American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Abstract In order to achieve a better understanding of the Wairakei geothermal reservoir and of others of similar type, the current investigation considers the problem of a hypothetical geothermal aquifer that depletes the fluid saturating its upper regions during production. This depletion leads to the characteristic depressurization and drawdown of the liquid surface observed at Wairakei. This effect is significant in that the subsequent appearance of a horizontal pressure gradient may cause the premature shutdown of the bore field due to the premature shutdown of the bore field due to the encroachment of colder ground water. 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 pressure equilibrium and its output at that time. Problems associated with the numerical procedures are considerable even though the 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. Introduction The numerical investigation of geothermal reservoirs has been performed on one of two levels - first, attempts to model exactly all the geological properties and fluid and thermo-dynamic behavior of the system or, second, fundamental models that include only the effect deemed significant. Although some success has been made with the first approach, for example, the comprehensive, two-phase, rock matrix interaction model of Garg et al. and the simpler model of Mercer and Faust, the current investigation favors the second approach. There are advantages in avoiding a full simulation since little information concerning the physical system is available from below the level of the well bases, so that it is only possible to represent conditions in the upper possible to represent conditions in the upper reservoir.