The evolution of geothermal drilling muds used in the Imperial Valley, California began with the first generation Sepiolite muds in 1976. These muds had adequate rheology, but poor fluid loss control and were sensitive to contamination by brine or cement. The second generation muds, 1978 to 1980, utilized Sepiolite with increased amounts of bentonite and lignite to improve fluid loss control. Rheological properties of the second generation fluid were unacceptable and were extremely sensitive to contamination. The third generation muds, have exhibited excellent rheological stability, good filtration control, and are resistant to contamination.

The principal ingredients of the third generation geothermal drilling fluid are bentonite as a viscosifying agent, a low molecular weight copolymer for high temperature deflocculation and rheological stability, a sulfonated lignite and a modified vinyl copolymer for high temperature filtration control. Compared with the sepiolite and lignite based fluids, the new formulation performs better under extreme conditions, such as temperatures in excess of 500°F (260°C), contamination levels of 200,000 ppm CO2-2, soluble calcium over 7000 mg/l, and chlorides over 20,000 mg/l. Even at these high contamination levels, the current mud system maintains rheological stability as well as adequate filtration control.

After extensive laboratory testing of various high temperature fluids, the third generation system was selected as the most promising fluid to be used in California's geothermal drilling operations. Under actual field conditions, the fluid confirmed the performance shown in the laboratory tests, but much lower additive concentrations were required in field muds than in the laboratory tests.

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