Patent ID: 7919437

Claim:
A wellbore fluid having a continuous aqueous phase, and comprising: (a) at least one polymer which has a number average molecular weight in the range of from 10,000 to less than 100,000, wherein the polymer is selected from a copolymer of at least one hydrophilic monomer and at least one hydrophobic monomer containing an alkyl group having at least 8 carbon atoms, wherein the hydrophilic monomer is selected from acrylic acid, methacrylic acid, maleic acid, maleic anhydride, fumaric acid and itaconic acid, and wherein sulfonate groups have been introduced into the copolymer by reaction of at least a portion of the carboxylic acid and/or anhydride groups with a compound of general formula R 1 2 N(CH 2 ) n SO 3 X (I) or a compound of general formula HO(CH 2 ) n SO 3 X (II) wherein each R 1 is independently selected from hydrogen, methyl or ethyl, X is either H or a Group IA metal, and n is an integer of from 1 to 6, and wherein the combined molecular weight of the pendant alkyl groups that are introduced into the copolymer by the hydrophobic monomer is in the range of from 15 to 70% by weight based on the molecular weight of the polymer such that the polymer has a solubility of at least 2% in water at 20° C.; and (b) at least one additive selected from the group consisting of additional fluid loss control agents, bridging particulate materials, finely dispersed additives for increasing the fluid density, thinners, and shale inhibitors.