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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
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