Geoscience Reference
In-Depth Information
seismicity is the fact that the hot subsurface rocks are significantly cooled by the injected
water, and the resulting thermal contraction reduces the confining pressures and allows the
local stresses to be released by limited movement on fracture surfaces.
The two strong motion recording instruments installed in 2003 near the neighboring
communities of Anderson Springs and Cobb commonly record moderate shaking, plus
about a dozen Mercalli VI (strong shaking) events each year (see also Chapter 1 for a
definition of the Mercalli scale). The one event of Mercalli VII intensity caused an average
acceleration of 21.0%g
3
at Anderson Springs and was related to a
M
3.03 seismic event
located at a depth of 4,750 feet only 1.2 miles west of the recording instrument.
The operators at The Geysers meet regularly with representatives of these two com-
munities, county government, federal and state regulatory agencies, the USGS, and the
Lawrence Berkeley National Laboratory to discuss the field operations and the recently
observed seismicity. Minor damage is occasionally caused by the induced seismicity at The
Geysers, generally as cracks to windows, drywalls, or tile walls or flooring in these communi-
ties. A system for receiving, reviewing, and approving such damage claims attributed to the
local seismicity was established 6 years ago, and the homeowners are reimbursed for their
costs to have the home damage repaired. To date these reimbursements for home repairs
total $81,000, and this system appears to be resulting in mutually satisfactory relationships.
Liquid-Dominated Geothermal Resources
In contrast to the development of the vapor-dominated geothermal resources, liquid-
dominated resources commonly use downhole pumps in the production wells to deliver
the thermal waters to surface facilities. Surface pumping facilities are needed to force the
injected waters back down into the reservoir. The liquid-dominated geothermal reservoirs
that have been commercially developed to produce electricity in the western United States
are listed in Table 3.1
(sources include the California Division of Oil, Gas and Geothermal
Resources [CDOGGR], the Nevada Commission on Mineral Resources, the Imperial Irri-
gation District, and various operators).
Several different methods are used to generate electricity in liquid-dominated geo-
thermal systems depending primarily on the temperature of the produced fluids; the flash
steam power cycle process and the binary cycle process are the most common (Figure 3.6).
The cause and extent of the induced seismicity related to the development of
liquid-dominated geothermal resources are different from those in the vapor-dominated
resources (Box 3.2). From the start of operations the amount of fluid produced from a
liquid-dominated reservoir is almost fully replaced by injection, which prevents a signifi-
3
ā%gā is motion measured as acceleration by an instrument, expressed as a percent of the acceleration of a falling object
due to gravity.
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