Energy Technologies: How They Work and Their Induced Seismicity Potential - Induced Seismicity Potential in Energy Technologies - page 45

cant decline in reservoir pressure. The temperature difference between the produced and
reinjected waters is also relatively limited, so less cooling of the reservoir results. Conse-
quently, if the surface and resulting bottom-hole pressures in the injection wells are limited
to be less than that necessary to induce fracturing, little cause exists for the operations to
produce significant induced seismicity. Monitoring at many of the liquid-dominated geo-
thermal fields has demonstrated a relative lack of induced seismicity. However, as described
below, the Coso geothermal field began as a strictly liquid-dominated field and has evolved
during extended production to become partly vapor dominated. This evolution has resulted
in reduction in fluid replacement and has caused the introduction of induced seismic events.
The Coso geothermal field provides a well-documented example of a complex resource
area that was liquid dominated before the start of development 25 years ago and that may
have evolved, following extensive production, into a resource that is now in part vapor domi-
nated (see Box 3.2). Coso near Ridgecrest, in southeast-central California, is in a region
of recent volcanism that is also seismically active. The first commercial geothermal power
plant began operating in 1987; since 1989 three plants have been in operation with a total
generating capacity of 260 MW, with about 85 production and 20 injection wells currently
in use (CDOGGR, 2011). The geothermal fluids (dominantly water) are at temperatures
in excess of 300°C (572°F) at depths of 1.5-2 km (~0.9-1.2 miles) (Feng and Lees, 1998).
The areal coincidence of the local seismicity at Coso with local surface subsidence,
identified by using synthetic aperture radar data, suggest that the Coso field operations have
caused reservoir cooling and thermal contraction, resulting in induced seismicity (Fialko
and Simons, 2000). More recently, Kaven et al. (2011), based in part on their investigation
of local changes in seismic velocities (V p :V s ratios), attribute the induced seismicity at Coso
to decreases in fluid saturation and/or fluid pressure within the active geothermal reservoir.
An important issue to emphasize with regard to potential changes in pore pressure
at vapor- and liquid-dominated geothermal power plants is the selection of conversion
cycle—whether flash cycle or binary cycle (see Figures 3.2 and 3.6). The cycle selection is
determined by the temperature and nature (physical state) of the geothermal fluids produced
to the surface. Those power-cycle differences are important to explain why evaporative
losses are significant at vapor-dominated resource power plants and moderate at flash cycle
power plants. Evaporative losses can result in pore pressure and thermal losses that in turn
can result in significant or moderate levels of induced seismicity. Equally important is to
explain why in the case of binary cycle power plants there are no evaporative losses and
generally little if any loss of pore pressure or fluid temperature, and therefore little if any
associated induced seismicity.