Thermally stable water-soluble polymer which is crosslinkable at high temperatures

1. A copolymer which is reversibly crosslinkable with polyvalent metal ions at temperatures of above 150° C. and comprises:

i) 80-90% by weight of one or more structural units of the formula A

where

R 1 is hydrogen or methyl,

R 2 is C 2 -C 10 -alkylene, and

Me + is an ammonium or an alkali metal ion,

ii) from 1 to 10% by weight of one or more structural units of the formula B

where R 3 and R 4 are each independently hydrogen, methyl or ethyl,

iii) from 1 to 10% by weight of one or more structural units of the formula C,

where n is a number from 1 to 6,

iv) from 0.1 to 5% by weight of structural units of the formula D

where X is hydrogen, an ammonium ion or an alkali metal ion, and optionally,

v) from 0 to 10% by weight of one or more structural units of the formula E

where R 5 is hydrogen, methyl and/or ethyl,

with the proviso that the content in the copolymers of structural units B) and C) is from 6 to 15% by weight.

2. The copolymer as claimed in claim 1 ,comprising from 82 to 88% by weight of the structural units of the formula (A).

3. The copolymer as claimed in claim 1 , comprising from 2 to 8% by weight of the structural units of the formula (B).

4. The copolymer of claim 1 , comprising from 2 to 8% by weight of the structural units of the formula (C).

5. The copolymer of claim 1 , containing from 0.5 to 3% by weight of the structural units of the formula (D).

6. The copolymer of claim 1 , containing from 0.1 to 8% by weight of the structural units of the formula (E).

7. The copolymer of claim 1 , in which R 2 is a C 4 -alkylene group.

8. The copolymer of claim 1 , in which R 3 and R 4 are simultaneously hydrogen.

9. The copolymer of claim 1 , in which the total content of structural units of the formula (B) and (C) is from 7.5 to 12.5% by weight.

10. The copolymer of claim 1 , in which the weight ratio of the structural units of the formula (B) to (C) is less than 3:1.

11. The copolymer of claim 1 , in which X is hydrogen.

12. The copolymer of claim 1 , in which R 5 is hydrogen.

13. The copolymer of claim 1 , in which the proportions of the structural units (A), (B), (C) and (D) add up to 100% by weight.

14. The copolymer of claim 1 , in which the proportions of the structural units (A), (B), (C), (D) and (E) add up to 100% by weight.

15. The copolymer of claim 1 , which has molecular weights between 50 000 and 2·10 7 g/mol.

16. A composition comprising a copolymer of claim 1 and a crosslinker which comprises a compound of a polyvalent metal ion.

17. The composition as claimed in claim 16 , wherein the polyvalent metal ion is an ion selected from the group consisting of zirconium, chromium, titanium, aluminum, and mixtures thereof.

18. A process for extracting mineral oil or natural gas from an underground formation having a borehole, said formation comprising the mineral oil or natural gas and water or saline water and mixtures thereof by altering the permeability of the underground formation for water or saline water, said process comprising:

introducing simultaneously a crosslinker selected from the group consisting of at least one polyvalent alkaline earth metal, a transition metal compound, and mixtures thereof, and an aqueous solution of the copolymer of claim 1 into the formation at a temperature above 200° C., and subsequently extracting from the borehole mineral oil or natural gas, or mixtures thereof.

19. A method for altering the permeability of an underground formation for water or saline water, said method comprising:

introducing into the formation at a temperature above 200° C.—a fluid comprising a crosslinker selected from the group consisting of at least one polyvalent alkaline earth metal, a transition metal compound, and mixtures thereof, and the polymer of claim 1 .