Patent ID: 7449430

Claim:
A water-based drilling mud consisting essentially of (1) from 50% to 90% of an aqueous phase wherein the aqueous phase contains from 0.1% to 20% of an oil soluble polymer in the form of a gel as a fluid loss reducer, wherein the oil soluble polymer is an unsubstituted styrene-acrylate copolymer or an unsubstituted styrene-butadiene copolymer, (2) from 0.0 1% to 0.5% of at least one pH controller, (3) from 0.1% to 5% of at least one viscosifier, (4) from 0.01% to 30% of at least one salt, (5) from 0.1% to 3% of at least one emulsifier, (6) from 4 to 60% of at least one weighting agent, (7) from 0% to 15% of at least one clay, and (8) at least one fluid loss control agent selected from the group consisting of carboxymethylcellulose and polyanionic cellulose.