Drilling fluid and method for drilling in coal-containing formations
A drilling fluid and method for drilling in a coal containing formation. The method includes: providing a mixed metal-viscosified drilling fluid including at least 1% potassium salt; circulating the drilling fluid through the well; and drilling into a coal seam.
1. A drilling fluid consisting of:
an aqueous mixture of 15 to 45 kg/m 3 bentonite, and a mixed metal viscosifier of inorganic magnesium/aluminum oxides and/or hydroxides at a weight ratio of 1:8 to 1:12, viscosifier to bentonite, the aqueous mixture having a pH above about pH 10; and
greater than 3% and up to 10% potassium sulfate,
wherein the drilling fluid retains a fluid state over time.
2. The drilling fluid of claim 1 further comprising a base to maintain the pH above about 10.0.
3. The drilling fluid of claim 1 wherein the drilling fluid consists of 20 to 40 kg/m 3 bentonite, mixed metal viscosifier at a weight ratio of 1:9.5 to 1:10.5, viscosifier to bentonite, a base to maintain the pH between about 10.5 to 13 and the potassium sulfate.
4. The drilling fluid of claim 1 wherein the drilling fluid consists of about 30 kg/m 3 bentonite, 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 up to 5% potassium sulfate.
5. The drilling fluid of claim 1 wherein the drilling fluid is prepared by:
mixing bentonite in water to form a bentonite mixture;
adding the mixed metal viscosifier to the bentonite mixture;
adjusting pH to greater than above about pH 10; and
adding the potassium sulfate.
6. The drilling fluid of claim 1 further comprising at least one of a fluid loss control additive and/or a lost circulation material.
7. The drilling fluid of claim 1 wherein the drilling fluid includes a yield point of greater than 10 Pa.
8. The drilling fluid of claim 1 wherein the pH is adjusted using caustic soda, caustic potash, potassium carbonate or soda ash.