Aqueous fracturing fluid

1. Aqueous viscoelastic fracturing fluid for use in the recovery of hydrocarbons, comprising a viscoelastic surfactant and a hydrophobically-modified polymer,

wherein the surfactant comprises a head group and a hydrophobic tail and is of the following formula:

R—X—Y—Z

where R is the hydrophobic tail of said viscoelastic surfactant, which is a fully or partially saturated, linear or branched, hydrocarbon chain of at least 18 carbon atoms; X is a degradable acetal, amide, ether or ester bond; Y is a spacer group, formed by a short fully or partially saturated hydrocarbon chain of at least one carbon atom; and Z is the head group of the surfactant which can be —NR 1 R 2 R 3 + , —SO 3 − , —COO − or —N + R 1 R 2 R 3 —COO − where R 1 , R 2 and R 3 are each independently hydrogen or a linear or branched saturated aliphatic chain of at least one carbon atom

and wherein the concentration of the hydrophobically-modified polymer is comprised between, approximately, its overlap concentration c* and, approximately, its entanglement concentration c e , and wherein the viscoelastic surfactant comprises a cleavable viscoelastic surfactant and the hydrophobically-modified polymer comprises one or more cleavable pendant hydrophobic chains.

2. The fluid of claim 1 , wherein the viscoelastic surfactant concentration is above its critical micelle concentration.

3. The fluid of claim 1 , wherein the viscoelastic surfactant concentration is below 10 wt %.

4. The fluid of claim 3 , wherein the viscoelastic surfactant concentration is below 5 wt %.

5. The fluid of claim 1 , wherein the viscoelastic surfactant concentration is below 20×c*, where c* is the overlap concentration of said viscoelastic surfactant.

6. The fluid of claim 5 , wherein the viscoelastic surfactant concentration is below 10×c*.

7. The fluid of claim 1 , further comprising a salt.

8. The fluid of claim 1 , wherein the viscoelastic surfactant forms worm-like, thread-like or rod-like micelles.

9. The fluid of claim 1 , wherein the viscoelastic surfactant is ionic.

10. The fluid of claim 1 , wherein the viscoelastic surfactant is a mono-, a di- or an oligomeric carboxylate.

11. The fluid of claim 1 , wherein the viscoelastic surfactant is a oleyl amide succinate C 22 H 40 NO 3 − Na + .

12. The fluid of claim 1 , wherein the hydrophobically-modified polymer has a principal backbone and said one or more cleavable pendant hydrophobic chains are grafted on said backbone.

13. The fluid of claim 12 , wherein the one or more cleavable pendant hydrophobic chains are grafted on the principal backbone at a substitution degree range comprised between 0.01 and 10 and, preferentially, between approximately 0.05 and approximately 5.

14. The fluid of claim 12 , wherein the principal polymer backbone is a polysaccharide, a polyanhydride, a polyacrylamide, a polyacrylate, a polyacrylate copolymer, a polyether, a polyester, a polyamide or a polyvinylalcohol.

15. The fluid of claims 12 , wherein the one or more cleavable pendant hydrophobic chains are fully or partially saturated linear or branched hydrocarbon chains.

16. The fluid of claim 1 , wherein the hydrophobically-modified polymer is a hydrophobically-modified chitosan.

17. The fluid of claim 1 , wherein the hydrophobically-modified polymer is a hydrophobically-modified polyanhydride.

18. The fluid of claim 1 , wherein said fluid is able to form a gel responsive to hydrocarbons.

19. The fluid of claim 18 , wherein gel is able to be broken down on contact or mixing with hydrocarbons.

20. The fluid of claim 19 , wherein said gel is able to be broken down, with a viscosity of less than 20 cP in less than 2 hours, under downhole conditions.

21. The fluid of claim 1 , wherein said fluid leak-off rate is below the leak-off rate of a pure viscoelastic surfactant fluid of equivalent rheology.