奥会津地熱フィールドにおける蒸気減衰対策

抄録

Although the electric power generation using geothermal energy is classified as renewable energy, in the process to reach some renewable level of production there is often a considerable decline in steam production, and it causes an economical problem at several geothermal power plants in the world, including Japan. In the Okuaizu geothermal field in Fukushima prefecture, The Yanaizu Nishiyama Geothermal Power plant of the Tohoku Electric Power Company started operating by 65MWe in 1995, and Okuaizu Geothermal Co., Ltd. produces the steam used by this operation. Despite the fact that the steam production decreased in 2000 soon after the production had started, which caused the power generation output to fall until 38MWe, it succeeded in recovering up to 45MWe by now. From now on, it is needed to take measures that do not let the electric power to go below 45MWe. On the whole, the production decline trend of the Okuaizu geothermal field as well as almost all the geothermal fields in the world is to be harmonic trend. However, since a complicated factor is involved in each geothermal well, in many cases, a decline trend cannot be approximated with exponential or harmonic function curve. That the natural decline curve accompanying the pressure fall of the whole reservoir, as well as the decline or the increase curve by temporary cause and the overlap that is peculiar to each well make prediction more difficult. The writer examined the decline curve of each well in the Okuaizu geothermal field and concluded that there are three typical curves in this field: (1) the parabola decline curve by the physical blockade in the well bore by carbonate or sulfate scale, (2) the sharply irregular trend by the physical blockade of the geothermal fluid passing in the reservoir by mud water and cuttings and (3) the harmonic decline curve accompanying the pressure fall of the whole reservoir. The measure for the case (1), which is the well bore stimulation using hydrochloric acid was done successfully. For the case (2), which is the well bore stimulation using river water, was done with fruitful result. Enhanced Geothermal Systems, which means injecting water into the reservoir, are going on of preliminary tests as the measure for the case (3); the return current is not recognized yet.