The geochemical effects of injecting fluids into geothermal reservoirs are poorly understood and may be significantly underestimated. Decreased performance of injection wells after 5 to 7 years of injection has been documented in several geothermal fields. In this study, the effects of injecting flashed geothermal fluids into the Coso and Salton Sea geothermal fields, California are investigated by comparing drill cuttings from the original injection wells with samples from wells drilled on the same pads after injectivities in the original wells had declined. At Coso the fluids injected into 68-20 had silica contents up to 940 ppm and are grossly supersaturated in silica with respect to quartz, the stable silica phase in the reservoir. X-ray diffraction and scanning electron microscope analyses of the reservoir rock penetrated by redrilled injection well 68-20RD indicate that loss of injectivity in 68-20 was caused by the deposition of silica as opal-A accompanied by trace amounts of calcite near the well bore. As the scale deposits mature, the original 2 m spheres coalesce into larger spheres, up to 10 m in diameter and plate-like sheets. At the Salton Sea the fluids injected into Elmore IW3 RD are hypersaline and metal-rich but relatively silica poor. Scale deposits in the reservoir rocks near the injection well consist of layered barite and fluorite, accompanied by minor anhydrite, copper arsenic sulfides, and traces of amorphous silica.

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