Low pH crosslinking of polymers
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.
1. A method of improving sweep efficiency of a CO 2 flood of a reservoir, said method comprising:
a) injecting a composition into said reservoir, said composition comprising a polymer with one or more acrylamide based monomers in the repeat unit, wherein one of said acrylamide based monomers has pendant hydroxyl groups, a fluid, and an optional phenolic crosslinker, wherein said polymer has a first viscosity at pH 8 and a second higher viscosity to form a gel when aged at a pH less than 5;
b) aging the composition to increase its viscosity or form a gel;
c) injecting a slug of CO 2 into said reservoir, wherein said CO 2 reduces a viscosity of oil; and producing said reduced viscosity oil,
wherein said CO 2 is co-injected with said composition.
2. The method of claim 1 , wherein said aging is at least 20 days.
3. The method of claim 1 , wherein said aging is at least 40 days.
4. The method of claim 1 , wherein the pH of the reservoir is acidic due to CO 2 injection.
5. The method of claim 1 , wherein the pH of the reservoir is acidic due to inclusion of an ester in said composition, wherein said ester produces acid upon hydrolysis to reduce the pH.
6. An improved method of CO 2 flooding, wherein CO 2 is injected into a reservoir to mobilize and produce oil, the improvement comprising injecting a composition into a reservoir, aging said composition to increase its viscosity or form a gel, injecting CO 2 into said reservoir thus reducing the viscosity of the oil, wherein said CO 2 is co-injected with said composition, and producing said oil, said composition comprising a polymer with one or more acrylamide based monomers in the repeat unit, wherein one of said acrylamide based monomers has pendant hydroxyl groups, a fluid, and an optional phenolic crosslinker, wherein said polymer has a first viscosity at pH 8 and a second higher viscosity to form a gel when aged at a pH less than 5.