Numerical modeling of the Ogiri geothermal system, Kyushu, Japan

A three-dimensional numerical reservoir model of the Ogiri geothermal field was developed through natural-state simulation and history matching using the code TOUGH2 (Pruess et al., 1999). The conceptual model of the Ogiri field shows that the geothermal system consists of a two-phase upper reservoir overlying a hot-water reservoir. Therefore, it was necessary to develop a numerical reservoir model that can represent the two region system. The constructed model was characterized by the presence of a high permeable zone that represents the main production zone of Ginyu fault and of the probable recharge zone at depth located in the eastern part of the field. In the natural-state simulation, the observed and calculated temperature profiles and pressures were matched well, confirming the validity of the conceptual model and providing initial conditions for the subsequent history matching simulations. In the exploitation modeling, a good agreement was obtained between simulated and measured temperature and pressure histories. Moreover, the constructed model reproduced the measured enthalpy histories in the 13 production wells reasonably well. Finally, the amounts of steam and internal energy in the geothermal reservoir system were evaluated quantitatively in both the preexploitation and exploitation stages and the causes of the depletion of the Ogiri geothermal reservoir were analyzed based on our modeling study.