Liquid breaker for acid fracturing fluids

1. A method of fracturing a zone of a subterranean carbonate formation penetrated by a well bore, comprising the steps of:

a) hydrating a polymer comprising acrylamide, an acid, and a liquid breaker comprising fluoroboric acid to produce a gelled acid;

b) adding a zirconium based crosslinking agent to crosslink the gelled acid to produce a crosslinked fluid of sufficient viscosity to facilitate fracturing of the formation; and

c) injecting the crosslinked fluid into the well bore and into contact with the formation under sufficient pressure to react the acid and the formation at conditions that decompose the fluoroboric acid to form fluoride ions that break zirconium—polymer bonding in the crosslinked fluid.

2. The method of claim 1 , wherein the fluoroboric acid is present in a range of about 10 ppm to about 150 ppm of the crosslinked fluid.

3. The method of claim 1 , wherein the acid comprises hydrochloric acid, formic acid, acetic acid, citric acid, or combinations thereof; the acrylamide comprises acrylamide, (meth)acrylamide, diacetone acrylamide, N-methylolacrylamide, a polyacrylamide derivative, a copolymer of acrylamide, a copolymer of (meth)acrylamide, a hydrophobically- modified polyacrylic acid/acrylate copolymer, or combinations thereof; and the crosslinking agent comprises zirconium lactate, zirconium acetate, zirconium lactate with alkanolamine, and combinations thereof.

4. The method of claim 1 , wherein the crosslinking agent is present in a range of about 4 gallons to about 20 gallons per 1000 gallons of the crosslinked fluid.

5. The method of claim 1 , wherein the acid is present in a range of about 3 wt. % to about 28 wt. %.

6. The method of claim 1 , wherein a viscosity of the crosslinked fluid is less than a viscosity of a same crosslinked fluid without the fluoroboric acid.

7. The method of claim 1 , wherein a viscosity of the crosslinked fluid decreases over time.

8. A method of fracturing a zone of a subterranean carbonate formation penetrated by a well bore, comprising the steps of:

a) combining a polymer comprising acrylamide, an acid, and a liquid breaker comprising hydrochloric acid, ammonium bifluoride, and boric acid to produce a gelled acid;

b) adding a zirconium-based crosslinking agent to crosslink the gelled acid to form a crosslinked fluid of sufficient viscosity to facilitate fracturing of the formation; and

c) injecting the crosslinked fluid into the well bore and into contact with the formation under sufficient pressure to react the acid and the formation at conditions, the hydrochloric acid, ammonium bifluoride, and boric acid react and produce fluoroboric acid, the conditions being so that the fluoroboric acid decomposes to form a fluoride ion that breaks zirconium—polymer bonding in the crosslinked fluid.

9. The method of claim 8 , wherein the fluoroboric acid is produced in situ in a range of about 10 ppm to about 150 ppm of the crosslinked fluid.

10. The method of claim 8 , wherein the acid comprises hydrochloric acid, formic acid, acetic acid, citric acid, or combinations thereof; the acrylamide comprises acrylamide, (meth)acrylamide, diacetone acrylamide, N-methylolacrylamide, a polyacrylamide derivative, a copolymer of acrylamide, a copolymer of (meth)acrylamide, a hydrophobically- modified polyacrylic acid/acrylate copolymer, or combinations thereof; and the crosslinking agent comprises zirconium lactate, zirconium acetate, zirconium lactate with alkanolamine, and combinations thereof.

11. The method of claim 8 , wherein the crosslinking agent is present in a range of about 4 gallons to about 20 gallons per 1000 gallons of the crosslinked fluid.

12. The method of claim 8 , wherein the acid is present in a range of about 3 wt. % to about 28 wt. %.

13. The method of claim 8 , wherein a viscosity of the crosslinked fluid is less than a viscosity of a same crosslinked fluid without the fluoroboric acid.

14. The method of claim 8 , wherein a viscosity of the crosslinked fluid decreases over time.