| Abstract |
In order to maintain the rated power output of a geothermal power plant, it is required to keep reservoir pressure and temperature constant. This is however difficult because both parameters usually change due to the interference between wells and to the breakthrough of reinjected water into production zones. Reservoir monitoring and numerical simulation are therefore indispensable to keep track of the actual operation condition of the reservoir and to forecast the tendency of future changes in its properties. In fact it is only through reservoir simulation that it is possible to analyze different scenarios to efficiently maintain the rated power output, and to optimize the position of drilling targets and the operation of production and reinjection wells. The first step is to construct a precise numerical model that satisfactorily explains the tendency of the past and present variations of pressure, temperature, and fluid behavior. Then, following steps are addressed to predict future changes of power output by coupling reservoir and wellbore simulators. The results of analyzing different exploitation schemes can provide the optimum well position that will prevent overproduction and/or cooling of production zones. An integrated reservoir management based on reservoir monitoring techniques and reservoir simulation has been conducted to maintain the rated power output of 110 MW at the Hatchobaru geothermal power plant which is located in northern part of Kyushu Island, Japan. The potential of the reservoirs in Hatchobaru has been studied by means of numerical simulation. A three-dimensional reservoir model was constructed and progressively updated. Now, it is capable of reproducing changes in pressure, temperature, gravity and tracers behavior simultaneously, which are recorded during the reservoir monitoring and periodical well tests. Numerical simulation showed that the reservoirs in Hatchobaru are capable of sustaining power facilities of the capacity around 120 MW. Therefore, the present exploitation is equivalent to the 92% of this power potential. However, in the case of no future make-up drilling, the power output is predicted to drop to 80 MW within 5 years. It was disclosed that the reason for this is the current overexploitation of one of the main productive faults. A scenario to maintain the rated power output was devised. This scenario permitted the optimization of future production and reinjection targets taking into consideration the power potentials of each productive fault. In this paper, studies of optimizing the scenario of power maintenance in the Hatchobaru geothermal power plant are introduced. |