Most recent interest in the injection of cold water into a geothermal reservoir has been related to the disposal of geothermal brines. Injection also offers the potential benefit of prolonging the useful life of a vapor-dominated system by providing additional water to extract energy out of the rock matrix. In a liquid-dominated reservoir injection may help to maintain pressures near the production wells by pushing the hot water toward them and preventing too much local boiling. Pressure maintenance can also be achieved for superheated steam zones, because injection will cause pressures to increase towards the saturation pressure (Schroeder et al. (1980)).

The general physical principles governing these processes are understood but no quantitative informat ion is available. The present work is aimed at helping to improve the qualitative and quantitative understanding of injection into a geothermal reservoir by considering a few idealized problems. First a vapor-dominated, single layer reservoir is considered, next a vapor-dominated, four layer reservoir, and finally a liquid-dominated, single layer reservoir. In each case varying injection rates are considered and in some cases the injection is changed at different times.

The SHAFT79 simulator ( see Pruess and Schroeder (1979) for example) is used to calculate there servoir behavior in each case. It is only with the advent of efficient geothermal reservoir simulators, such as SHAFT79 and other codes ( fee Coats (1977), Faust and Mercer (1979) and Brownell et al. (1975), for example), that it is possible to calculate the behavior of a two-phase reservoir during injection. The condensation of steam and the movement of thermal and hydrodynamic fronts through the reservoir as a cold zone around an injection well expands are severe tests of the capabilities of a simulator and are very difficult phenomena to model accurately. Previous work by the authors (0í Sullivan and Pruess (1980), Schroeder et al. (1980)) has demonstrated the accuracy of SHAFT79 in modeling injection problems.

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