Injection and Energy Recovery in Fractured Geothermal Reservoirs

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Abstract
Numerical studies of the effects of injection on the behavior of production wells completed in fractured two-phase geothermal reservoirs are presented. In these studies the multiple-interacting-continua (MINC) method is employed for the modeling of idealized fractured reservoirs. Simulations are carried out for a five-spot well pattern with various well spacings, fracture spacings, and pattern with various well spacings, fracture spacings, and injection fractions. The production rates from the wells are calculated using a deliverability model. The results of the studies show that injection into two-phase fractured reservoirs increases flow rates and decreases enthalpies of producing wells. These two effects offset each other so that injection tends to have small effects on the usable energy output of production wells in the short term. However, if a sufficiently large fraction of the produced fluids is injected, the fracture system may become liquid-filled and an increased steam rate is obtained. Our studies show that injection greatly increases the long-term energy output from wells because it helps extract heat from the reservoir rocks. If a high fraction of the produced fluids is injected, the ultimate energy recovery will increase many-fold. Introduction At present, reinjection of geothermal brines is employed or being considered at most high-temperature geothermal fields under development. At many geothermal fields, primarily those in the U.S. or Japan, reinjection is a primarily those in the U.S. or Japan, reinjection is a necessity because environmental considerations do not permit surface disposal of the brines (unacceptable permit surface disposal of the brines (unacceptable concentrations of toxic minerals). At other fields (e.g., The Geysers, CA) reinjection is used for reservoir management to help maintain reservoir pressures and to enhance energy recovery from the reservoir rocks. The effectiveness of injection in maintaining reservoir pressures has been illustrated at the Ahuachapan geothermal field in El Salvador. During the last decade various investigators have studied the effects of injection on pressures and overall energy recovery from geothermal fields. Theoretical studies have been carried out by Kasameyer and Schroeder, Lippmann et al., O'Sullivan wad Pruess, Schroeder et al., and Pruess, among others. Site-specific studies were reported by Morris and Campbell on East Mesa, CA; Schroeder et al. and Giovannoni et al. on Larderello, Italy; Bodvarsson et al. on Baca, NM; Tsang et al. on Cerro Prieto, Mexico; and Jonsson and Pruess et al. on Krafla, Iceland. These studies have given valuable insight into physical processes and reservoir response during injection. However, there is limited understanding of injection effects in fractured reservoirs, especially high-temperature, two-phase systems. Fundamental studies and quantitative results for the design of injection programs in such systems are greedy needed. The objectives of the present work are to investigate the effects of injection on the behavior of fractured two-phase reservoirs. Several questions will be addressed.How will injection affect flow rates and enthalpies of the production wans?Can injection increase the short-term usable energy output of well?What are the long-term effects of injection?How is the efficiency of injection dependent on factors such as well spacing and fracture spacing? Reliable answers to these questions should be valuable for field operators in the design of injection systems for two-phase fractured reservoirs. Approach In the present work we consider wells arranged in a five-spot pattern (Fig. 1). Because of symmetry we only need to model one-eighth of a basic element as shown in Fig. 1; however, our results always are presented for the full five spot. The "primary" (porous medium) mesh shown in Fig. 1 consists of 38 elements; some of the smaller ones close to the wells are not shown. The mesh has a single layer, so that gravity effects are neglected. The fractured reservoir calculations are carried out by the MINC method, which is a generalization of the double-porosity concept introduced by Barenblatt et al. and Warren and Root. The basic reservoir model consists of rectangular matrix blocks bounded by three sets of orthogonal infinite fractures of equal aperture b and spacing D (Fig. 2a. M the mathematical formulation the fractures with high transport and low storage capacity are combined into one continuum and the low-permeability, high storativity matrix blocks into another. The MINC method treats transient flow of fluid (steam and/or water) and heat between the two continua by means of numerical methods. Resolution of the pressure and temperature gradients at the matrix/fracture interface is achieved by partitioning of the matrix blocks into a series of interacting partitioning of the matrix blocks into a series of interacting continua. SPEJ P. 303

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