Geological Setting of Reinjection Wells in the Otake and the Hatchobaru Geothermal Field, Japan

Abstract

The reinjection wells at Otake and Hatchobaru in Japan have been studied by geology and alteration mineralogy. At Otake, all reinjection wells met a fault plane at depths from 300 to 500 m, corresponding to the depth of the main production zone. The reinjection of 70°C water had the beneficial effect to increase the vapor output in some production wells, but a productive well nearby the fault was damaged. At Hatchobaru, both reinjection and production wells encountered at about 1,000 m deep the same unconformity which was the main reservoir. The reinjection more or less affected the cooling of all production wells. These facts suggest that the communication between the two kinds of well through these permeable zones should be avoided. In the case where both wells are drilled in the same formation (whose permeability is smaller than 2 x 10-³ cm/sec as in Otake), the minimum acceptable distance between reinjection and production wells will be 150 m, as an approximate standard. In conclusion, important factors concerning reinjection are summarized.