Patent ID: 7439209

Claim:
A drilling fluid system comprising: a brine comprising an amount of at least one bridging agent and a quantity of cationic copolymer comprising a first ratio of cationic derivatives of acrylamide monomers to acrylamide monomers, said cationic derivatives of acrylamide monomers having the following general structure: wherein x is from about 1 to about 3; R 1 is selected from the group consisting of hydrogen and alkyl groups having from about 1 to about 3 carbon atoms; and R 2 , R 3 , and R4 are selected from the group consisting of hydrogen; cyclic, linear, and branched alkyl. and heteroalkyl groups having from about 1 to about 6 carbon atoms wherein said heteroalkyl groups comprise one or more heteroatoms selected from the group consisting of nitrogen and oxygen; cyclic, linear, and branched alkenyl and heteroalkenyl groups having from about 1 to about 6 carbon atoms and resonance hybrids thereof; carboxylic acid groups and salts, esters, and amides thereof comprising cyclic, linear and branched alkyl groups, heteroalkyl groups, alkenyl groups, and heteroalkenyl groups having from about 1 to about 6 carbon atoms wherein said hetero groups comprise one or more heteroatoms selected from the group consisting of nitrogen and oxygen; aromatic groups; alkanols and alkenols having from about 1 to about 6 carbon atoms; alkanolamides and alkenol amides having from about 1 to about 6 carbon atoms; and combinations thereof; wherein said quantity of said cationic copolymer and said amount of said bridging agent are effective to form a filter cake which provides effective filtration control and rheology in said drilling fluid system at temperatures of at least about 250° F. for at least about 16 hours.