Wellbore fluid having a continuous aqueous phase, and comprising 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. The hydrophilic monomer is selected from acrylic acid, methacrylic acid, maleic acid, maleic anhydride, fumaric acid and itaconic acid, and 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 R12N(CH2)nSO3X (I) or a compound of general formula HO(CH2)nSO3X (II). Each R1 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. 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. The fluid further comprises at least one additive selected from additional fluid loss control agents, bridging particulate materials, finely dispersed additives for increasing the fluid density, thinners, and shale inhibitors.