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Use of Geothermal Resources: Environmental Considerations 231
Table 12.1
brine concentrations of selected elements (from indicated
references) and Their estimated annual Value.
brine concentration
resource (mg/kg) Value (million $)
Silica (Si) 537 50.2
Lithium (Li) 327 248.2
Gold (Au) 0.08 0.66
Silver (Ag) 1.4 5.18
Manganese (Mn) 1560 48.8
Zinc (Zn) 790 7.7
Source: Entingh and Vimmerstedt (2005).
Note: Si: Henley, R. W., Truesdell, A. H., Barton, P. B., and Whitney, J. A.,
Society of Economic Geologists, Reviews in Economic Geology, Vol. 1,
1–267, 1984; Li: McKibben, M. A. and Hardie. L. A., Geochemistry of
Hydrothermal Ore Deposits, New York: Wiley, 877–935, 1997; Au:
Gallup, D., Ore Geology Reviews 12:225–36, 1998; Ag: Ellis, A. J., and
Mahon, W. A. J., Chemistry and Geothermal Systems. New York:
Academic Press, 1977; Mn and Zn: Skinner, B. J., White, D. E., Rose, H.
J., and Mays. R. E., Economic Geology, 62:316–30, 1967.
seIsmIcITy
Seismic activity associated with geothermal applications results from several effects: injection of
cool water into hot geothermal reservoirs, extraction of fluid from reservoirs, and high-pressure
injection of fluid to enhance reservoir permeability. The seismic events associated with these pro-
cesses are generally very small in magnitude. From a seismological perspective, the magnitude of
the vast majority of events is less than 2.0 and they usually are not felt. However, larger magnitude
events have been recorded. The largest event that was associated with geothermal power production
was a magnitude 4.6 at The Geysers field in California in 1982 (Peterson et al. 2004). To mitigate
the impact of seismicity for the use of geothermal energy, and to understand the associated risks, the
mechanics of energy release associated with earthquake events must be understood. The following
discussion addresses the fundamentals of the relevant rock mechanics issues. For detailed discus-
sions, see the source material recommended at the end of this chapter.
mechanics of seismic evenTs
shear stress, normal stress, and Frictional strength
As discussed in the Sidebar for Chapter 4, rock failure occurs when the internal strength of a rock is
exceeded by the stress to which it is subject. For real rocks in a geothermal setting, evaluating rock
strength and its relationship to local stresses can be complex. Most rocks in such a setting will possess
several sets of fractures, each with specific characteristics. Among the important characteristics will be
the length of the fractures, their roughness and planarity, the extent to which they have been cemented
by secondary minerals deposited by fluids migrating along the fractures, and their orientation. How
the rock responds to an imposed stress field will depend upon the interplay among these variables, as
well as the magnitude of the stress, its orientation, and the rate at which stress is applied.
The criterion for failure is based on the ratio of the shear stress, τ, to the normal stress σ . For our
n
purposes, we will define the frictional strength of a material as
μ f = |τ| / σ , (12.2)
n