The present situation of the geothermal energy developments in Japan is such that six geothermal power stations are being operated and one under construction. Although there are still many geothermal areas to be explored, it will take a long time to realize them. For such a geothermal field development, it is expected to develop a simulation model which can give rational data for planning strategies. With a view to it, the model should be able to simulate physical phenomena in the geothermal field. The system shown in this paper is composed mainly of two principal physical models; Reservoir Model and Well Model. The former is based on the mathematical description of fluid flow through porous media and the latter through vertical pipes. The final purpose of this study is to develop simulation models for optimizing the scheme of geothermal field development.

Introduction

The present situation of the geothermal energy development in Japan is such that six geothermal power stations are being operated and one under construction. Although there are still many geothermal areas to be explored, it will take a long time to realize them. Up to this day, the target of geothermal exploration has mainly been the areas determined by surface geological survey and the existing geothermal reservoirs are located not deeper than 1,500m depth. Recent geothermal energy development shows a trend from the study on vapor dominated or liquid dominated hydrothermal resources in shallow zones to that on hydrothermal resources in the deeper zones (3,000m - 4,000m depth). An exploration well of 3,000m depth class has been drilled in the northern part of Kyushu.

For such a geothermal field development, it is expected to develop a simulation model which can give rational data for planning strategies. In view of this, the model should be able to simulate physical phenomena through the fife of the geothermal field. The physical phenomena through the fife of the geothermal field. The development system is composed mainly of two principal physical models; Reservoir Model and Well Model. The former is based on the mathematical description of fluid flow through porous media and the latter through vertical pipes. The equations of mass balance and energy balance are solved simultaneously by numerical methods. The input data to this system is compiled in the exploration investigation stage. Static physical characteristics of objective reservoir and allocations and dimensions of production/ injection wells are provided in the models. The trial purpose of this study is to develop a simulation model for optimizing the scheme of geothermal field development.

Three-dimensional Reservoir Model

This model simulates the behaviors of fluid flow in a geothermal reservoir, assuming water influx and heat conduction from heat sources under the reservoir.

Basic Equations: The following three equations describe a system, where mass transfer and heat conduction occur.

Mass conservation equation

------------(1)

Energy balance equation

------------(2)

Equation of state (in case of water-steam equilibrium)

------------(3)

where, = =1

The boundary conditions for solving the above equations are= 0 nd = 0 for mass and heat flow, respectively. Both mass and heat production terms are also considered to account for water encroachment and heat flow through the boundaries.

Potential equilibrium and heat equilibrium are adopted for the Potential equilibrium and heat equilibrium are adopted for the initial conditions which in turn imply no mass flow and steady state heat flow.

Difference Equations: Equations 1 through 3 ar-e approximated by the finite difference equations as follows.

-----------(4)

-----------(5)

-----------(6)

The right hand sides of the eqs. 4 and 5 are expressed in terms of delta P using Z. The eqs. 4 and 5 can be written in the following matrix form with delta Sw, delta T and delta P as independent variables.

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