Analysis of cause and mechanism for injection-induced seismicityat the Geysers Geothermal Field, California Page: 3 of 9
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used for the analysis of seasonal injection cycles. We emphasize that the two-dimensional cross-
section model and the production and injection rates we are using are not meant to be a precise
model of the Geysers system, but rather an analog model capable of representing fundamental
processes of THM coupling.
ANALYSIS OF 44 YEARS OF PRODUCTION/INJECTION
The simulation of 44 years of steam-production and injection resulted in a reservoir-wide
pressure and temperature decline of a few MPa and a few degrees, respectively, as well as
subsidence of about 0.5 to 1 meter. These numbers are in general agreement with field
observations at the Geysers (Mossop and Segall,1997). In the simulations, a rock-mass bulk
modulus of 3 GPa was adopted, which approximately corresponds to values back-calculated by
Mossop and Segall (1997), using analytical strain analyses. The thermal expansion coefficient
was set to 3x105 Co, which corresponds to values determined on core samples of the reservoir
rock at high (250 C) temperature (Mossop and Segall, 1997).
Figure 3 shows calculated liquid saturation and changes in fluid pressure and temperature after
44 years of production/injection. Figure 3a shows that the injection caused formation of a wet
zone that extends all the way to the production well as well as downwards, about 1,000 m below
the injection well. Figure 3c indicates a local cooling effect wherever the water flows, especially
where the liquid reaches the production well. The injection has a significant effect on the fluid
pressure at depths towards the bottom of the model, where pressure depletion is prevented
(Figure 3b).
Figures 4a and b depict changes in vertical and horizontal effective stresses, respectively. The
stress change in the rock mass is caused by both production-induced depletion and injection-
induced cooling. The depletion and cooling causes a general shrinkage of the reservoir, which in
turn give rise to increased horizontal stresses near the ground surface (Figure 4a). The main
effect of water injection is a reduction of vertical effective stress within the zone of cooling. The
cooling shrinkage near the wells is stronger in the vertical direction because the zone of cooling
is elongated vertically.
Figure 4c shows the calculated distribution of failure potential, which is represented by the
parameter Aa'im = 46'1 - A4'1 described above. In Figure 4c, red and yellow contours are the
zones that are most prone to failure, whereas blue contours are zones that are least prone to
failure. The figure indicates that failure (and induced seismicity) caused by production/injection
would occur both near the ground surface and close to the wells, and at depth below the wells
(Figure 4c).
ANALYSIS OF SEASONAL INJECTION CYCLES
We analyzed the effects of seasonal injection cycles corresponding to 2005 production/injection
rates. Our initial conditions are those achieved at the end of the 44-year simulation period, from
1960 to 2004. Thus, in this case we study mechanical changes that occur during 12 months with
respect to the mechanical state at the end of December 2004.
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Rutqvist, Jonny & Oldenburg, Curtis. Analysis of cause and mechanism for injection-induced seismicityat the Geysers Geothermal Field, California, article, June 14, 2007; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc900043/m1/3/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.