Subterranean formations

1. A method of fracturing a subterranean formation susceptible to formation of a undesirable gel-like goo, the method comprising:

contacting the formation with a fluid (A) comprising a friction reducer polymer (PP) which includes a pendent group which includes a moiety

O—S═O  (I), and

reducing and/or minimizing undersirable gel-like goo within the subterranean formation,

wherein the sulphur atom of the moiety is covalently bonded, optionally via a linking atom or group, to the backbone of the polymer, and

wherein formation water includes Total Dissolved Solids (TDS) of at least 5000 ppm; and

formation rock within the subterranean formation includes rock having an Fe(II)S 2 -containing mineral at a level of greater than 0.1 wt %.

2. The method according to claim 1 , wherein said subterranean formation is: (i) one that is previously known to form said undesirable gel-like goo; or (ii) one that is similar in minerology to a formation known to form said undesirable gel-like goo.

3. The method according to claim 1 , wherein moiety (I) is part of a moiety of formula

4. The method according to claim 3 , wherein moiety (I) and/or moiety (II) is pendent from a repeat unit of formula

wherein the carbon atom attached to the oxygen atom is covalently bonded to a moiety which includes said moiety (I) and/or said moiety (II), wherein R 1 , R 2 and R 3 independently represent a hydrogen atom or an optionally-substituted alkyl group.

5. The method according to claim 4 , wherein said polymer (PP) includes 2 to 10 atoms in a linking chain between the carbon atom attached to the oxygen atom in moiety (III) and the sulphur atom in moiety (I) and/or (II), wherein a nitrogen atom is included in said linking chain.

6. The method according to claim 3 , wherein said moiety (I) is part of a moiety

XO—S═O  (IV)

and said moiety (II) is part of a moiety

wherein X is a hydrogen atom, an alkali metal or NH 4 ;

wherein said polymer (PP) includes a repeat unit which includes a moiety

wherein the carbon atom bonded to R 2 is bonded to a pendent group which includes said moiety (I) and/or said moiety (II) and R 1 , R 2 and R 3 independently represent a hydrogen atom or an optionally-substituted alkyl group.

7. The method according to claim 6 , wherein said repeat unit of formula (VI) is part of a moiety of formula

wherein L 1 is a linking atom or group and wherein, optionally, L 1 includes 3 to 6 atoms in a chain between the carbon atom bonded to group R 2 and the sulphur atom.

8. The method according to claim 7 , wherein said polymer (PP) includes a repeat unit of formula (VII) wherein L 1 represents

9. The method according to claim 1 , wherein polymer (PP) includes a repeat unit derived or derivable from a monomer of the formula

10. The method according to claim 1 , wherein said moiety (I) is part of a moiety —SO 3− and is part of a zwitterion and said polymer (PP) includes a repeat unit which includes a moiety

wherein the carbon atom bonded to R 7 is bonded to a pendent group which includes said SO 3− moiety and R 6 , R 7 and R 8 independently represent a hydrogen atom or an optionally-substituted alkyl group; and, optionally,

said repeat unit of formula (XIII) is part of a moiety of formula

wherein L 2 is a linking group, wherein L 2 represents

wherein n2 is an integer in the range 1 to 5, n3 is an integer in the range 1 to 6 and R 9 and R 10 independently represent C 1-4 alkyl groups.

11. The method according to claim 1 , wherein polymer (PP) is a copolymer which includes an acrylamide repeat unit and/or a repeat unit comprising a vinylpyrrolidone moiety.

12. The method according to claim 1 , wherein polymer (PP) includes a repeat unit of formula

wherein R 11 and R 12 are independently selected from a hydrogen atom and an optionally-substituted, alkyl group.

13. The method according to claim 4 , wherein said polymer (PP) includes 5 to 100 mol % of first repeat units of formula (III).

14. The method according to claim 4 , wherein said polymer (PP) includes at least some acrylamide repeat units (herein “second repeat unit(s)”) which are different to first repeat units III, wherein said polymer (PP) includes 0 to 99 mol % of said second repeat units; and the ratio defined as the mol % of said second repeat units divided by the mol % of said first repeat units is in the range 1 to 50.

15. The method according to claim 14 , wherein in said polymer (PP), the sum of the mol % of said first repeat units and said second repeat units is at least 70 mol %.

16. The method according to claim 1 , wherein said polymer (PP) is a copolymer of acrylamido-methyl-propane sulfonate and acrylamide which includes 5 to 40 mol % of acrylamido-methyl-propane sulfonate repeat units and 60 to 90 mol % of acrylamide repeat units.

17. The method according to claim 16 , wherein said polymer (PP) does not include any phosphonate moiety; and/or does not include any cationic moiety except as part of a zwitterion; and/or does not include any free quaternary ammonium groups.

18. The method according to claim 16 , wherein said polymer (PP) has a molecular weight in the range 1,000,000 to 50,000,000 Daltons; and/or a polydispersity in the range 1-7.

19. The method according to claim 18 , wherein said fluid (A) includes 100-10,000 ppm of polymer (PP); and includes up to 95 wt % water.

20. The method according to claim 1 , wherein, at some stage in the method, one or a plurality of proppants is incorporated into fluid (A), wherein the proppant has a size of at least 140 US Mesh and/or said fracturing fluid includes 2.9 to 54 wt % of proppants.

21. The method according to claim 1 , wherein the only polymer for friction reduction in said fluid (A) is said friction reducer polymer (PP) and/or said fluid (A) does not include:

any cationic polymer containing a hydrolysable group within a group pendent from the polymer backbone; and/or

any anionic polymer comprising a pendent group that will complex with iron; and/or

any polymer which includes a P-containing functional group such as a phosphonic acid-based group.

22. The method according to claim 1 , wherein the method comprises fracturing a subterranean formation in a method comprising contacting the subterranean formation with a fluid (A) and fracturing the subterranean formation by pumping the fluid (A) into a wellbore at a pressure sufficient to create a fracture network.

23. The method according to claim 1 , wherein said method is a method of slick water fracturing of said subterranean formation.

24. The method according to claim 1 , wherein formation water includes Total Dissolved Solids (TDS) of at least 5000 ppm; and

the formation rock includes rock having an Fe(II)S 2 -containing mineral at a level of at least 1 wt %.

25. The method according to claim 1 , wherein formation water includes Total Dissolved Solids (TDS) of at least 5000 ppm; and

ICP Flowback water analysis yields a fluid comprising at least 100 ppm Fe.