Novel polymer compositions are disclosed, along with their use as well service fluids, for example as completion fluids, work-over fluids or drilling fluids, comprising water soluable copolymers having sulfonate groups and carboxylate groups, along with alkali metal salts of carboxylic acid. Exemplary copolymer has 5 to 95 wt. % structural units derived from 2-acrylamido-2-methylpropanesulfonic acid or salt thereof, and 5 to 95 wt. % structural units derived from acrylic acid or salt thereof. A salt of the polymer may be used, such as the sodium, potassium, ammonium and calcium salts. Exemplary alkali metal salts of the polymer composition include sodium, potassium and cesium salts of formic acid and/or acetic acid in amounts suitable to develop high temperature viscosity suitable for such well servicing fluids. The polymer composition is hydrateble/soluble in a brine of sodium and/or potassium and/or cesium salts of formic and/or acetic acid.