Organophilic clays and drilling fluids containing them

1. A method for preparing an organophilic clay, comprising:

a) homogenizing a mixture comprising:

i) a smectite-type clay;

ii) 0 to 15% by weight water and/or polar solvent; and

iii) a quaternary ammonium salt comprising at least 75 mol-% bis-(2-hydroxypropyl)-dimethylammonium methylsulfate fatty acid ester;

wherein the homogenizing is performed at a temperature above the melting point of the quaternary ammonium salt; and

b) drying the resulting homogenized mixture;

wherein the organophilic clay comprises the smectite-type clay ion exchanged with quaternary ammonium ions, wherein at least 75 mol-% of said quaternary ammonium ions are bis-(2-hydroxypropyl)-dimethylammonium fatty acid esters and at least 90% of the fatty acid moieties of said bis-(2-hydroxypropyl)-dimethylammonium fatty acid esters have a structure RC(O) with group R being a linear alkyl or alkenyl group containing from 9 to 21 carbon atoms.

2. The method of claim 1 , wherein, the resulting homogenized mixture of paragraph b) is subjected to grinding after it has been dried.

3. The method of claim 2 , wherein, after grinding, particles larger than 75 μm are removed by sieving.

4. The method of claim 1 , wherein the quaternary ammonium salt has a melting point of from −10 to 80° C.

5. The method of claim 1 , wherein the organophilic clay, further comprises (2-hydroxypropyl)-(1-methyl-2-hydroxyethyl)-dimethylammonium fatty acid esters, wherein the molar ratio of bis-(2-hydroxypropyl)-dimethylammonium fatty acid esters to (2-hydroxypropyl)-(1-methyl-2-hydroxyethyl)-dimethylammonium fatty acid esters is from 4:1 to 20:1.

6. The method of claim 5 , wherein the organophilic clay comprises water or an alcohol.

7. The method of claim 5 , wherein the resulting homogenized mixture of paragraph b) is subjected to grinding after it has been dried.

8. The method of claim 7 , wherein, after grinding, particles larger than 75 μm are removed by sieving.

9. The method of claim 5 , wherein the quaternary ammonium salt has a melting point of from −10 to 80° C.

10. The method of claim 1 , wherein the smectite-type clay comprises bentonite.

11. The method of claim 1 , wherein the bentonite comprises at least 60% by weight montmorillonite.

12. The method of claim 10 , wherein, in the bis (2 hydroxypropyl)-dimethylammonium fatty acid esters having the structure RC(O), said R groups have an average chain length of from 16 to 18 carbon atoms.

13. The method of claim 12 , wherein R groups have an iodine value of from 0 to 120, calculated for the free fatty acid RCOOH.

14. The method of claim 13 , wherein said bis-(2-hydroxypropyl)-dimethylammonium fatty acid esters on average have a molar ratio of fatty acid moieties to ammonium nitrogen of from 1.2 to 2.0.

15. The method of claim 1 , wherein, in accordance with paragraph a)ii), the mixture comprises the polar solvent.

16. The method of claim 15 , wherein the polar solvent is a monohydric, dihydric or polyhydric alcohol having from 1 to 4 carbon atoms.

17. The method of claim 15 , wherein the polar solvent is selected from the group consisting of: ethanol, 2-propanol, ethylene glycol, propylene glycol, and glycerol.

18. The method of claim 1 , wherein, in accordance with paragraph a)ii), the mixture comprises water and the polar solvent in a combined amount of 0.1 to 15% by weight.

19. The method of claim 18 , wherein water and the polar solvent are in a combined amount of 8 to 12% by weight.

20. The method of claim 1 , wherein the mixture of paragraph a) is homogenized with a pug mill or an extruder.