EFFECTS OF INJECTION INTO THE HIGH-TEMPERATURE RESERVOIROF THE NW GEYSERS - A CAUTIONARY TALE

Abstract

The energy contained in the high-temperature reservoir (HTR) of the NW part of The Geysers (Sonoma Co. Calif.) is large but underexploited due to
high gas and corrosive chloride concentrations in produced steam. Although chloride can be mitigated, the gas contents exceed the rating of available power plants. As a result, most current exploitation of the HTR is limited to wells that produce steam from both the HTR and the overlying normal reservoir to form a mixture with a moderate gas content.

Injection of water into the HTR potentially could eliminate high gas in the steam as well as increasing production rates. These effects would result from dilution of gas by near gas-free evaporated injectate, and increase of pressure resulting from vaporization of injectate at high temperatures. We foresee only beneficial effects of injection into an isolated, unexploited HTR, but wish to caution that the effects of injection into an exploited HTR or into connected reservoirs may have some unfavorable aspects. The zone of increased pressure (produced by injection into a HTR) would extend more rapidly than the zone of reduced gas contents and, in an exploited reservoir
with steam production from both the high-temperature and the normal reservoirs, the injection could temporarily raise the gas content in the mixed steam to unusable concentrations.