Page 247 - Geothermal Energy Renewable Energy and The Environment

236 Geothermal Energy: Renewable Energy and the Environment

7.0

6.0
Maximum
observed
5.0 event
Magnitude 4.0

3.0

2.0

1.0
0.001 0.01 0.1 1 10 100 1,000
2
Area (km )
FIGUre 12.5 The relationship between earthquake moment magnitude and rupture area (in km ), based on
2
the empirical relationship of Wells and Coppersmith (1994). The shaded area encloses the region that encom-
passes the majority of seismic and microseismic events observed in geothermal developments. The dashed
line indicates the largest observed earthquake at a geothermal site and its inferred rupture area.
On the basis of that research it may be possible to establish what additional research should be done
to minimize uncertainties and data deficiencies.
In conjunction with analysis of the stress field, the mechanical properties of the rock types in the
area should be evaluated. Such studies include an analysis of the orientation, mechanical proper-
ties, and mineralogy of fracture sets. Establishing the approximate values for μ and σ n effective in situ
f
provide a basis for estimating the likelihood of inducing rupture.
The third important effort for characterizing a site is a thorough analysis of the largest fault
or fracture features that have the potential to rupture. Establishing the location and size of these
features can help delineate the most advantageous location to minimize risk associated with a geo-
thermal installation.
The combination of quantitatively evaluating the stress field, rock properties, and potential
structures that could fail will provide an important and credible method for reducing seismic risk.
Concomitantly, it is important to consistently monitor microseismic activity before, during, and
after construction and operation activities have commenced in order to identify unusual activity that
may indicate unexpected effects. Often such effects can be mitigated by modification of the rate,
timing, and operational parameters for injection.

seismiciTy associaTed wiTh fluid exTracTion
Seismicity associated with extraction of geothermal fluids can be understood by considering the
influence pore pressure has on rock behavior. In areas of limited recharge, removal of a geothermal
fluid will change the in situ stress field by lowering the pore pressure in a restricted region. That
effect will locally reorient the principle stress directions. Rupture may occur if a fracture set exists
in the area that is oriented appropriately and μ is exceeded when the stress field changes orienta-
f
tion and the effects of changing pore pressure propagate through the local geological framework.
Mitigation of any potential negative impacts can be accomplished by following the same data col-
lection, analysis, and monitoring activities as described for injection of cool water.