In the course of the Geothermal Development Promotion Survey Projects of NEDO, eight wells have been drilled in the Akinomiya geothermal field located in the northern area of Japan. FMI logs were conducted at all of the wells and DTFs (Drilling-induced Tensile Fractures) were observed at all wells, except N9-AY-5 which is only 150 m deep. The objective of the present paper is to analyze the DTP data obtained from the boreholes in order to estimate stress fields and to interpret permeable fractures based on the stress fields. This is our first attempt to analyze DTF data observed at so many wells in one geothermal area and to study the variation of the stress field and the direction of permeable fractures in the area. We have applied an inversion technique to estimate the stress fields. The estimated stress fields in the Pre-Tertiary basement rock, whose depth was rather shallow, yielded a Normal faulting stress regime. On the other hand, the stress fields in the Torageyama formation and in the boundary between the Torageyama formation and the Pre-Tertiary basement rock, where the basement rock lays rather deep, yielded a Reverse faulting stress regime. Overall directions of the maximum principal stresses in the horizontal plane are approximately east-west and the minimum are approximately north-south. The stress field interpreted using all the DTF data in a combined analysis yielded a Normal faulting regime with the orientation of the stress axes at N 44.6° W, 78.8° NW for σ 1 (the maximum compressive principal stress), N 88.9° E, 7. 7° SW for σ2 (the intermediate compressive principal stress) and S 0.0° W, 8.0° S for σ3 (the minimum compressive principal stress). The average stress ratios for σ 1 σ2 and σ3 are 1.0 : 0.6 : 0.51. Permeable regions interpreted based on the stress field derived above were characterized by approximately east-west strike with 50° N and 70° S dip. The character of the permeable fractures is consistent with lost circulation structures detected by the PTS log conducted at N9-AY-3 and an investigation of faults surveyed on the surface in the Akinomiya geothermal field.
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