Copolymers useful for gelling acids

1. A method for fracturing a subterranean formation, the subterranean formation being in fluid communication with the surface through a well bore, comprising:

(a) creating a fracture in a subterranean formation; and

(b) injecting into the fracture an etching agent, wherein the etching agent includes a gelled acid comprising an acid gelled using a copolymer having a backbone, the organic component of the gelled acid having the general formula:

wherein:

(a) A is an H or other terminating group;

(b) R 1 is an OH or NH 2 ;

(c) R 2 is an O or NH;

(d) Z is an integer having a value of from 1 to 4;

(e) X and Y are present in a ratio (X:Y) of from 3:2 to 4:1;

(f) structures I and II are present as blocks or randomly distributed along the copolymer backbone;

(g) D − is an anion of a mineral acid; and

wherein the copolymer has a molecular weight of from about 1,000,000 to about 10,000,000.

2. The method of claim 1 wherein, in the general formula for the copolymer, R 1 is NH 2 .

3. The method of claim 1 wherein, in the general formula for the copolymer, Z is an integer having a value of from 2 to 3.

4. The method of claim 3 wherein, in the general formula for the copolymer, Z is an integer having a value of 2.

5. The method of claim 4 wherein the copolymer has a molecular weight of from about 1,000,000 to about 6,000,000.

6. The method of claim 1 wherein the mineral acid is selected from the group consisting of sulfuric, nitric, hydrochloric, and phosphoric acid.

7. The method of claim 6 wherein the mineral acid is selected from the group consisting of sulfuric and hydrochloric acid.

8. The method of claim 1 wherein the etching agent includes an additive selected from the group consisting of emulsifiers, chelators, surfactants, proppants, delay additives, biocides, corrosion inhibitors, and mixtures thereof.

9. The method of claim 1 wherein the etching agent includes a proppant.

10. The method of claim 1 wherein the copolymer is prepared using a formulation comprising:

(a) a first vinyl component selected from the group consisting of acrylamide, acrylic acid, and mixtures thereof; and

(b) a second vinyl component selected from dimethylaminoethyl methacrylate, dimethylaminoethyl methacrylamide, dimethylaminopropyl methacrylamide, and mixtures thereof;

and additionally comprising a crosslinking agent.

11. The method of claim 10 wherein the crosslinking agent is bis-acrylamide.

12. The method of claim 11 wherein the bis-acrylamide is present in a concentration of less than about 250 parts per million.

13. The method of claim 11 wherein the bis-acrylamide is present in a concentration of less than about 200 parts per million.

14. The method of claim 11 wherein the bis-acrylamide is present in a concentration of less than about 100 parts per million.