Impact of injection on reservoir performance in the NCPA steam field at The Geysers

A managed injection program implemented by the NCPA in The Southeast Geysers reservoir continues to positively impact reservoir performance. Injection effects are determined by the application of geochemical and geophysical techniques to track the movement of injectate. This information, when integrated with reservoir pressure, flowrate, and thermodynamic data, is used to quantify the overall performance and efficiency of the injection program. Data analysis indicates that injected water is boiling near the injection wells, without deeper migration, and is recovered as superheated steam from nearby production wells. Injection derived steam (IDS) currently accounts for 25 to 35 percent of total production in the NCPA steamfield. Most importantly, 80 to 100% of the injectate is flashing and being recovered as steam. The amount of IDS has increased since 1988 due to both a change in injection strategy and a drying out of the reservoir. However, significant areas of the reservoir still remain relatively unaffected by injection because of the limited amount of injectate presently available. That the reservoir has been positively impacted in the injection areas is evidenced by a decrease in the rate of pressure decline from 1989 through 1992. Correspondingly, there has been a reduction in the rate of steam flow decline in the areas' production wells. Conversely, little evidence of reservoir cooling or thermal breakthrough is shown even in areas where IDS accounts for 80 percent or more of production. Finally, since injection water is a relatively low-gas source of steam, noncondensible gas concentrations have been reduced in some steam wells located within the injection dominated areas