Inversion of drilling-induced tensile fracture data obtained from a single inclined borehole

We investigate the feasibility of estimating an in situ three-dimensional stress field by using data of drilling-induced tensile fractures (DTFs) observed in a single borehole. The principal assumptions in this investigation are that the rock is isotropic, homogeneous and elastic. A DTF is a longitudinal crack consisting of many small parallel cracks which are oblique to the borehole axis. A DTF is characterized by its circumferential position (θ_mD) along the borehole surface and the inclination (γ_m) of the small cracks with respect to the borehole axis. We show how it is possible to estimate the three-dimensional stress field by using the variation of Θ_mD and γ_m with respect to borehole orientation (i.e. azimuth and inclination of a borehole). Based on the variation of Θ_mD and γ_m as functions of borehole orientation which changes with depth, an inverse problem is formulated to estimate the three-dimensional stress field. Tests with synthetic data sets (Θ_mD and γ_m) show that it is feasible to estimate the three-dimensional stress field and that the statistical approach is appropriate for the inversion practically. Finally, we discuss a DTF data set (Θ_mD and γ_m) measured in a real borehole in the northern area of Japan Main Island and apply the inversion technique to estimate the stress field.