Viscosifier agent for oilfield fluids in hard conditions

1. A viscosifier for oilfield fluids, comprising:

at least one zwitterionic polymer, prepared by inverse emulsion polymerization of:

monomers A, each comprising a betaine group;

nonionic monomers B; and

monomers C, each comprising a metal ion-crosslinkable group, wherein monomers C are each selected from the group consisting of acrylate monomers bearing COOH or COO − groups, monoesters of C4-C10 monoethylenically unsaturated dicarboxylic acids, and mixtures thereof, wherein the polymer comprises from 1 to 10% by weight of monomers C, based on the total weight of the polymer;

with a molar ratio of the monomers A to the monomers B between 4/96 and 40/60; and

at least one metal ion compound for crosslinking the metal ion-crosslinkable groups of monomers C.

2. The viscosifier of claim 1 , wherein the polymer exhibits an intrinsic viscosity greater than 600 mL/g.

3. The viscosifier of claim 1 , wherein the monomers A are monomers selected from the group consisting of the following monomers and their mixtures:

alkylsulphonates or phosphonates of dialkylammonioalkyl acrylates, dialkylammonioalkyl methacrylates, dialkylammonioalkyl acrylamides, or dialkylammonioalkyl methacrylamides, each comprising a betaine group,

heterocyclic betaine monomers,

betaines resulting from the reaction of ethylenically unsaturated anhydrides and dienes, and

phosphobetaines.

4. The viscosifier of claim 3 , wherein the monomers A comprise monomers having one of the following formulae:

in which:

R 1 is hydrogen or methyl,

R 2 and R 3 , which are identical or different, are hydrogen or alkyls having from 1 to 6 carbon atoms,

Y 1 is a divalent group of formula —O— or NR 2 ,

Z — is SO 3 − ,

m is 2 or 3, and

n is 1-6.

5. The viscosifier of claim 4 , wherein the monomers A comprise monomers having one of the following formulae:

6. The viscosifier of claim 5 , wherein the monomers A are SPP and/or SPE monomers.

7. The viscosifier of claim 1 , wherein the monomers B are selected from the group consisting in the following monomers and the mixtures thereof:

hydroxyethyl acrylate,

hydroxyethyl methacrylate,

hydroxypropyl acrylate

hydroxypropyl methacrylate,

acrylamide,

methacrylamide,

N-methylolacrylamide,

dimethylacrylamide,

dimethylmethacrylamide,

poly(ethylene and/or propylene oxide), if appropriate random or in the block form,

α-methacrylates,

vinyl alcohol, and

vinylpyrrolidone.

8. The viscosifier of claim 7 , wherein the monomers B are acrylamide.

9. The viscosifier of claim 1 , wherein the monomers A are SPP and/or SPE and the nonionic monomers B are acrylamide, with a AM/SPP molar ratio of between 85/15 and 95/5.

10. The viscosifier of claim 1 , wherein the at least one metal ion compound comprises a cation of Fe(III) or Al(III).

11. The viscosifier of claim 1 , wherein the at least one metal ion compound comprises a cation of Zr(IV) or Ti(IV), or a borate.

12. The viscosifier of claim 1 , wherein monomers C are each selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, [alpha]-chloro-acrylic acid, crotonic acid, maleic acid, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, aconitic acid, fumaric acid, monomethyl maleate, and mixtures thereof.

13. The viscosifier of claim 1 , wherein the monomers A are monomers selected from the group consisting of the following monomers and their mixtures:

14. A process for increasing the viscosity of an oilfield fluid, comprising adding to the fluid:

at least one zwitterionic polymer, prepared by inverse emulsion polymerization of:

monomers A, each comprising a betaine group;

nonionic monomers B; and

monomers C each comprising a metal ion-crosslinkable group, wherein monomers C are each selected from the group consisting of acrylate monomers bearing COOH or COO — groups, monoesters of C4-C10 monoethylenically unsaturated dicarboxylic acids, and mixtures thereof;

with a molar ratio of the monomers A to the monomers B between 4/96 and 40/60,

wherein the polymer comprises from 1 to 10% by weight of monomers C, based on the total weight of the polymer, and

at least one metal ion compound for crosslinking the metal ion-crosslinkable groups of monomers C.

15. The process of claim 14 , wherein the oilfield fluid is a high total dissolved solids fluid.

16. The process of claim 15 , wherein the oilfield fluid is a heavy brine.

17. The process of claim 14 , wherein the molar ratio of the monomers A to the monomers B is between 7/93 and 30/70.