ドリリングインデューストフラクチャーによる原位置3次元地下応力場の評価

抄録

Feasibility was examined for estimating in-situ stresses by using the so-called drilling-induced tensile fracture (DTF) of a borehole. DTF is macroscopically a longitudinal crack consisting of many small parallel cracks which are oblique to the borehole axis, and is characterized by the circumferential position (θ_mN) along the borehole wall and the inclination (γ_m) of the small cracks with respect to the borehole axis. First, we studied the stress field induced by the stress concentration on the borehole wall due to a far stress field and examined the effect of factors such as the far stress field, the tensile strength of a surrounding rock and the Poisson's ratio, on the characteristics (θ_mN, γ_m) of DTF. Then we discussed the feasibility of estimating in-situ stress by using a set of data of θ_mN and γ_m observed along a deviated borehole. Finally, we attempted to apply the basic concept of estimating the stress field by using θ_mN and γ_m to well TG-2 which was drilled by NEDO (New Energy and Industrial Technology Development Organization) in the Yunomori geothermal field. The application was fairly successful.