Heat, fractures and fluid are the fundamental elements of geothermal resources. From a different viewpoint, existence of natural hydrothermal convection to form convective geothermal resources, and fluid production from wells, are the important factors for geothermal development. The fractures play two different roles in geothermal development. The first contribution is for the onset of the natural convection in geothermal systems, and the second is for flow paths to connect wells to the reservoir for fluid production. Since the ascending velocity of natural convection in geothermal reservoirs is order of 10-9m/s, fractures of all sizes, from very small to very large permeabilities, contribute to the first role. However, in-flow velocity of single-phase liquid within fractures in the vicinity of the well face ranges from 10-1 to 101m/s for example. Thus, only fractures of very high permeability can contribute to the second role. We extend this discussion based on field data.
Gravity changes at 26 stations have been monitored since May 1991 to study reservoir behavior in the Takigami geothermal field, central Kyushu, Japan. The apparent gravity changes were observed and caused by the variation of the shallow ground water level. We removed the effect of shallow ground water level change on gravity by applying a statistical technique to obtain the gravity change due to the mass change associated with the production and reinjection of geothermal fluids. The multivariate regression analysis was used with the gravity and the precipitation data. As a result, we obtained a good correlation between them. It became possible to remove the effects caused by the ground water level change at each observation point by using the precipitation data and calculate the residual gravity variation. The increase of the residual gravity up to 20 microgals in the reinjection zone, and the decreases, up to 40 microgals in the production zone, were obtained after the commencement of the geothermal power plant. The rate of decrease was very high at the beginning of the production but it became smaller, implying that underground fluid flow reached to a new equilibrium state. These residual gravity changes reflect very well the changes in mass balance in the geothermal reservoir.
The characteristics of the tracer response are examined for the evaluation of geothermal reservoirs. In this study, we simulate the tracer response in a geothermal reservoir which has two planar dominant flow paths connected with a vertical flow path. Fluid flow and tracer concentration analyses are made with basic equations including the effect of the deformation of planar flow paths due to tectonic stresses and fluid pressure. Fluid pressure distributions, fluid velocities, and tracer responses are calculated numerically. From the results of simulation, the effects of the vertical distance between two planar flow paths and the distance between injection and production wells on the tracer response are discussed. By comparing with the results of single large dominant planar flow path, it is found that the maximum tracer concentrations at the production well are about 50% lower than those for the single planar flow path.