In situ stress field and permeable fractures in the Akinomiya Geothermal Field, Japan

In the course of the Geothermal Development Promotion Survey Projects of NEDO (the New Energy and Industrial Technology Development Organization of Japan), eight wells have been drilled in the Akinomiya geothermal field in northern Japan. Formation microimaging (FMI) logs were conducted at all of the wells and drilling-induced tensile fracture (DTF) were observed at seven wells. The objective of the present paper is to analyze the DTF data obtained from the boreholes in order to estimate stress fields and to interpret permeable fractures based on the stress fields. An inversion technique estimates the stress fields. The estimated stress fields in the Pre-Tertiary basement rock yielded a normal faulting stress regime. However, the stress fields in the Torageyama formation and in the boundary between the Torageyama formation and the Pre-Tertiary basement rock yielded a reverse faulting stress regime.