Page 249 - Geothermal Energy Renewable Energy and The Environment

238 Geothermal Energy: Renewable Energy and the Environment

1000 Events / 100 m 3
290

2000 210

Depth (m) 3000 GPK-2 110
GPK-3

GPK-4
4000
10

5000

0 1000 2000 3000
Distance (m)
FIGUre 12.6 (See color insert following page 17.0..) Density contour map of the number of seismic events
associated with stimulation of the crystalline rock reservoir at the European Hot Dry Rock (EGS) project in
Soultz-sous-Forêts, France. The contour map reflects the results from two periods of hydrofracturing carried
out on the wells labeled GPK2 and GPK3. Also shown are individual events (dots) associated with hydrof-
racturing from GPK4. The other wells shown in the figure were used for seismic monitoring. (Modified from
Baria, R., R. Jung, R., Tischner, T., Nicholls, J., Michelet, S., Sanjuan, B., Soma, N., Asanuma, H., Dyer, B.,
and Garnish, J., Creation of an HDR Reservoir at 5000 m Depth at the European HDR Project, Proceedings of
the Thirty-First Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California.
SGP-TR-179, 2006.)

pressures. Analysis of these responses, along with thorough monitoring of the seismic behavior
after shut-in can provide guidelines for injection rates, volumes, and pressures that will minimize
seismic risk.

GroUnd sUbsIdence
Withdrawal of fluid from the subsurface without reinjection can impact the local hydrology and
subsurface stress regime. In locations where the rock framework has high strength, water in inter-
connected pores and fractures experiences little or no load from the rock. In such a case, the only
force acting on the water at depth is from the overlying water mass and the pressure is hydrostatic.
Such sites are usually composed of granite, gneiss, or other crystalline rock. Water removed from
such a location will have no discernible effect on the elevation of the land surface.
Rocks with low strength, and which therefore are compressible, will exert some force on water
in pores and fractures. Since water is incompressible it will exert a corresponding force on the
impinging rock that can be as high as the lithostatic pressure. Under such circumstances the water
becomes an intrinsic element in the subsurface structural framework. Rocks that behave this way
are usually unconsolidated sediments, porous volcanic rocks or rocks with a high clay content. If
water is removed from such a setting, the overlying rock mass will settle to some extent, depending
upon the amount of water removed and the compressibility of the rock. On the ground surface this
effect will be recognized as subsidence.