The Geysers is a very seismically active area. Over 99% of seismic events at The Geysers are of magnitude 3.0 or smaller, referred to as “microseismic events” or microearthquakes. Most of these microseismic events are generally not felt by humans unless you happen to be very close by.
A network of closely spaced and very sensitive seismometers installed at The Geysers allows detection of micro seismicity down to a very low level. Naturally occurring conditions and small changes in stresses, such as those caused by local changes in reservoir temperature or pressures at depth, can cause small shifts in the rocks resulting in detectable microseismicity. The magnitude, location and depth of microseismic events are used by geoscientists and engineers to study the steam reservoir and its response to steam production and injection.
A direct relationship between water injection and microseismicity has been firmly established. However, the relationships of fieldwide steam production and water injection to stronger but less frequent seismic events are more complicated. Importantly, the frequency of seismic events greater than magnitude 3.0 has been trending downward since about 1990.
Calpine is continually studying The Geysers’ microseismicity and seeking ways to limit the frequency and reduce the impact of events. The most promising approaches to date involve a more uniform distribution of water injection, including injection rate reductions for particular areas, the addition of many low-rate injectors and relocation of water injection farther away from residential areas.
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