Reinjection experiments in the strongly fluid-depleted reservoir of Larderello have revealed the possibility of increasing production rates and overall heat extraction by injection into high permeability, low pressure zones of the reservoir (Giovannoni et al., 1981; Cappetti et a]., 1983; Bertrami et al., 1985). A large fraction (over 80%) of the injected water was recovered as steam in the most favorable area and, despite the short distance between injection and producing wells (the minimum distance being about 150 m), no significant temperature change has been observed in the latter, after 3 years of injection at a rate ranging from 10 to 50 kg/s (Bertrami et al., 1985). The physical processes involved in cold water injection into a ģsuperheatedī fractured reservoir are not yet fully understood, and this insufficient knowledge of the fundamental mechanisms limits the possibility of forecasting future reservoir behavior and optimizing the heat extraction process. Numerical simulation can be a very effective tool in the study of the complex phenomena involved, allowing a rapid examination of different situations and conditions, a systematic investigation of the effects of various parameters on reservoir performance, and some insight into long term behavior. We have performed simulation experiments on simple one-dimensional, porous and fractured reservoir models in order to study the migration of injected water, thermodynamic conditions in the boiling zone, heat extraction, and vapor generation. A two-dimensional radial porous medium model, with some characteristics typical of the high productivity zones of Larderello, has, also been applied for studying the- evolution. of the shape and the thermodynamic conditions of the injection plume in the presence of gravity, reservoir heterogeneities and anisotropy.

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