Some injection wells at the Coso geothermal field, California have experienced decreases in injection rates after 5 to 7 years of service due to mineral deposition in fractures surrounding the wells. Scanning electron microscope images and X-ray diffraction analyses of the scale deposits from injection well 68-20RD show that the scale consists of spheres and plates of amorphous silica (opal-A) and traces of calcite.

To further assess the fluid-rock interactions, and their effects on well performance, geochemical modeling using the program TOUGHREACT has been initiated. A one-dimensional model was used to simulate injection into a fracture zone with 99% porosity in diorite host rock. The modeling predicts the deposition of amorphous silica as the dominant mineral precipitated, followed by minor calcite and a sharp decrease in porosity 1 to 2 years after initiation of injection. These predictions are consistent with the observations. Furthermore, the modeling predicts silica deposition will occur close to the wellbore, consistent with the observation that silica is found only in the redrilled well closest to the original injector.

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