The project "Deep-Seated Geothermal Resources Survey" has been performed by the New Energy and Industry Technology Development Organization (NEDO). This project investigated the development of deep geothermal resources. As part of this project we have investigated the feasibility of developing numerical models of deep-seated geothermal reservoirs. Of particular interest are the conditions beneath the explored area of existing reservoirs. The numerical models match not only temperature and pressure measurements but also a number of chemical and geological conditions of the developed reservoir. These conditions appear to be more sensitive to the deep reservoir properties than temperature and pressure.
For this work, we have used a version of TOUGH2 (Pruess, 1991) that has been modified to include the transport of reacting chemicals (White, 1995). We model a simplified conceptual model of the Kakkonda geothermal field which is located northern part of Japan. Exploratory drilling beneath the produced area of the field found temperatures that exceeded 350oC.
Firstly a model is developed with thepermeabilities and geothermal inflow adjusted to provide a good match to temperature data taken from the shallow part of the reservoir. Next we add sources of volcanic gases (H2S, CO2, HCl) at the top of the two-phase area that forms over the intrusion. Finally we calculate reservoir chemistry and adjust gas source strength to match reservoir chemical conditions. A model with permeability of the order of 10-16m2 to a depth of about 3km is capable of representing the observed hydrothermal alteration reservoir chemistry and temperature distribution. A reasonable match to measured isograds of alteration such as lumontite, wairakeite was obtained. The key to matching alteration is the existence of a source of magmatic vapor. Reservoir chemistry is sensitive to the distance of this source from the observed reservoir.

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