Method of increasing efficiency in a hydraulic fracturing operation

1. A method of fracturing a multi-zone subterranean formation penetrated by a well which comprises:

(a) injecting into a first zone of the subterranean formation in a single-step a buffered HF sandstone acidizing solution, wherein the buffered HF sandstone acidizing solution is injected at a pressure sufficient to create or enlarge at least one fracture in the first zone;

(b) injecting a pad fluid, a fracturing fluid or both a pad fluid and a fracturing fluid and displacing the buffered HF sandstone acidizing solution farther into the created or enlarged fracture

wherein:

(i) a pre-flush is not injected into the first zone before step (a); and

(ii) a post-flush is not injected into the first zone after step (a);

(c) sealing the first zone from other zones in the formation;

(d) repeating steps (a), (b) and (c) in at least one other zone of the multi-zone subterranean formation.

2. The method of claim 1 , wherein the pH of the buffered HF sandstone acidizing solution is between from about 1.9 to about 4.8.

3. The method of claim 2 , wherein the pH of the buffered HF sandstone acidizing solution is between from about 2.5 to about 4.5.

4. The method of claim 1 , wherein the buffered HF sandstone acidizing solution comprises a phosphonate of the formula:

wherein R1, R2 and R3 are independently selected from hydrogen, alkyl, aryl, phosphonates, phosphates, acyl, amine, hydroxy and carboxyl groups and R4 and R5 are independently selected from hydrogen, sodium, potassium, ammonium or an organic radical.

5. The method of claim 4 , wherein the buffered HF sandstone acidizing solution comprises an acid selected from the group consisting of citric acid, acetic acid, formic acid and mixtures thereof.

6. The method of claim 5 , wherein the buffered HF sandstone acidizing solution comprises from about 1 to about 50 weight percent citric acid, up to about 20 weight percent HF and from about 0.5 to about 50 weight percent phosphonate compound.

7. A method of fracturing a multi-zone subterranean formation penetrated by a horizontal well or a vertical well which comprises:

(a) injecting into a first zone of the subterranean formation a buffered HF sandstone acidizing solution, wherein the buffered HF sandstone acidizing solution is injected at a pressure sufficient to create or enlarge at least one fracture in the first zone wherein the at least one fracture is defined by a fracture face and a fracture wall;

(b) etching or dissolving at least a portion of the fracture face or fracture wall of step (a) with the buffered HF sandstone acidizing solution;

(c) injecting a pad fluid, a fracturing fluid or both a pad fluid and a fracturing fluid into the first zone and displacing at least a portion of the buffered HF sandstone acidizing solution further into the created or enlarged fracture;

(d) sealing the first zone;

(e) repeating steps (a), (b), (c) and (d) in at least one other zone in the multi-zone subterranean formation.

8. The method of claim 7 , wherein the buffered HF sandstone acidizing solution has a pH between from about 1.9 to about 4.8.

9. The method of claim 7 , wherein the buffered HF sandstone acidizing solution comprises a phosphonate of the formula:

wherein R1, R2 and R3 are independently selected from hydrogen, alkyl, aryl, phosphonates, phosphates, acyl, amine, hydroxy and carboxyl groups and R4 and R5 are independently selected from hydrogen, sodium, potassium, ammonium or an organic radical.

10. The method of claim 9 , wherein the buffered HF sandstone acidizing solution further comprises an organic acid selected from the group consisting of citric acid, formic acid, acetic acid and mixtures thereof.

11. The method of claim 10 , wherein the buffered HF sandstone acidizing solution comprises from about 1 to about 50 weight percent citric acid, up to about 20 weight percent HF and from about 0.5 to about 50 weight percent phosphonate compound.

12. A method of decreasing tortuosity in multiple zones of a subterranean formation penetrated by a multi-zoned completed well which comprises:

(a) pumping with one or more pumps into a first zone of the subterranean formation a buffered HF sandstone acidizing solution, wherein the buffered HF sandstone acidizing solution is pumped at a pressure sufficient to create or enlarge at least one fracture in the first zone and further wherein a pre-flush is not pumped into the first zone;

(b) advancing the buffered HF sandstone acidizing solution farther into the created or enlarged fracture by pumping with one or more pumps of step (a) a pad fluid, a fracturing fluid or both a pad fluid and a fracturing fluid into the first zone;

(c) sealing the first zone;

(d) repeating steps (a), (b) and (c) in at least one other zone in the multi-zone subterranean formation,

wherein tortuosity in each zone is reduced by the creation or enlargement of the fracture by the acidizing solution.

13. The method of claim 12 , wherein the buffered HF sandstone acidizing solution comprises from about 1 to about 50 weight percent citric acid, up to about 20 weight percent HF and from about 0.5 to about 50 weight percent phosphonate compound.

14. The method of claim 12 , wherein a post-flush is not injected into the first zone after step (a).

15. The method of claim 12 , wherein the one or more pumps are run continuously throughout steps (a) through (c).