Theoretical Feasibility Studies of Reservoir Monitoring Using Geophysical Survey Techniques

Abstract

A steady-state three-dimensional model of a hypothetical (but realistic and typical) geothermal reservoir system was developed using a numerical reservoir simulator. The region covers 81 square kilometers and extends to three kilometers depth. Under natural-state conditions, a shallow water/steam region underlies the central part of the anomaly, but the reservoir is otherwise liquid-dominated. Next, a reservoir exploitation strategy was devised, and enough fluid was produced and injected to operate a 50 MW geothermal power station, causing underground pressures, temperatures and steam saturations to evolve. Then, various mathematical ìpostprocessorsî were applied to this computed 27-year reservoir history to appraise changes that might be observable using various surface measurement techniques. Methods examined include microgravity surveys, electrical resistivity surveys (both DC resistivity and MT surveys), self-potential (SP) surveys and seismic surveys. The results suggest that these techniques have considerable promise. By supplementing conventional data sets usually employed in reservoir engineering studies, they offer the potential to improve the quality of mathematical reservoir models, thereby permitting more accurate forecasts of reservoir performance, facilitating efficient reservoir management, and reducing the costs of geothermal electricity.