SPATIAL RELATIONSHIPS BETWEEN DYNAMIC RESERVOIR CHARACTERISTICS AND INDUCED SEISMICITY IN THE NORTHERN GEYSERS, CALIFORNIA

摘要

The Geysers is the largest-volume geothermal injection operation in the world, with injection volumes in the northern section of the field exceeding a total of 1.93E+9 kilopounds of water over the history of the field and total steam production in excess of 2.07E+10 kilopounds. The northwest Geysers area contains several high-volume injection wells located at the southwest section of a ring of seismicity that coincides with ongoing injection and production activity. The highest-volume producers in the NW Geysers reside on one side of low seismicity region (LSR) opposite the highest volume injectors; this region is locally referred to as the "doughnut hole." In the absence of simulation results to infer fluid flow paths, a number of hypotheses for the low-seismicity region have been suggested, including (1) an absence of flow or diminished flow through the low-seismicity region (reduced pore pressure), (2) cooling due to high volumes of cold injectate that have moved through the region and the ensuing diminished contribution of thermal contraction-induced microearthquakes, (3) near-complete release of stress due to a history of brittle failure in regions affected by flow. This article explores the first two ideas in the form of long-term trends in temperature and pressure at numerous production wells in the region, as well as fluid volume change and the spatial comparison of seismicity with injection and production at a number of wells. The low-seismicity region contains very few wells whose producing temperature and pressure could be included in calculations, but data from those wells suggests that temperature is decreasing near a high-volume injector in the LSR this well, and that pressure is both increasing and decreasing near this well. Instances of decoupled temperature and pressure are common for the field, and manifest as constant-pressure temperature change in the P-vs-T curves of wells in and outside of the LSR. The maximum fluid volume change, defined as amount injected minus the amount produced, overlaps spatially with the high-volume injectors in the southern LSR, and the minimum volume change occurs west of the LSR and in the northern LSR, suggesting that water produced from these regions is supplied by more than the adjacent low-volume injectors. Additional information comes from double-difference tomographic inversion for seismic velocity and wellbore water level. The combination of these varied data suggest that fluid is accumulating in the LSR, and that the diminished appearance of seismicity in this region may be related to the presence of this fluid.