Natural Convection System at the Ohnuma-Sumikawa Geothermal Field, Northeast Japan

Abstract

A number of wells were drilled, and downhole temperature and pressure surveys were conducted to identify the major feed point of geothermal fluid, and to determine the subsurface distribution of pressure and temperature in and around the developing area. The isothermal map reveals that the heat source in the area is probably seated beneath the volcanic chain. The pressure distribution provides important insights concerning the general characteristics of the naturalstate flow pattern in the area. Based on the subsurface isobars map, the natural-state convection system in the area is estimated as follows: (1) It is virtually certain that a vapor-dominated flow region is present in the southern part of the reservoir system at a depth extending from the lower part of the caprock. (2) The up-flow zone of the geothermal fluid is located in the southern part of the area. The down-flow zone is in the northern part of the developing area, as a whole, forming a large convection cell. A study of the natural-state flow pattern based on field data is being examined to design the most suitable field development scheme.