Imaging Injected Water in The Geysers Reservoir Using Microearthquake Data

Abstract

Based on proprietary and public data from the last 14 years I find good spatial and temporal correlation between injection and microearthquakes (MEQ) activity at the Geysers. Comparison with geochemical and steam production data suggests that MEQ clusters associated with injection wells form a rough three dimensional image of injected liquid in the formation. These images can help to track injected water, estimate reservoir thickness, and provide early warning of possible water breakthrough situations. The spatial correlation though not simple, can be seen in maps of seismicity deeper than 4000 ft subsea. Every injector or group of injectors can be associated with an MEQ cluster. Where the clusters extend far from the injectors, the producing wells tend to show the "heavy" isotopic signature of flashed injectate. Some of the "heavy" steam producers coincident with extended MEQ clusters are round in zones of higher reservoir steam pressure than the nearly injection wells. A simple explanation is that those MEQ's occur where injected water flows as a liquid, driven by hydraulic pressure or by gravity. Temporal correlations between injection and nearby seismicity are generally events. Based on visual inspection, lag times between changes in injection rate and seismicity typically range from days to weeks. The MEQ's appear to represent shear slip triggered along surfaces already stressed tectonically to near the failure point.