Pressure Interference Tests at the Oguni Geothermal Field, Northern Kyushu, Japan

Abstract

The subsurface stratigraphy in the Oguni geothermal field (northwestern Hohi geothermal region) consists of a sequence of indurated sediments and volcanics overlying a granitic basement. The Hohi formation and the upper part of the Shishimuta formation constitute the principal geothermal aquifers. The feedzone pressures indicate that the northwestern Hohi region consists of two pressure zones, i.e., a high-pressure zone in the southern part of the Oguni geothermal field, and a low-pressure zone in the central and northern parts of the northwestern Hohi area.. To delineate the permeability structure for the Oguni Geothermal Field, Electric Power Development Co., Ltd. performed numerous pressure transient tests. Analyses of pressure interference data indicate that the low-pressure zone has a transmissivity of 100ñ250 darcy-meters. In contrast to the high transmissivity of the low-pressure zone, the high-pressure zone has only a modest transmissivity. The pressure interference data are consistent with the presence of one or more no-flux boundaries between the low- and high-pressure zones.