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Abstract

A composition designed for dissolution or dispersion in water to give an aqueous treatment fluid having a yield point of zero, at a temperature between about 85.degree. and about 160.degree. F., which is compatible with cement slurries and with drilling muds and a method of using same to displace a drilling mud from a borehole which penetrates a subterranean formation, particularly immediately preceding the injection of a fluid such as a cement slurry, and which also exhibits good fluid loss control and may be adapted for use with either water-based or oil-based drilling muds. The composition comprises three components, a water soluble viscosifier which increases the viscosity of water under mixing and handling conditions but which becomes substantially insoluble in water when exposed to temperatures encountered downhole in a wellbore; a second component, a thickener, which is soluble or dispersible in and which increases the viscosity of water at temperatures experienced under downhole conditions; and a third component, a dispersant, which enhances the dispersibility of the first component in water and thereby permits it to go smoothly into solution. The composition optionally comprises a nonionic surfactant which may be added to improve the compatibility with an oil-based drilling mud of an aqueous treatment fluid prepared from the composition. The composition also optionally comprises a weighting agent to impart increased density to a treatment fluid prepared therefrom. When combined with water, the resultant aqueous treatment fluid, at a temperature between about 85.degree. and about 160.degree. F., has a kinematic viscosity of greater than about 0.1 and less than about 7.5 centipoise-gallons/pound.

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