Numerical Modeling of a Desaturating Geothermal Reservoir

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Abstract
To achieve a better understanding of hot-water geothermal reservoirs, the problem of a hypothetical geothermal aquifer that depletes the fluid saturating its upper regions during production is considered. This depletion leads to the characteristic depressurization and drawdown of the liquid surface observed typically at the Wairakei geothermal field in New Zealand. This effect is significant in that the subsequent appearance of a horizontal pressure gradient may cause premature shutdown of the bore field because of the encroachment of colder groundwater. In fact, the results of the modeling show a different behavior, emphasizing the vertical movement of the saturating hot fluid as a more important effect. The model gives an indication of the time the reservoir will take to reach pressure equilibrium, and its output at that time. Problems associated with the numerical procedures are considerable even though the model is single-phase and axisymmetric, and the value of such an intricate numerical model is tenuous. By using a simpler model, replacing the moving surface by an encroaching region of cold water, it is possible to obtain a lower bound for the useful life of the system and also to examine the possible effects of recharging the system.