Inproceedings,
Effect of high rate injection on seismicity in The Geysers
GRC Transactions, Geothermal Resources Council, 34, page 1203--1208. Davis, California, Geothermal Resources Council, (2010)
Abstract
Projects to massively augment Geysers injection in first the southeast
(1997) and later (2003) the northwest Geysers have had very different
effects on induced seismicity based on a comparison of two 9.6 km2
study areas. Each project startup resulted the following year in
a new all-time peak in fieldwide injection and correlated with a
new all-time peak in the number of M>=1.2 earthquakes. With the startup
of the Southeast Geysers Effluent Pipeline (SEGEP) project in late
1997, injection rates approximately doubled in the southeast Geysers.
In the southeast study area (SESA) a gradual increase from about
five to 20 events per month occurred over the ensuing five year period
and has sub- sequently declined to about 10 events per month. No
correlation was seen between annual injection peaks and the frequency
of M>=1.2 events. In the northwest Geysers a high temperature reservoir
(HTR) up to 360 deg C underlies the normal 240 deg C reservoir and
heavily influences seismic response to injection. Seismicity extends
deep into the HTR. Annual winter peaks in injection have for decades
been followed a few months later by peaks in seismicity as measured
by the monthly count of M>=1.2 earthquakes. An approximate tripling
of injection rate in the Northwest Study Area (NWSA) in late 2003
by startup of the Santa Rosa Geysers Recharge Project (SRGRP) was
followed by a commensurate increase in seismicity. Since 2007 NWSA
annual injection peaks are not followed by peaks in seismicity, even
though peak injection rates have remained high and relatively constant.
Moreover, since 2007 the monthly M>=1.2 count has fallen dramatically
and apparently is no longer influenced by injection rates. This may
have implications regarding the current state of the HTR in the NWSA.
In the northwest Geysers annual peaks in injection induced seismicity
may be related to the amount of heat loss from reservoir rock, particularly
in the HTR. During peak (i.e. winter) injection, the volume of saturated
fracture porosity expands as the water front rapidly moves outward
into hot, dry rock fractures. While the water front is expanding
the saturated volume of rock, most heat flow from the rock is absorbed
in raising the temperature of the water to the boiling point. Boiling
is inhibited as the expand- ing water front results in generally
higher hydrostatic pressures and cooler water temperatures within
the saturated volume. At the outset of the dry season lower injection
rates result in lower hydrostatic pressures and an increased rate
of injectate heating and boiling. The water front stabilizes, then
begins to recede as a result of boiling. Eventually, the rock is
sufficiently cooled from heating and boiling of injectate that thermal
contraction increasingly triggers microearthquakes (MEQs). The üncoupling"
in the NWSA of annual peaks in injection and seismicity may indicate
that separate injection water plumes have begun to coalesce, thereby
deactivating the triggering mechanism.
