Inproceedings,
Seismic evidence for a long-lived Enhanced Geothermal System (EGS) in the northern Geysers reservoir
GRC Transactions, Geothermal Resources Council, 27, page 727--732. Davis, California, Geothermal Resources Council, (2003)
Abstract
Historic Geysers earthquake and injection data show an area of approximately
8 km2 underlain by a cluster of injection-induced microearthquakes
(MEQs) in the depth range 3-5 km below sea level. The cluster lies
far below the normal 240C isothermal reservoir and in the underlying
High Temperature Zone (HTZ), where temperature gradients can exceed
100C per km. This deep cluster has existed, more or less in its current
form, since 1976 or earlier. Above the cluster is a gap, 0.5 to 1
km thick, where few MEQs occur. Above the gap is a more typical pattern
of Geysers seismicity, including plumes of MEQs associated with injection
wells. A conceptual model shows that this pattern could be governed
largely by the temperature contrast between injected water and the
rock, and implies that significant volumes of injected water have
descended into the HTZ, reaching depth as great as 5 km bsl. In this
part of the field, there is also a long history of production of
flashed injectate (as evidenced by geochemical studies). Thus the
interpreted deep injectate flow through the HTZ may have contributed
significantly to the prolonged productivity of this part of the reservoir.
Overall, the results indicate that for much of the history of the
northern Geysers field, injection has created and maintained an Enhanced
Geothermal System (EGS) that has been making a valuable contribution
to production and sustainability. This success offers the promise
of expanded development, and redevelopment, of marginal field areas
which otherwise might be uneconomic.
