A MODEL OF THE KAKKONDA GEOTHERMAL RESERVOIR

Reservoir pressures, temperatures and chloride concentrations are inte rpreted from well measurements in the Kakkonda geothermal field (Japan ), to obtain a natural state conceptual model and to determine changes in the reservoir subsequent to production and reinjection. In this mo del the system is viewed as two reservoirs, a shallow reservoir extend ing from the surface to about 800 m below sea level and, partially iso lated from this, a deep reservoir extending down to 3000 m below sea l evel. The modelling of the changes has taken two forms, one a lumped-p arameter mathematical model leading to an expression for the average r eservoir chloride concentration as a function of time, and the other a three-dimensional numerical simulation model. The mathematical model is fitted to pressure and chloride concentration changes. The numerica l model is fitted to the natural state pressures, temperatures and chl oride concentrations, and is verified by comparing the predicted varia tion of production enthalpy with the measured changes. The numerical m odel is also used to explore a simplified development scenario, involv ing a production/injection flow doublet, thus allowing a clearer inter pretation of thermal and chemical travel times across the reservoir.