of 200-600 nm.

4) The composition of claim 1, wherein said nanogel has an average particle size of 400 nm.

5) The composition of claim 1, wherein said polymer is made from monomers selected from the group of vinyl, allyl, styrene, and acrylamide monomers and their derivatives, conjugated with a dicarboxylate or tricarboxylate.

6) The composition of claim 5, wherein said dicarboxylate or tricarboxylate is citrate, succinate, aspartate, glutamate, malate, oxalate, malonate, glutarate, adipate, or pimelate, or a derivative thereof.

7) The composition of claim 1, wherein said polymer is carboxylated polysaccharide, carboxylated guar, or carboxymethyl cellulose.

8) The composition of claim 1, wherein said polymer having releasable carboxylate groups is a polymer or copolymer of succinate, aspartate, malate, oxalate, malonate, glutarate, adipate, or pimelate, carbonate or a derivative thereof.

9) The composition of claim 1, wherein said composition comprises polyvinyl alcohol (PVA) succinate, N-hydroxylmethyl acrylamide (NHMA) succinate, allyl alcohol succinate and allylamine succinate, PVA malate, NHMA malate, allyl alcohol malate or allylamine malate.

10) The composition of claim 1, wherein said multivalent metal ion is chromium, zirconium, iron, aluminum, and titanium.

11) The composition of claim 1, comprising PVA succinate and chromium or zirconium.

12) The composition of claim 1, comprising PVA malate and chromium or zirconium.

13) The composition of claim 1, comprising polyaspartate and chromium or zirconium.

14) The composition of claim 1, comprising polyglutamate and chromium or zirconium.

15) The composition of claim 1, wherein a carboxylate group to multivalent metal ion molar ratio is from 3:1 to 15:1.

16) The composition of claim 1, wherein the multivalent metal ion is present at 50-5000 ppm.

17) A degradable composition comprising polyvinyl alcohol (PVA) succinate or PVA malate complexed with multivalent metal ion comprising chromium, zirconium, iron, aluminum, titanium or combinations thereof, said degradable composition lasting at least 5 days at 85° C. in a brine solution having 23 g/l NaCl, and thereafter degrading and releasing said multivalent metal ion.

18) A degradable composition comprising polyasparate or polyglutamate complexed with multivalent metal ion comprising chromium, zirconium, iron, aluminum, titanium or combinations thereof, said degradable composition lasting at least 20 days at 88° C. in a brine solution having 23 g/l NaCl, and thereafter degrading and releasing said multivalent metal ion.

19) A degradable composition comprising polyvinyl alcohol (PVA) succinate or PVA malate or polyasparate complexed with multivalent metal ion comprising chromium, zirconium, iron, aluminum, titanium or combinations thereof, said degradable composition lasting at least 30 days at 65° C. in a brine solution having 23 g/l NaCl, and thereafter degrading said nanogel and releasing said multivalent metal ion.

20) A degradable composition comprising polyvinyl alcohol (PVA) succinate or PVA malate complexed with multivalent metal ion comprising chromium, zirconium, iron, aluminum, titanium or combinations thereof.

21) A degradable composition comprising polyasparate or polyglutamate complexed with multivalent metal ion comprising chromium, zirconium, iron, aluminum, titanium or combinations thereof.

22) A degradable composition comprising polyvinyl alcohol (PVA) succinate or PVA malate or polyasparate complexed with multivalent metal ion comprising chromium, zirconium, iron, aluminum, titanium or combinations thereof.

23) A delayed gelling composition comprising the composition of claims 1 admixed with an injection fluid admixed with a carboxylate containing polymer.

24) The delayed gelling composition of claim 22, said carboxylate containing polymer comprising partially hydrolyzed polyacrylamide, copolymers of N-vinyl-2-pyrrolidone and sodium acrylate, tetrapolymers of sodium-2-acrylamido-2-methylpropanesulfonate, acrylamide and N-vinyl-2-pyrrolidone and sodium acrylate; and copolymers of sodium-2-acrylamido-2-methylpropanesulfonate and sodium acrylate; carboxylated polysaccharide; carboxymethylcellulose; carboxylated guar; and combinations thereof.

25) An improved method of sweeping for oil or gas, said method requiring blocking thief zones with a polymer, and sweeping a reservoir for oil or gas, the improvement comprising injecting the composition of claim 23 into a reservoir, aging said composition until the viscosity increases, and sweeping the reservoir for oil or gas.

26) An improved method of producing oil or gas, said method requiring injecting a polymer into a reservoir and producing an oil or gas, the improvement comprising injecting the composition of claim 23 into a reservoir, aging said composition until the viscosity increases, and producing said oil or gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] FIG. 1 depicts the synthesis of PVA succinate.

[0048] FIG. 2 shows the basic concept of multivalent metal ion-loaded nanogel for gelation.

[0049] FIG. 3 shows synthesis of NHMA succinate, allyl alcohol succinate and allylamine succinate.

[0050] FIG. 4 displays examples of temporary carboxyl groups, including polycarbonate-containing carboxyl groups, polyaspartate and polyglutamate, that could be used to make degradable nanogels pursuant to this disclosure.

[0051] FIG. 5 shows the gelation of 0.5% HPAM in Brine A crosslinked with 100 ppm Cr(III) as Cr-nanogel-27, taking 12 days of aging to begin gelling at 85° C. This polymer would set to a gel within a few hours at 85° C. if the crosslinker was only Cr(III)-acetate. Using Cr-nanogel-28 and Cr-nanogel-30, gelation occurred at 10 days at 85° C. Therefore, the highest ratio of COOH/Cr (Cr-Nanogel-27) took the longest time to gel. This is because more COOH groups bind the Cr ions stronger, making them less available for crosslinking HPAM.

[0052] FIG. 6 shows the gelation of 0.5% HPAM in Brine A crosslinked with Cr-nanogel-27 (PVAS-25-6:1) and Cr-nanogel-29 (PVAS-6-6:1) containing 100 ppm Cr(III) aged at 85° C. As this plot shows, it took over 10 days of aging at 85° C. for this system to begin the gelation process. The two nanogels are made with different molecular weight polymers, but are somewhat different based on the vinyl acetate content as well, making it difficult to provide conclusive statements. However, the two polymers had the similar delays, indicating that the molecular weight of the polymer used to make the nanogel had little effect, at least at these conditions.

[0053] FIG. 7 shows the gelation results for a solution of 0.5% HPAM in Brine A crosslinked with 100 ppm Cr(III) in the form of Cr-nanogel-32 (PVAS-6-6:1 made by inverse emulsion) described below. This gelant began to gel in about 10 days of aging at 85° C. The same gelant solution began to gel after 10 weeks of aging at 65° C. Typically reaction rate double for every 10° C. rise in temperature. Thus, 85° C. is expected be about four times faster than 65° C.

[0054] FIG. 8 shows the viscosity versus aging time for gelation of 0.5% HPAM in Brine A exposed to 100 ppm Cr in the form of Cr-nanogel-33 (PVAS-25-6:1, made by inverse emulsion) aged at 65° C. and 85° C. This gelant required about 65 days of aging at 65° C. and about 5 days at 85° C. to begin gelling. In this instance, the Cr-nanogel-33 was made in NaOH solution, which may have affected gelation time.

[0055] FIG. 9 summarizes gelation tests results for 0.5% B29 polymeric microparticle in Brine A exposed to 100 ppm Cr(III) in the form of Cr-nanogel-32 (PVAS-6-6:1 inverse emulsion) aged at 65° C. and 85° C. While the gelant aged at 65° C. took over 7 weeks of aging to begin gelling, the gelant aged at 85° C. began to gel in about 5 days of aging.

[0056] FIG. 10 shows viscosity versus aging time for 0.5% B29 polymeric microparticle in Brine A exposed to 100 ppm Cr(III) in the form of Cr-nanogel-33 (PVAS-25-6:1, inverse emulsion) aged at 65° C. and 85° C. While the gelant aged at 85° C. began to gel in about 5 days of aging, the gelant aged at 65° C. took over 5 weeks of aging to exhibit a substantial increase in viscosity.

[0057] FIG. 11 shows viscosity versus aging time for 0.5% HPAM with Zr-nanogel-43 [Zr(IV) concentration of 120 ppm] in Synthetic Brine A without NaHCO.sub.3 at 65 and 88° C. While the PVAS-6-6:1 gelant aged at 65° C. took over around three weeks of aging to begin gelling, the gelant aged at 85° C. began to gel in about 2-5 days of aging.

[0058] FIG. 12 shows viscosity versus aging time for 0.5% HPAM with 100 ppm Cr(III) as Cr-PAsp nanogel-2 in Synthetic Brine B at 88° C. and 106° C. While the gelant aged at 88° C. took over around 34 days of aging to begin gelling the gelant aged at 106° C. began to gel in about 10 days of aging.

[0059] FIG. 13 shows viscosity versus aging time for 0.5% B29 with 100 ppm Cr(III) as Cr-PAsp nanogel-2 in Synthetic Brine B at 88 and 106° C. While the gelant aged at 88° C. took over around 28 days of aging to begin gelling the gelant aged at 106° C. began to gel in about 8 days of aging.

[0060] FIG. 14 shows viscosity versus aging time for 0.5% B29 with 100 ppm Cr(III) as CrCl.sub.3-PVAS in Synthetic Brine A at 65 and 85 ° C. While the gelant aged at 65° C. took over around 62 days of aging to begin gelling, the gelant aged at 85° C. began to gel in about 9 days of aging.

[0061] FIG. 15 shows viscosity versus aging time for 0.5% B29 with 100 ppm Cr(III) as CrCl.sub.3-PAsp (CrCl.sub.3-PAsp-1 and CrCl.sub.3-PAsp-2 are the same formulation, used to prove the reproducibility of gelation delay) in Synthetic Brine A at 100 and 120° C. While the gelant aged at 120° C. took about 1 day of aging to begin gelling the gelant aged at 100° C. began to gel in about 3-4 days of aging.

[0062] FIG. 16 shows viscosity versus aging time for 0.5% B29 with 100 ppm Cr(III) as CrCl.sub.3-PAsp (CrCl.sub.3-PAsp-1 and CrCl.sub.3-PAsp-2 are the same formulation, used to prove the reproducibility of gelation delay) in Synthetic Brine A at 85° C. The gelant took over around 39 days of aging to begin gelling aged at 85° C.

[0063] FIG. 17 shows viscosity versus aging time for 0.5% HPAM with 100 ppm Cr(III) as CrCl.sub.3-PAsp (CrCl.sub.3-PAsp-1 and CrCl.sub.3-PAsp-2 are the same formulation, used to prove the reproducibility of gelation delay) in Synthetic Brine A at 100 and 120° C. While the gelant aged at 120° C. took about 1 days of aging to begin gelling the gelant aged at 100° C. began to gel in about 2-3 days of aging.

[0064] FIG. 18 shows viscosity versus aging time for 0.5% HPAM with 100 ppm Cr(III) as CrCl.sub.3-PAsp (CrCl.sub.3-PAsp-1 and CrCl.sub.3-PAsp-2 are the same formulation, used to prove the reproducibility of gelation delay) in Synthetic Brine A at 85° C. The gelant took over around 30 days of aging to begin gelling aged at 85 ° C.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0065] The disclosure provides novel compositions and methods, including any one or more of the following embodiments:

[0066] A degradable composition is provided, comprising a polymer having labile or releasable carboxylate groups complexed with a multivalent metal ion, said degradable composition lasting at least 5 days at 85° C. in a brine solution having 23 g/l NaCl, and thereafter degrading said composition and releasing said multivalent metal ion. Degradation is thus to be ascertained by release of the multivalent metal ion, which is ascertained by a second polymer gelling, as shown in the viscosity increase experiments described herein.

[0067] In preferred embodiments, the composition is a nanogel, but this is not an absolute requirement. Nanogels include particles of less than one micron, preferably 200-500, and most preferably 400 nm.

[0068] The polymer used to form nanogel can be made from monomers selected from the group of vinyl, allyl, styrene, and acrylamide monomers and their derivatives, or any polysaccharide, conjugated with a dicarboxylate or having naturally appended carboxylate groups. Any dicarboxylate (or tricarboxylate) can be used, including citrate, succinate, aspartate, glutamate, malate, oxalate, malonate, glutarate, adipate, pimelate, and the like, or a derivative thereof.

[0069] In some embodiments, the degradable nanogel having carboxylate groups is a polymer or copolymer of citrate, succinate, aspartate, glutamate, malate, oxalate, malonate, glutarate, adipate, pimelate, carbonate, and the like, or derivatives thereof.

[0070] In some preferred embodiments, the nanogel comprises polyvinyl alcohol (PVA) succinate, N-hydroxylmethyl acrylamide (NHMA) succinate, allyl alcohol succinate and allylamine succinate, PVA malate, NHMA malate, allyl alcohol malate or allylamine malate. In other embodiments, the polymer is polyaspartate or polyglutamate, or the like.

[0071] The multivalent metal ion is any such ion whose presentation needs be delayed, and for reservoir use for tertiary crosslinking includes chromium, zirconium, iron, aluminum, and titanium.

[0072] In some embodiments, the nanogel is PVA succinate and chromium or zirconium. In others, it is PVA malate and chromium or zirconium. In yet others, it is polyaspartate and chromium or zirconium, or polyglutamate and chromium or zirconium. Other exemplary nanogels are selected from Table 1.

[0073] Preferably, the carboxylate group to multivalent metal ion molar ratio is increased to delay release of the multivalent cation, and preferred embodiments include ratios from 3:1 to 15:1, or even 20:1. A 6:1 ratio was useful for the delays shown herein, but higher ratios may be preferred for hotter reservoirs.

[0074] The multivalent metal ion is present at amounts needed for the application, but in reservoir applications a lower amount is preferred as being more cost effective. Amounts thus range from 10-10,000 ppm or 50-5000 ppm, or about 1-200 ppm, such ppm given as the final weight/weight basis of the complete injection fluid.

[0075] Other embodiments provide a degradable nanogel comprising PVA succinate or PVA malate or polyasparate complexed with multivalent metal ion comprising chromium, zirconium, iron, aluminum, titanium or combinations thereof, said degradable nanogel lasting at least 5 days to 10 days at 85° C. in a brine solution having 23 g/1 NaCl, and thereafter degrading said nanogel and releasing said multivalent metal ion. Other degradable nanogels last at least 10 days at 85° C., and/or at least 30 days at 65° C.

[0076] Delayed gelling compositions are also provided, comprising any degradable nanogel herein described, admixed with an injection fluid admixed with a carboxylate containing polymer.

[0077] Any carboxylate containing polymer can be used in the injection fluid, provided such polymer can be crosslinked with the metal ion in the nanogel. Such polymers include, e.g., partially hydrolyzed polyacrylamide, copolymers of N-vinyl-2-pyrrolidone and sodium acrylate, tetrapolymers of sodium-2-acrylamido-2-methylpropanesulfonate, acrylamide and N-vinyl-2-pyrrolidone and sodium acrylate; and copolymers of sodium-2-acrylamido-2-methylpropanesulfonate and sodium acrylate; and combinations thereof.

[0078] Improved methods of sweeping for oil are also provided. In one embodiment, wherein prior methods required blocking thief zones with a polymer, and sweeping a reservoir for oil, the improved method comprising injecting any delayed gelling composition herein described into a reservoir, aging said composition until the viscosity increases, and sweeping the reservoir for oil.

[0079] Improved methods of producing oil or gas are also provided, prior methods requiring injecting a polymer into a reservoir and producing oil or gas, the improved methods comprising injecting any of the delayed gelling compositions herein described into a reservoir, aging said composition until the viscosity increases, and producing said oil or gas.

[0080] The following experiments were performed to synthesize multivalent metal ion loaded degradable nanogels for use as delayed crosslinking agents to produce gels with anionic polymers deep into oil-bearing formations.

[0081] PVA succinate, 6 k. A representative poly(vinyl alcohol succinate), herein referred to as PVA succinate, 6 kDa was prepared through the reaction of poly(vinyl alcohol, Mw 6 k, 80 mol % degree of hydrolysis), (PVA-6) and succinic anhydride using triethylamine (TEA) as catalyst in N-methyl-2-pyrrolidone (NMP) as solvent. First, 10 g PVA-6 was dissolved in 120 g NMP at 80° C. while stirring. Second, the solution was maintained at 60° C. reaction temperature, and 15 g TEA and 15 g succinic anhydride in 40 g NMP were added while stirring. After 22 hours at 60° C., PVA succinate 6 k (PVAS-6) was purified by precipitation in ether and dried under vacuum. FIG. 1 shows the chemical composition of PVAS.

[0082] PVA succinate 25K. A representative poly(vinyl alcohol succinate), herein referred to as PVA succinate, 25 kDa was prepared through the reaction of poly(vinyl alcohol, Mw 25 k, 88 mol % degree of hydrolysis) (PVA-25) and succinic anhydride using TEA as catalyst in NMP as solvent. First, 10 g PVA-25 was dissolved in 135 g NMP at 80° C. while stirring. Second, the solution was maintained at 60° C. reaction temperature, and 18 g TEA and 18 g succinic anhydride in 45 g NMP were added while stirring. After 22 hours at 60° C., PVAS-25 was purified by precipitation in ether and dried under vacuum.

[0083] Cr-Nanogel-27, 28 and 30 with PVAS-25 and CrAc. A representative Cr(III)-loaded nanogel herein referred to as Cr-nanogel-27 was prepared through mixing PVAS-25 with Cr(III) as CrAc in Reverse Osmosis (RO) water while stirring. 135 mg PVAS-25 was dissolved in 4.84 g RO water and 29 mg Cr-Acetate was added into the above solution while stirring. The carboxyl groups/Cr (III) molar ratio is 6:1. Cr(III) loading in Cr-nanogel-27 was around 1500 ppm. Cr-nanogel-28 and Cr-nanogel-30 having lower carboxyl groups/Cr (III) molar ratio were prepared using the same procedure. Detailed information regarding these nanogels is listed in Table 1.

[0084] Cr-Nanogel-29 with PVAS-6 and CrAc. A representative Cr(III)-loaded nanogel herein referred to as Cr-nanogel-29 was prepared through mixing PVAS-6 with Cr(III) as CrAc in RO water while stirring. 143 mg PVAS-6 was dissolved in 4.83 g RO water and 29 mg CrAc was added into the above solution while stirring. The carboxyl groups/Cr(III) molar ratio was 6:1. Cr(III) loading in Cr-nanogel-29 is around 1500 ppm. Detailed information regarding Cr-nanogel-29 is listed in Table 1.

[0085] Cr-Nanogel-31 with PVAS-6 and CrAc. Cr-Nanoge1-31 was made of PVAS-6 and Cr-acetate in Synthetic Brine A for the compatibility test with brine. Cr-Nanoge1-31 looked homogeneous and its gelation delay with HPAM was similar to other nanogels (data not shown), but a lot of bubbles appeared during dissolving PVA succinate in Brine A due to CO.sub.2 release resulting from reaction of PVA succinate carboxyl groups with NaHCO.sub.3 in Brine A.

[0086] Cr-nanogel-32 with PVAS-6 and CrAc by inverse-emulsion. A representative Cr(III)-loaded nanogel herein referred to as Cr-nanogel-32 was prepared using PVAS-6 and CrAc by inverse-emulsion in order to prepare small size particles. In such process, an aqueous mixture containing 794 mg PVAS-6, 158 mg CrAc and 6.0 g RO water as the dispersed phase and an oil mixture of 3.5 g kerosene, 557 mg Span 83 and 313 mg polyoxyethylene sorbitol hexaoleate (PSH) as a continuous phase were prepared. The inverse-emulsion was prepared by mixing the aqueous phase and the oil phase, followed by rapid homogenization using a sonicator. The carboxyl groups/ Cr(III) molar ratio was 6:1. Cr(III) loading in Cr-nanogel-32 was around 3600 ppm. The mean particle size, measured in RO water by dynamic light scattering experiments employing a ZetaPALS zeta potential analyzer (Brookhaven Instruments Corp.), was around 400 nm. Detailed information regarding Cr-nanogel-32 is listed in Table 1.

[0087] Cr-Nanogel-33 with PVA succinate, 25 k and CrAc by inverse-emulsion. A representative Cr(III)-loaded nanogel herein referred to as Cr-nanogel-33 was prepared using PVAS-25 and CrAc by inverse-emulsion. In order to increase the solubility and ionization degree of PVA succinate, the partial carboxyl groups of PVA succinate were transformed to sodium carboxylate. In such process, an aqueous mixture containing 368 mg PVAS-25, 62 mg NaOH, 79 mg CrAc and 3.0 g RO water as the dispersed phase and an oil mixture of 1.7 g kerosene, 279 mg Span 83 and 157 mg PSH as continuous phase were prepared.

[0088] The inverse-emulsion was prepared by mixing the aqueous phase and the oil phase, followed by rapid homogenization using a sonicator. The carboxyl groups/Cr(III) molar ratio was 6:1. Cr(III) loading in Cr-nanogel-33 was around 3600 ppm. The mean particle size was around 400 nm. Detailed information regarding Cr-nanogel-33 is listed in Table 1.

[0089] Zr-Nanogel-43 with PVA succinate, 6 k and ZrLa. In order to compare chromium ions against zirconium ions, a representative Zr(IV)-loaded nanogel herein referred to as Zr-nanogel-43 was prepared through mixing PVAS-6 with Zr(IV) as Zr-lactate (ZrLa) in RO water while stirring. 328 mg PVAS-6 was dissolved in 3.9 g RO water and 2.0 g NaOH solution and its pH was adjusted to 6.11, and 550 mg ZrLa (5.5% Zr(IV)] was added into the above solution while stirring. The carboxyl groups/Zr(IV) molar ratio was 6:1. Zr(IV) loading in Zr-nanogel-43 was 4463 ppm. Detailed information regarding Zr-nanogel-43 is listed in Table 1.

[0090] Cr-PAsp Nanogel-2 with PolyAspartic acid (PAsp) (Mw =4-6 k) and CrAc by inverse-emulsion. In order to test nanogels based on other sources of carboxylate ions, we made a nanogel with polyaspartate (PAsp) in place of PVAS. A representative Cr(III)-loaded PAsp nanogel herein referred to as Cr-PAsp nanogel-2 was prepared using PAsp with Cr(III) as CrAc by inverse-emulsion. In such process, an aqueous mixture containing 921 mg PAsp, 232 mg CrAc and 4.6 g NaOH solution as the dispersed phase and an oil mixture of 2.41 g kerosene, 385 mg Span 83 and 217 mg PSH as a continuous phase were prepared. The inverse-emulsion was prepared by mixing the aqueous phase and the oil phase, followed by rapid homogenization using a sonicator. The carboxyl groups/Cr(III) molar ratio was 7:1. Cr(III) loading in Cr-PAsp nanogel-2 was around 6837 ppm. The mean particle size was around 400 nm. Detailed information regarding Cr-PAsp nanogel-2 is listed in Table 1.

[0091] Several gelation tests were performed on the various nanogels made herein to demonstrate the suitability of nanogels containing multivalent cations as crosslinking agents with delayed gelation times. The following examples show slower gelation rates with these crosslinkers compared with multivalent cation complexes typically used in gelation of partially hydrolyzed polyacrylamides.

[0092] Gelation of Cr-Nanogel-27, -28 and -30 with HPAM. In an oxygen-free glove box, 12.50 g of 2% HPAM solutions were added into 34.17 g of deoxygenated Synthetic Brine A in a beaker with stirring. Then 3.33 g of Cr-nanogel-27, Cr-Nanogel-28 or Cr-Nanogel-30 was added into the above mixture under stirring (final Cr(III) concentration was 100 ppm, final HPAM concentration was 0.5%). Finally the initial viscosity was recorded.

[0093] A Brookfield Digital Viscometer Model LVDV-II+PCP was used to monitor the viscosity changes of gelant and control solutions and determine the gel time of the gelant solutions. The gelation process was monitored as a function of time starting from the point of visual homogeneous dispersion. The gelation time was defined as the time when the viscosity of the gel solution increases abruptly to a value greater than 1000 cP (100% scales) at a shear rate of 2.25 s.sup.−1. The temperature of the viscometer was controlled at the stated temperatures during the measurements.

[0094] The composition of Synthetic Brine A used in gelation experiments is listed in Table 2. A second Brine B composition used in later experiments is listed in Table 3. The various solutions were then divided into 6 ml vials and incubated at the indicated temperature(s). The viscosities of the samples were monitored as a function of aging time.

TABLE-US-00003 TABLE 2 Composition of Synthetic Brine A Component Concentration, g/kg NaCl 22.982 KCl 0.151 CaCl.sub.2•2H.sub.2O 0.253 MgCl.sub.2•6H.sub.2O 1.071 NaHCO.sub.3 2.706 Na.sub.2SO.sub.4 0.145 Water To 1000 g pH 8

TABLE-US-00004 TABLE 3 Composition of Synthetic Brine B Component Concentration, g/kg NaCl 18.420 KCl 0.424 CaCl.sub.2•2H.sub.2O 0.550 MgCl.sub.2•6H.sub.2O 0.586 SrCl.sub.2•6H.sub.2O 0.061 NaHCO.sub.3 3.167 Na.sub.2SO.sub.4 0.163 Water To 1000 g pH 8

[0095] The results are shown in FIG. 5. As this figure shows, the delayed release of Cr(III) gelation agent from Cr-nanogel-27, -28 and -30 produced gels with HPAM is at a much slower rate than the prior art complexed multivalent cations used alone to gel HPAM.

[0096] Additionally, the highest carboxyl/Cr(III) ratio (6:1) held the Cr(III) tighter and gelled slower with HPAM. The other two ratios of 4.5 and 3 thus probably release Cr(III) easier, allowing more rapid gelation with HPAM. Thus, one way the gel time can be increased is by increasing the number of carboxylate groups in the nanogel.

[0097] Gelation of Cr-Nanogel-29 with HPAM. In an oxygen-free glove box, 12.50 g of 2% HPAM solution was added into 34.17 g of deoxygenated Synthetic Brine A in a beaker with stirring. Then 3.33 g of Cr-nanogel-29 containing CrAc and PVA succinate 6 k was added into the above mixture under stirring (final Cr(III) concentration was 100 ppm, final HPAM concentration was 0.5%). Finally, the initial viscosity was recorded. The solution was then divided into 6 ml vials and incubated at 85° C. The viscosities of the samples were monitored as a function of aging time.

[0098] The results are shown in FIG. 6, which compares Cr-nanogel-29 (PVAS-6-6:1) and Cr-nanogel-27 (PVAS-25-6:1). As this figure shows, the delayed release Cr(III) gelation agent forms gels with HPAM at a much slower rate than the prior art complexed multivalent cations used alone to gel HPAM, which took only hours. However, the two nanogels made with different molecular weight PVAS took about the same time to gel, indicating that the molecular weight of the polymer used to make the nanogel is not a significant factor in delay time, at least under these conditions.

[0099] Gelation of Cr-Nanogel-32 with HPAM. In an oxygen-free glove box, 2.08 g 30% inverting surfactant was dissolved in 106.27 g of deoxygenated Synthetic Brine A in a beaker with stirring. Then 4.15 g Cr-Nanogel-32 and 37.50 g of 2% HPAM were added into the above mixture under stirring (final Cr(III) concentration was 100 ppm, final HPAM concentration was 0.5%). Finally the initial viscosity was recorded. The solution was then divided into 6 ml vials and incubated at 65 and 85° C. The viscosities of the samples were monitored as a function of aging time. The results are shown in FIG. 7. As this figure shows, the lower temperature helped to greatly delay gel times for the Cr-Nanogel-32 (PVAS-6-6:1, 400 nm) from 10 days at 85° C. to about 80 days at 65° C.

[0100] Gelation of Cr-Nanogel-33 with HPAM. In an oxygen-free glove box, 1.73 g 30% inverting surfactant was dissolved in 88.56 g of deoxygenated Synthetic Brine A in a beaker with stirring. 3.46 g Cr-nanogel-33 25 k and 31.25 g of 2% HPAM were added into the above mixture under stirring (final Cr(III) concentration was 100 ppm, final HPAM concentration was 0.5%), and then the initial viscosity was recorded. The solution was then divided into 6 ml vials and incubated at 65 and 85° C. The viscosities of the samples were monitored as a function of aging time. The results are shown in FIG. 8. The lower temperature delayed gel time, from 5 days at 85° C. to 65 days at 65° C. using this Cr-nanogel-33 (PVAS-25-6:1, 400 nm).

[0101] Gelation of Cr-Nanogel-32 with B29. We also sought to confirm that the delayed gelling effect was general, not limited to HPAM polymers. B29 is an expandable microparticle made in part with labile crosslinkers and with stable crosslinkers. The degree of polymerization is quite high, resulting in a very small microparticle that can be easily pumped and penetrate the fine pores of the reservoir. Once there, the high temperature and/or pH results in loss of the labile crosslinker bonds and the remaining polymer absorbs water, swelling greatly in situ. While viscous, these polymers are still subject to washout, and thus further crosslinking in situ is desirable. We therefore sought to determine if our delayed gelling agents could also be used with such microparticles.

[0102] In an oxygen-free glove box, 2.22 g 30% inverting surfactant was dissolved in 93.34 g of deoxygenated Synthetic Brine A in a beaker with stirring. Then 2.77 g Cr-nanogel-32 and 1.67 g 30% B29 were added into the above mixture under stirring (final Cr(III) concentration was 100 ppm, final B29 concentration was 0.5%) and finally the initial viscosity was recorded. The solution was then divided into 6 ml vials and incubated at 65 and 85° C. The viscosities of the samples were monitored as a function of time.

[0103] The results are shown in FIG. 9. The delayed release of Cr(III) from Cr-nanogel-32 (PVAS-6-6:1-400 nm) and slow popping of B-29 polymeric microparticles releasing HPAM results in slower gel formation. Delay ranged from 7 days at 85° C. to 80 days at 65° C. B29 is largely the same as HPAM once it is popped, but its degree of hydrolysis is a bit lower (5%), thus it gels a little slower.

[0104] Gelation of Cr-Nanogel-33 with B29. In an oxygen-free glove box, 2.23 g 30% inverting surfactant was dissolved in 93.32 g of deoxygenated Synthetic Brine A in a beaker with stirring, and then 2.78 g Cr-nanogel-33 and 1.67 g of 30% B29 were added into the above mixture under stirring (final Cr(III) concentration was 100 ppm, final B29 concentration was 0.5%). Finally the initial viscosity was recorded. The solution was then divided into 6 ml vials and incubated at 65 and 85° C. The viscosities of the samples were monitored as a function of aging time and results are shown in FIG. 10. As shown, the delay times for Cr-nanogel-33 (PVAS-25-6:1, 400 nm) were somewhat reduced as compared with Cr-nanogel-32 (PVAS-6-6:1-400 nm) from about 5 days at 85° C. to about 35 days at 65° C. While preparing Cr-Nanogel-33, we dissolved PVAS-25 in NaOH solution, because it was difficult to dissolve it in water, before adding the CrAc. Thus, the NaOH probably accelerated Cr release from the nanogel.

[0105] Gelation of Zr-Nanogel-43 with HPAM. In an oxygen-free glove box, 25 g of 2% HPAM solutions were added into 72.29 g of deoxygenated Synthetic Brine A without NaHCO.sub.3 in a beaker with stirring. Then 2.71 g of Zr-nanogel-43 containing ZrLa and PVAS-6-6:1 was added into the above mixture under stirring (final Zr(IV) concentration was 120 ppm, final HPAM concentration was 0.5%), and the initial viscosity recorded. The solution was then divided into 6 ml vials and incubated at 85° C. The viscosities of the samples were monitored as a function of aging time. The results are shown in FIG. 11. As this figure shows, the delayed release of Zr(IV) results in gel formation with HPAM at a much slower rate than the prior art complexed multivalent cations used alone to gel HPAM. The lower temperature helped to greatly delay gel times for the Zr-Nanogel-43 from 2 to ˜5 days at 88° C. to around three weeks at 65° C.

[0106] Gelation of Cr-PAsp Nanogel-2 with HPAM. In an oxygen-free glove box, 50 g of 1% HPAM solutions in Synthetic Brine B were added into 47.81 g of deoxygenated Synthetic Brine B with 0.73 g 30% inverting surfactant in a beaker with stirring. Then 1.46 g of Cr-PAsp nanogel-2 containing CrAc and PAsp was added into the above mixture under stirring (final Cr(III) concentration was 100 ppm, final HPAM concentration was 0.5%), and the initial viscosity recorded. The solution was then divided into 6 ml vials and incubated at 88° C. The viscosities of the samples were monitored as a function of aging time. The results are shown in FIG. 12. As shows, the delayed release of Cr(III) results in gel formation with HPAM at a much slower rate than the prior art complexed multivalent cations used alone to gel HPAM. The lower temperature helped to greatly delay gel times for the Cr-PAsp nanogel-2 from 10 days at 106° C. to 34 days at 88° C. Also, Cr-PAsp nanogel-2 with HPAM had much longer gelation delay than all PVA succinate nanogels, because PAsp hydrolyzed much slower than PVA succinate, probably due to greater stability of the amide bonds over ester bonds. Based on these results, we predict that polyglutamate, and other polymers having pendant carboxylates and amide bonds should produce a long gel delay time.

[0107] Gelation of Cr-PAsp Nanogel-2 with B29. In an oxygen-free glove box, 1.67 g of 30% B29 were added into 95.3 g of deoxygenated Synthetic Brine B with 1.57 g 30% inverting surfactant in a beaker with stirring. Then 1.46 g of Cr-PAsp nanogel-2 was added into the above mixture under stirring (final Cr(III) concentration was 100 ppm, final B29 concentration was 0.5%), and the initial viscosity recorded. The solution was then divided into 6 ml vials and incubated at 88° C. or 106° C., viscosities were monitored and results are shown in FIG. 13. The delayed release of Cr(III) results in gel formation with HPAM at a much slower rate than the prior art complexed multivalent cations used alone to gel HPAM. The lower temperature helped to greatly delay gel times for the Cr-PAsp nanogel-2 from 8 days at 106° C. to 28 days at 88° C. Also, Cr-PAsp nanogel-2 with B29 had much longer gelation delay than all PVA succinate nanogels, because PAsp hydrolyzed much slower than PVA succinate, and this effect is independent of the injection polymer used.

[0108] Preparation of CrCl.sub.3-PVAS and CrCl.sub.3-PAsp nanogels using CrCl.sub.3 as Cr(III) source. A representative Cr(III)-loaded PVAS nanogel herein referred to as CrCl.sub.3-PVAS was prepared through mixing PVAS-6 with Cr(III) as CrCl.sub.3 in Reverse Osmosis (RO) water while stirring. 573 mg PVAS-6 was dissolved in 11.44 g RO water with 0.60 g of 10.19% NaOH and 3.42 g Cr(III) solution (8761 ppm Cr(III)) was added into the above solution while stirring. The carboxyl groups/Cr(III) molar ratio is 6:1. Cr(III) loading in CrCl.sub.3-PVAS was around 1869 ppm.

[0109] CrCl.sub.3-PAsp nanogel was prepared using the same procedure. A representative Cr(III)-loaded PAsp nanogel herein referred to as CrCl.sub.3-PAsp was prepared through mixing PAsp with Cr(III) as CrCl.sub.3 in Reverse Osmosis (RO) water while stirring. 653 mg PAsp was dissolved in 7.88 g RO water with 3.53 g of 10.19% NaOH and after pH was adjusted to 7.63 by addition of 2.57 g 1 N HCl, 5.45 g Cr(III) solution (9036 ppm Cr(III)) was added into the above solution while stirring. The carboxyl groups/Cr(III) molar ratio is 6:1. Cr(III) loading in CrCl.sub.3-PAsp was around 2452 ppm.

[0110] Gelation of CrCl.sub.3-PVAS with B29. In an oxygen-free glove box, 0.83 g 30% inverting surfactant was dissolved in 92.15 g of deoxygenated Synthetic Brine A in a beaker with stirring. 5.35 g CrCl.sub.3-PVAS and 1.67 g of 30% B29 were added into the above mixture while stirring (final Cr(III) concentration was 100 ppm, final B29 concentration was 0.5%) and initial viscosity recorded. The solution was then divided into 6 ml vials and incubated at 65 and 85° C., and viscosities monitored as a function of time. FIG. 14 shows viscosity versus aging time for 0.5% B29 with 100 ppm Cr(III) as CrCl.sub.3-PVAS in Synthetic Brine A at 65 and 85° C. While the gelant aged at 65° C. took over around 62 days of aging to begin gelling the gelant aged at 85° C. began to gel in about 9 days of aging. FIG. 15 shows viscosity versus aging time for 0.5% B29 with 100 ppm Cr(III) as CrCl.sub.3-PAsp (CrCl.sub.3-PAsp-1 and CrCl.sub.3-PAsp-2 are the same formulation, used to prove the reproducibility of gelation delay) in Synthetic Brine A at 100 and 120° C. While the gelant aged at 120° C. took about 1 day of aging to begin gelling the gelant aged at 100° C. began to gel in about 3-4 days of aging.

[0111] Gelation of CrCl.sub.3-PAsp with B29. In an oxygen-free glove box, 0.83 g 30% inverting surfactant was dissolved in 93.42 g of deoxygenated Synthetic Brine A in a beaker with stirring. Then, 4.08 g CrCl.sub.3-PAsp and 1.67 g 30% B29 were added into the above mixture while stirring (final Cr(III) concentration was 100 ppm, final B29 concentration was 0.5%) and initial viscosity was recorded. The solution was then divided into 6 ml vials and incubated at 85, 100 and 120° C. The viscosities of the samples were monitored as a function of time. FIG. 16 shows viscosity versus aging time for 0.5% B29 with 100 ppm Cr(III) as CrCl.sub.3-PAsp (CrCl.sub.3-PAsp-1 and CrCl.sub.3-PAsp-2 are the same formulation, used to prove the reproducibility of gelation delay) in Synthetic Brine A at 85° C. The gelant took over around 39 days of aging to begin gelling aged at 85° C.

[0112] Gelation of CrCl.sub.3-PAsp with HPAM. In an oxygen-free glove box, 100 g of 1% HPAM solution in Synthetic Brine A was added into 91.84 g of deoxygenated Synthetic Brine A in a beaker with stirring. Then 8.16 g of CrCl.sub.3-PAsp was added into the above mixture under stirring (final Cr(III) concentration was 100 ppm, final HPAM concentration was 0.5%) and initial viscosity was recorded. The solution was then divided into 6 ml vials and incubated at 100 and 120° C. The viscosities of the samples were monitored as a function of aging time. FIG. 17 shows viscosity versus aging time for 0.5% HPAM with 100 ppm Cr(III) as CrCl.sub.3-PAsp (CrCl.sub.3-PAsp-1 and CrCl.sub.3-PAsp-2 are the same when aged at the same temperature. These results indicate the reproducibility of gelation delay in Synthetic Brine A at 100 and 120° C. While the gelant aged at 120° C. took about 1 day of aging to begin gelling the gelant aged at 100° C. began to gel in about 2-3 days of aging. FIG. 18 shows viscosity versus aging time for 0.5% HPAM with 100 ppm Cr(III) as CrCl.sub.3-PAsp (CrCl.sub.3-PAsp-1 and CrCl.sub.3-PAsp-2 are the same formulation, used to prove the reproducibility of gelation delay) in Synthetic Brine A at 85° C. The gelant took over around 30 days of aging to begin gelling aged at 85° C.

[0113] The following references are incorporated by reference in their entirety for all purposes.

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