Method for treating a subterranean formation
Disclosed are acid fracturing methods for subterranean siliceous formations employing a viscous dissolving fluid to create a permeable conduit that bypasses the near wellbore damage. The viscous dissolving fluids used according to the invention are acid-fracturing treatment fluids containing a dissolving agent viscosified with a material such as a viscoelastic surfactant, and the fluid is injected into a sandstone formation at a pressure sufficient to create a hydraulic fracture. Also, the viscous dissolving fluid leaks off into the faces of the fracture thus “stimulating” the formation. Hence when the hydraulic fracture has closed there is a part of the formation that has been effectively stimulated, creating a pathway for fluids to preferentially flow to the wellbore. The formation may then be flowed back to produce a substantial volume of the treatment fluid.
1. A method of stimulating a sandstone subterranean formation comprising:
a. preparing an aqueous viscous dissolving fluid comprising:
i. viscosifier incorporated in an amount suitable to provide measured fluid viscosity from about 20 mPa-s to about 400 mPa-s at a shear rate of 100 s −1 over a temperature range from about 80° F. to about 300° F.;
ii. at least one formation dissolving agent; and,
iii. a chelant selected from the group consisting of aminopolycarboxylic acid, and aminopolycarboxylic acid salt;
b. injecting the viscosified fluid into a wellbore penetrating the formation at a pressure that exceeds the fracture initiation pressure of the formation;
c. relieving the initiation pressure and allowing a hydraulic fracture to at least partially close; and
d. flowing back the wellbore to produce a substantial amount of the viscosified fluid.
2. The method of claim 1 further comprising adding a proppant to the fluid wherein the proppant is placed in the fracture.
3. The method of claim 2 further comprising adding a proppant to the fluid wherein the proppant is placed in the fracture simultaneous with fracturing and acidizing the formation.
4. The method of claim 3 wherein the viscosified fluid further comprises a breaker.
5. The method of claim 2 wherein a tip screenout is induced in said step of injecting a viscous carrier fluid containing proppant at a rate and pressure sufficient to fracture said formation.
6. The method of claim 1 wherein the fluid comprises about 0.1% to 6% by weight of fluoride generating compound, provided the fluoride source is not generated by adding hydrofluoric acid to the fluid.
7. The method of claim 1 wherein the viscosified fluid pH is about 5 or less.
8. The method of claim 1 wherein the measured fluid viscosity is from about 40 mPa-s to about 400 mPa-s at a shear rate of 100 s −1 over a temperature range from about 80° F. to about 300° F.
9. The method of claim 1 wherein the dissolving agent comprises a chelant.
10. The method of claim 1 wherein the formation dissolving agent comprises a component selected from the group consisting of hydrofluoric acid and a hydrofluoric acid precursor.
11. The method of claim 1 wherein the viscosifier is a viscoelastic surfactant.
12. The method of claim 11 wherein the viscoelastic surfactant is a betaine.
13. The method of claim 11 wherein the concentration of the viscoelastic surfactant is between about 0.1% and about 10% by weight based upon total fluid weight.
14. The method of claim 13 wherein the concentration of the viscoelastic surfactant is between about 0.2% and about 2% by weight based upon total fluid weight.
15. The method of claim 1 further comprising injecting a second portion of the viscosified fluid into a wellbore penetrating the formation a sand at a pressure that exceeds the fracture initiation pressure of the formation, prior to relieving the initiation pressure and allowing a hydraulic fracture to at least partially close.
16. The method of claim 1 further comprising injecting a proppant-transporting carrier fluid into the wellbore, the proppant-transporting carrier fluid comprising a viscosifier and a proppant, wherein the proppant-transporting carrier fluid is injected at a pressure that exceeds the fracture initiation pressure of the formation.
17. The method of claim 1 wherein the fluid further comprises a material effective precursor to eliminate fines migration.
18. A method of stimulating a sandstone subterranean formation comprising:
a. preparing an aqueous viscous dissolving fluid comprising:
i. viscoelastic surfactant based viscosifier incorporated in an amount suitable to provide measured fluid viscosity from about 40 mPa-s to about 400 mPa-s at a shear rate of 100 s −1 over a temperature range from about 80° F. to about 300° F.;
ii. at least one formation dissolving agent; and,
iii. a chelant selected from the group consisting of aminopolycarboxylic acid, and aminopolycarboxylic acid salt;
b. injecting a first volume of the viscosified fluid into a wellbore penetrating the formation a sand at a pressure that exceeds the fracture initiation pressure of the formation;
c. injecting at least one further volume of the viscosified fluid into a wellbore penetrating the formation a sand at a pressure that exceeds the fracture initiation pressure of the formation;
d. relieving the initiation pressure and allowing a hydraulic fracture to at least partially close; and
e. flowing back the wellbore to produce a substantial amount of the viscosified fluid.
19. The method of claim 18 where the at least one further volume of the viscosified fluid further comprises a proppant.
20. A method of stimulating a sandstone subterranean formation comprising:
a. preparing an aqueous viscous dissolving fluid comprising:
i. viscoelastic surfactant based viscosifier incorporated in an amount suitable to provide measured fluid viscosity from about 40 mPa-s to about 400 mPa-s at a shear rate of 100 s −1 over a temperature range from about 80° F. to about 300° F.;
ii. at least one formation dissolving agent; and,
iii. a chelant selected from the group consisting of aminopolycarboxylic acid, and aminopolycarboxylic acid salt;
b. injecting the viscosified fluid into a wellbore penetrating the formation a sand at a pressure that exceeds the fracture initiation pressure of the formation;
c. injecting a proppant-transporting carrier fluid into the wellbore, the proppant-transporting carrier fluid comprising a polymer viscosifier and a proppant at a pressure that exceeds the fracture initiation pressure of the formation;
d. relieving the initiation pressure and allowing a hydraulic fracture to at least partially close; and
e. flowing back the wellbore to produce a substantial amount of the viscosified fluid.