ABSTRACT The Santa Rosa -Geysers Recharge Project (SRGRP) is a public-private collaboration that is bringing 42,000 m 3 per day of tertiary-treated municipal wastewater via a 65-km pipeline for injection at the 750-mw Calpine portion of the Geysers geothermal field. Since start-up in November 2003, over 11 million m 3 have been delivered and injected, as of August 31, 2004. This amounts to a 40% increase over pre-SRGRP injection rates. Reservoir modeling and experience with previous injection suggest that incremental steam production derived from the SRGRP injection will gradually increase and peak after three years at approximately 42% of the mass injection rate, yielding 85 gross mw, or 76 mw net of the 9 mw used to pump the wastewater to the injection wells. The benefit is calculated relative to the declining fieldwide production trend that would be expected without SRGRP. Early results are consistent with this projection, indicating a net benefit of approximately 16 mw after eight months of SRGRP injection (relative to the projected generation trend without SRGRP). Initial results from a tracer study showed recovery of 10% of the injected tritium slug within an eleven-week sample window. Analyses of non-condensible gases (NCG) in produced steam shows concentrations decreasing by as much as 70% in production wells in the high-NCG northwestern portion of the field. The project Environmental Impact Report (EIR) studied the possibility of increased seismicity induced by the increased injection. The study concluded that the effects on nearby residents would be "less than significant", because the induced seismicity is almost entirely in the form of microearthquakes that are detected by seismological instruments but not felt by people. After ten months of SRGRP operation, the results have been consistent with the EIR projections. Activity in the magnitude range 1.5 to 3.0 is up 29% compared with pre-SRGRP seismicity, but there has actually been a slight decrease in the occurrence rates of earthquakes of magnitude 3.0 and greater
