Hot dry rock geothermal energy extraction experiments in artificial reservoirs created by hydraulic stimulation in naturally fractured crystalline rocks have been undertaken in several countries over the last 20 years. The experiments have had mixed results in terms of fluid recovery, system impedance, and heat extraction. Numerical models have not yet delivered a generally agreed understanding of the processes and sensitivities involved in reservoir creation and circulation. In this paper a two‐dimensional fracture network model is described, which attempts to address the problems of both reservoir creation and circulation using rock mass characterization and in situ stress data as the primary inputs with a view to encapsulating our present understanding of how such systems work. The model is a prototype for a three‐dimensional version currently under development and is intended mainly for engineering sensitivity studies. The basis of the model lies in approximations of fracture mechanical behavior drawn from the rock mechanics literature, a very simplified analysis of the operative physical processes, and mapping of the connectivity of fracture networks to a fine resolution regular grid. Taken together, these permit the approximate resolution of what is normally a supercomputer problem on a personal computer. The model is applied to field data gathered at Fenton Hill, New Mexico; Hijiori, Yamagata Prefecture, Japan; and Rosemanowes, Cornwall, England.

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Abstract. A three-dimensional (3-D) stochastic network model for simulating a hot dry rock (HDR) or hot wet rock (HWR) engineered geothermal system formed in fractured crystalline rock is presented. The model addresses the problems of fracture network characterization from in situ field data, such as fracture orientation, size, spacing, and other mechanical properties. The model can simulate the changes that occur within the rock mass during stimulation (i.e., large volume fluid injection at pressures sufficient to allow shear slip on natural fractures). It can also be used to simulate steady state circulation of the heat exchange system thus created and includes provision for predicting tracer response curves and heat extraction history. The model has been applied to data gathered during the stimulation and circulation of a 2.2-km-deep HDR reservoir at Hijiori, Japan. The predicted shape of the stimulated and shear-propped fractures closely matched the distribution of seismic source distribution of acoustic emission (AE), regardless of realization of the fracture network, suggesting that the geometry of the stimulated volume can be robustly predicted from knowledge of the fracture population and in situ stresses. However, hydraulic behavior and tracer tests during the circulation could only be satisfactorily simultaneously reproduced by a small subset of realizations. These selected realizations, obtained by matching initial circulation and tracer data, are considered to give the best prospect of satisfactory long-term thermal modeling. The success in simultaneously modeling diverse data (hydraulic, microseismic, and tracer) lends confidence to the thermal predictions. The results indicate that a large improvement in the long-term thermal performance of the Hijiori reservoir could result from increasing well spacing from 100 to 150 rn without major degradation of the hydraulic performance.

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