We investigated the dissolution behavier of silica glass under super-critical condition (400-500°C, 40-96 MPa, pure water) to simulate the deep geothermal reservoir. We focused on the dependence of pressure and temperature and presence of iron plate in the system.The results of experiment are as follows: (1) The silica glass dissolved at constant rate. The rate increased with pressure and temperature. Siconcentration was higher at lower temperature in 400-500°C, 60 MPa inverse of the rate. From calculated activation energy, the hydration of glass surface controled this system. (2) In the case of addition the iron plate in the system, the dissolution rate of silica glass increased at 500°C, 40-96 MPa than the case of without iron plate. In surface of iron plate, fayalite and magnetite were produced and Si concentration was same as the case of without iron plate. Therefore, increasing of dissolution rate of silica glass was due to consuming of aqueous silica by production of fayalite.
In the Hatchobaru geothermal area, gravity changes at 46 stations have been monitored since May 1990 to study the effectiveness of gravity monitoring technique whether it has good correlation to reservoir behavior or not. A clear coincidence can be recognized between gravity and pressure changes monitored by capilary tubes which set into several observation wells. From the tendency of gravity changes, the Hatchobaru geothermal area can be divided into three zones such as northern, central and southern zones. In the northern and central zones, the gravity changes showed steep increase just after when Hatchobaru No.2 unit commenced its commercial operation, which might be caused by sudden rein jection due to No.2 unit operation. In the southern part, gravity has decreased without any initial increase like northern and central zones, which indicates that reinjected water has not circulated upto the production layer located around southern zone. After 3 years, the tendency of gravity changes looks to be settled which may show that the reservoir is mostly balanced after about 90 MW production and recovers its quantity balance year by year.
Fractured reservoirs have received considerable attention over the last decade in geothermal reservoir engineering. Typical models for the analysis of pressure-transient test data generally rely on the 2D radial-flow model. However, a simple 2D model is often insufficient for the analysis of data from naturally fractured systems. It is required to consider complex boundary effects, double-porosity behavior and 1D and 3D models for the characterization of highly heterogeneous reservoirs. Recently, pressure-transient response of fracture networks with fractal structure was investigated by Chang and Yortsos (1990); and mathematical solutions for reservoirs of arbitrary dimensions were obtained. On the basis of their predictions, we interpreted falloff data from the injection tests of well YT-2 drilled in the Yutsubo geothermal field. Fractal structure is indicated by parallel linear plots of pressure and pressure derivative vs time on a log-log scale with a slope of 0.26. This fractal network of fractures (1.48<D<1.85) has radial structure around the well and is thought to be formed during cold water in-jection into a hot low-permeability formation.
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