Foaming compositions for enhanced oil recovery

1. A composition C, suitable as a foaming composition for enhanced oil recovery, which comprises, in an aqueous medium M:

a surfactant or a mixture of surfactants capable of creating a foam in a presence of a gas; and

a polymer P or a mixture of polymers P, wherein the polymer P is based on acrylamide or acrylamido units, and has an average molecular mass M w ranging from 1,000,000 to 20,000,000 g/mol;

wherein

the composition C has a viscosity, as measured at 25° C. under a shear rate of 10s −1 , of is less than 6 mPa·s;

wherein

a total concentration of the polymer P or of the mixture of polymers P in the composition C ranges from 0.0025 to 0.1 wt % by mass relative to a total mass of the composition C and is greater than a concentration limit whereby an apparent viscosity of a foam obtained from the composition C is at least 10% greater than an apparent viscosity of a foam obtained from a composition C′ at least under certain conditions, wherein the composition C′ is a composition equal in all respects to the composition C, but deprived only of the polymer P or the mixture of polymers P; and

wherein the composition C′ has a viscosity, as measured at 25° C. under a shear rate of 10s −1 , four times less than a viscosity of a composition C″, wherein the composition C″ is a composition equal in all respects to the composition C′, but deprived of the surfactant or the mixture of surfactants.

2. The composition C according to claim 1 , wherein a content of the polymer P or of the mixture of polymers P is selected in such a manner that an apparent viscosity of the foam obtained from the composition C in a sandpack test is at least 10% higher than an apparent viscosity of a foam obtained from the composition C′ under same conditions, wherein the sandpack test is:

carried out by co-injecting a composition to be tested with dinitrogen and with a gas fraction of 0.9, wherein the composition to be tested is the composition C or the composition C′;

using Ottawa sand with a permeability greater than 1 darcy, and

with a back pressure of 50 bars at a temperature of 25° C. and at an interstitial speed of 100 ft/d.

3. The composition C according to claim 1 , wherein the polymer P is at least one polymer chosen from:

polyacrylamides, and copolymers comprising acrylamide units and acrylic acid units;

polymers of 2-Acrylamido-2-MethylPropane Sulfonic acid (AMPS);

copolymers comprising acrylamide monomer units and AMPS monomer units;

polymers comprising acrylamide monomer units, AMPS monomer units and N-vinylpyrrolidone NVP units; and

mixtures thereof.

4. The composition C according to claim 3 , wherein the polyacrylamides are partially hydrolyzed.

5. The composition C according to claim 1 , wherein a content of the surfactant or the mixture of surfactants is between 0.05% and 2% by mass relative to the total mass of the composition.

6. The composition C according to claim 1 , comprising a mixture of surfactants containing:

at least one anionic surfactant;

at least one surfactant of zwitterionic or amphoteric type; and

optionally at least one nonionic surfactant.

7. The composition C according to claim 6 , wherein the at least one anionic surfactant is of sulfonate type.

8. The composition C according to claim 6 , wherein the at least one surfactant of zwitterionic or amphoteric type is selected from the group of betaine or sultaine types.

9. The composition C according to claim 6 , wherein the at least nonionic surfactant is of ethoxylated alcohol type.

10. A foam, comprising the composition C according to claim 1 , wherein the foam has a gas fraction (fg) greater than or equal to 0.7.

11. A method of assisted oil recovery from an underground formation, comprising of:

injecting the composition C according to claim 1 into the underground formation, via at least one injection well, wherein the composition C is in contact with a gas within the underground formation; then

recovering, through at least one production well, a fluid conveying oil leaving the underground formation.

12. The method according to claim 11 , wherein the composition C is selected in such a manner that when injected into the subterranean formation with the gas, with a gas fraction of 0.9, induces an apparent viscosity of at least 10% greater than an apparent viscosity induced by the composition C′, under same conditions.

13. The method according to claim 12 , wherein, the composition C induces an apparent viscosity of at least 10% greater than an apparent viscosity induced, under the same conditions, by the composition C′.

14. The method of claim 11 , wherein the gas is selected from the group of water vapor, dinitrogen, CO 2 , or hydrogen gas.

15. The method of claim 11 , wherein injecting the composition C into the subterranean formation is selected from the group of: injecting the composition C as an expanded foam which contains the gas, injecting the composition C in a non-foamed form and the gas as a joint injection, injecting the composition C followed by injecting the gas, and injecting the composition C into the subterranean formation previously containing the gas.