THE GEOCHEMICAL EFFECTS OF INJECTION INTO GEOTHERMAL RESERVOIRS

The geochemical effects of injecting fluids into geothermal reservoirs are poorly understood and may be significantly underestimated. Decreased performance of injection wells, most likely caused by mineral deposition, has been documented in several geothermal fields after only 5 to 7 years of service. In order to mitigate the effects of mineral deposition in injection wells, the geochemistry and mineralogy of the deposits, as well as the fluid-rock interactions, must first be understood. 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 had declined. In both cases the fluids, which are injected at temperatures of 105-120^oC, are grossly supersaturated in silica with respect to quartz, the stable silica phase in the reservoirs. The Coso geothermal field is developed in Mesozoic granitic rocks of the Sierra Nevada Batholith on the western edge of the Basin and Range. Fluid temperatures prior to injection ranged from approximately 205-240^oC. White to grey dense opaline and porous botryoidal silica scale deposits have been observed in the redrilled Coso injection well 60-20RD, but not in the original well 68-20. The Salton Sea geothermal system is developed in Quaternary deltaic sandstones and shales of the Salton Trough. Scale deposits in injection well Elmore IW3 RD at the Salton Sea consist of dense red, orange and white vitreous opaline silica and metal sulfides. The injected fluids are hypersaline and metal-rich. Two-phase liquid-rich fluid inclusions trapped in euhedral calcite crystals from the injected interval at approximately 2300 meters depth have yielded homogenization temperatures of 344^oC and salinities of 21 to 25 weight percent NaCl + CaCl₂ equivalent. These inclusions record the pre-injection in-situ conditions.