Energy Technologies: How They Work and Their Induced Seismicity Potential - Induced Seismicity Potential in Energy Technologies - page 49

BOX 3.2
Induced Seismicity at the Coso Liquid-Dominated Geothermal Field
Locally induced seismicity recorded in the area of the Coso geothermal field development between
1996 and 2008 in map view (Figure 1, top) and cross section (Figure 1, bottom) shows clustering relative
to the location and depth of the geothermal wells shown in blue. The number of seismic events of magnitude
0.5 and greater is plotted; these events total 10,200.
The history of geothermal fluid (dominantly water) production, water injection, and recent seismic his-
tory at the Coso field from 1977 through 2011 is shown in Figure 2. Starting in 1987, annual production
reached a maximum of 121 billion lb* in 1990 and had decreased to 68 billion lb by 2009, while annual
injection has declined from a maximum of 80 billion lb to 27 billion lb (CDOGGR, 2011). The relatively low
reinjection rate for a liquid-dominated resource is because of cooling tower evaporative losses that result
from the produced fluids containing an increased steam fraction as reservoir pressures have declined over
the almost 25 years of operation.
Using the catalog of data available from the Southern California Earthquake Data Center, the history
of local seismicity at the Coso field from 1977 to 2009 is shown in Figure 2.
With reference to Figure 2, the number of events of M 1.5 and greater averaged 5 per year during
the 10 years prior to development, then doubled in the first 5 years after 1987, reaching maxima of 51 in
1995, 55 in 1998-1999, and 64 in 2001 before declining to a current level of about 20 per year. The peaks
in 1995 and in 1998-1999 were attributed by Bhattacharyya and Lees (2002) to triggering in response to
significant ( M > 5.0) nearby earthquakes at Ridgecrest and in the Coso range. Additionally, the number of
earthquakes of M 3.0 and greater is shown near the bottom of the chart. Single events occurred in 1978,
1995, 1998, 1999, and 2007, with three in 2009. The single earthquake in 2007 was a M 4.11 event, as
shown near the top of the chart.
*Note that where at least part of the production is in the form of steam as well as liquid water, “pounds” is needed
as the single unit to describe both the quantity of production and injection because gallons or cubic meters cannot be used
in reference to steam.
(Box continues)