Model Studies of the Depletion of Two-Phase Geothermal Reservoirs

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Abstract
The simulator SHAFT79 of Lawrence Berkeley Laboratory has been used to study the depletion of different types of geothermal reservoirs. Investigations of idealized systems include effects of gravity and fluid injection. Pressure decline is analyzed as a function of cumulative production. The main conclusions are as follows. 1. The well-known p/Z method for estimating fluid reserves is not applicable to two-phase geothermal reservoirs. 2. There is a strong tendency toward spatially uniform boiling. This causes a pressure decline, which allows in many cases estimates of the total reservoir volume and of the total heat content of the reservoir rock. 3. Propagation of a boiling front through a deep water table, as a consequence of fluid production, gives rise to a peculiar pattern of pressure decline. This may allow prediction of the distance of the water table from the producing wells and of the vertical thickness of the water zone, thereby giving important clues to estimating fluid reserves. 4. The pressure effects of injection of colder fluid depend strongly on (one- or two-) phase conditions in the reservoir, injection rate, and absolute permeability. Average pressure actually may decline in two-phase reservoirs rather than increase as a result of injection. Introduction There are only a few geothermal areas in the world where a significant portion of the in-situ reservoir volume is believed to contain vapor instead of liquid. Although the liquid-dominated systems are far more prevalent, the vapor-dominated systems are more readily exploitable and currently provide the major source of steam for geothermal electrical power. Several studies have shown that most of the mass reserves of vapor-dominated reservoirs consist of liquid water. This usually is concluded on the basis of the cumulative production of steam and the corresponding pore volume that would be required if the reservoir fluid were saturated or superheated steam. The major uncertainty is not whether liquid exists in vapor-dominated systems but whether there are significant amounts dispersed throughout the reservoir. This, of course, will be site specific to some extent. With respect to the mass reserves in place, vapor-dominated reservoirs more appropriately should be termed "two-phase" reservoirs. Liquid-dominated systems eventually develop into two-phase reservoirs after significant production has taken place. This has occurred at Wairakei, Ahuachapan, and Cerro Prieto. Thus, the important fundamental questions to be answered about two-phase geothermal reservoir phenomenology are applicable to all hightemperature hydrothermal resources. This paper investigates, for a variety of idealized model reservoirs, the phenomena occurring during production from and injection into two-phase geothermal systems. Using numerical simulation and an analytically solvable lumped-parameter reservoir model, we study systems with uniform initial conditions as well as systems with steam/water interfaces. Our main objective is to identify the signature of reservoir characteristics in pressure decline curves for different production and injection strategies. SPEJ P. 280^

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