Numerical simulators developed for geothermal reservoir engineering applications generally only consider systems which are saturated with liquid water and/or steam. However, most geothermal fields are in hydraulic communication with shallow ground water aquifers having free surface !water level ), so that production or injection operations will cause movement of the surface, and of the a i r in the pore spaces above the water level. I n some geothermal fields the water level is located hundreds of meters below the surface !e.g. Olkaria, Kenya; Bjornsson, 1978), so that an extensive unsaturated zone is present. I n others the caprock may be very leaky or nonexistent [e.g., Klamath Falls, Oregon (Sammel, 1976); Cerro Prieto, Mexico; (Grant et al., 19841 in which case there is good hydraulic communication between the geothermal reservoir and the shallow unconfined aquifers. Thus, there is a need to explore the effect of shallow free-surface aquifers on reservoir behavior during production or injection operations. In a free-surface aquifer the water table moves depending upon the rate of recharge or discharge. This results in a high overall storativity ; typically two orders of magnitude higher than that of compressed liquid systems, but one or two orders of magnitude lower than t h at for liquid-steam reservoirs. As a consequence, various data analysis methods developed for compressed liquid aquifers (such as conventional well test analysis methods) are n o t applicable to aquifer with a free surface (Bodvarsson and Zais, 1982). We have developed a numerical simulator for the modeling of air-steam-water systems. paper we apply the simulator to various problems involving inject i o n into or production from a geothermal r e servoir in hydraulic communication with a shallow free-surface aquifer. First, we consider a one-dimensional column problem and study the water level movement during exploitation using different capillary pressure functions. Second, a two-dimensional radial model is used to study and compare reservoir depletion for cases with and without a free-surface aquifer. Finally, the contamination of a shallow freesurface aquifer due to cold water injection is in v e s t i q at e d. is to obtain an understanding of the response of are servoir in hydraulic communication with a unconfined aquifer during exploitation or injection and to determine under which circumstances convent i o n a l modeling techniques ! fully saturated systems) can be applied to such systems.

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