Patent ID: 7651980

Claim:
A drilling fluid system comprising: an aqueous base consisting essentially of freshwater; bridging agent comprising from about 0.7 to about 29 kg/m 3 (0.25 lb/bbl to about 10.0 lb/bbl) water soluble polymer having a 5% by weight Brookfield viscosity in freshwater of from about 50 cP to about 100 cP, said water soluble polymer comprising from about 10 to about 20 mole % monomer of acrylamide, from about 50 to about 60 mole % monomer of 2-acrylamido-2-methylpropane sulfonic acid or alkali metal salts thereof, and from about 25 to about 35 mole % N-vinyl pyrrolidone monomer; and, from about 21.42 to 57.15 kg/m 3 (7.5 to 20 lb/bbl) plugging agent comprising sulfonated asphalt and lignite; wherein, after 16 hours of static aging of the drilling fluid system at about 260° C. (500° F.), the drilling fluid system exhibits a HPHT filtrate of 25 cm 3 /min or less after 30 minutes at about 149° C. (300° F.).