Numerical Modeling of the Onikobe Caldera Hydrothermal System, Northeastern Honshu, Japan

Abstract

An active geothermal system is present in the southern part of the Onikobe caldera. A numerical simulation study was carried out to characterize this high permeability reservoir. Our numerical model of the system in its natural state is based on mapped geology, fracture directions, fumarole locations, resistivity structure and drilling data. Repetitive calculations were carried out, varying the principal unknown parameters from one calculation to the next to optimize agreement between mathematical calculation and field observations. After a dozen long term simulations, a model consistent with measured subsurface temperature, pressure, and surface heat flow data was finally obtained. Next, we analyzed the sensitivity of the final mathematical reservoir model to perturbations in the formation permeability distribution. Varying the horizontal permeability in the fractured zones had little effect on the natural state model. When the model was used to simulate the effects of fluid production, however, the results were found to be sensitive to horizontal permeability.