Physical Characteristics of the Ohnuma Geothermal Reservoir Estimated by a Lumped Parameter Model of the Fluid Chemical Changes

Abstract

The authors have been investigating a history of the physico-chemical condition of the Ohnuma geothermal field during 20 years since the start of operation. Concentration of CI and dilution of tracer, I, are characteristic in its history. Several geophysical parameters of the geothermal reservoir are estimated by simulating it, based on a lumped parameter model, to fit to the observed chemical data. Equations required for this model are energy and mass balances. In this model, a part of reinjected hot water and fluid from the exterior of the reservoir are mixed with the geothermal fluid in the reservoir, and the reinjected water is heated up during its residence in the reservoir by heat extraction from the ambient hot rock.
The results of estimation are summarized as follows;
1) Mean residence time of fluid in the reservoir is approximately 1 year.
2) Effective porosity of the reservoir is about 1%.
3) 75% of the reinjected water is discharged again through production well.
4) Heat extraction from the reservoir rock to fluid is effective to reach 60% of rock mass.
5) Temperature of water reinjected to the reservoir rises more than 100°C during residence in the reservoir.
6) Average reservoir permeability is approximately 0.069 kg/(s.kPa) as in-flow index.