Abstract
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 (θmN) along the borehole wall and the inclination (γm) 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 (θmN, γm) of DTF. Then we discussed the feasibility of estimating in-situ stress by using a set of data of θmN and γm observed along a deviated borehole. Finally, we attempted to apply the basic concept of estimating the stress field by using θmN and γm 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.
