Drilling fluid and method for drilling in coal-containing formations

1. A method for drilling in a coal containing formation, the method comprising:

providing a mixed metal-viscosified drilling fluid including at least 1% potassium salt;

circulating the drilling fluid through a well; and

drilling into the coal seam.

2. The method of claim 1 wherein providing the mixed metal-viscosified drilling fluid includes providing an aqueous-based drilling fluid including 15 to 45 kg/m3 bentonite, mixed metal viscosifier at a weight ratio of 1:8 to 1:12, viscosifier to bentonite, a base to maintain the pH above about 10.0 and at least 1% potassium salt.

3. The method of claim 1 wherein providing the mixed metal-viscosified drilling fluid includes providing an aqueous-based drilling fluid including about 30 kg/m3 bentonite, a mixed metal viscosifier in a quantity of about 1:10 mixed metal viscosifier to bentonite with a pH controlled to greater than pH 11 and 1 to 5% potassium salt.

4. The method of claim 1 wherein providing the mixed metal-viscosified drilling fluid includes:

mixing bentonite in water to form a bentonite mixture;

adding a mixed metal viscosifier to the bentonite mixture;

adjusting pH to greater than above about pH 10;

adding the potassium salt.

5. The method of claim 4 further comprising adding any of fluid loss control additives and/or lost circulation materials.

6. The method of claim 4 wherein the pH is adjusted using caustic soda, caustic potash, potassium carbonate or soda ash.

7. The method of claim 1 wherein providing the mixed metal-viscosified drilling fluid includes obtaining a drilling fluid with a yield point greater than 10 Pa.

8. The method of claim 1 continuing circulation with entrained coal fines in the drilling fluid.

9. The method of claim 8 wherein rheology of the drilling fluid is substantially maintained.

10. The method of claim 1 wherein circulating the drilling fluid is initiated prior to drilling into the coal seam.

11. The method of claim 1 wherein circulating the drilling fluid is maintained while a coal seam is open to the drilling fluid.

12. The method of claim 1 wherein circulating the drilling fluid is initiated substantially at surface.

13. The method of claim 1 wherein the potassium salt is selected from the group consisting of potassium sulfate, potassium chloride, potassium acetate and potassium formate.

14. The method of claim 1 wherein the potassium salt is potassium sulfate.