Land subsidence, sometimes observed during oil field production, is potentially a serious problem in geothermal energy production, particularly of liquid-dominated hydrothermal and geopressured fields. In the Imperial Valley--one of the most promising liquid-dominated geothermal regions in the United States--extensive subsidence could damage irrigation canals and other surface structures. Even if the subsidence is confined to the production area, special measures may be necessary to protect the geothermal extraction and electrical generating equipment. Subsidence is, in general, caused by the compaction of the semi-consolidated and unconsolidated strata of the reservoir as the effective overburden stress is increalsed due to fluid withdrawal. In some oil fields (e.g., the Wilmington oil field in the Los Angeles basin), injection of water into the formation has been successfully employed to reduce subsidence. Subsidence is potentially a more serious threat in geothermal production due to the much larger volume of fluid required to produce a given amount of energy. undoubtedly useful, is not a universal remedy to subsidence, for several reasons. First, while some of the compaction is elastic and may be recovered, it is well known that irreversible pore collapse (permanent dleformation) also accompanies fluid withdrawal. Second, due to the nature of the electrical generating process, only a fraction of the produced fluids will be available for reinjection; the reinjection fluid may, of course, be aug,mented by surface water to make up the volume deficit. Third, reinjection (especially of concentrated brines which are characteristic of some Imperial Valley geothermal anomalies) may not always be practical at (or near) the same horizontal and vertical location as production. Reinjection at a sufficient lateral distance from the producing well may result in uneven surface displacement. more, subsurface fluid injection may, by increasing pore pressures, tend to increase levels of seismic activity. Many geothermal reservoirs (including those in the Imperial Valley and the geopressured systems in the Gulf Coast) lie in regions of extensive faulting-- thus, the danger of earthquake triggering cannot be discounted.

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