Development of borehole tangential deformation gage and its application for determining the stress in rock

We developed a new reusable gage designed upon a concept completely different to that of the conventional ones. The gage allows us to measure the tangential deformation of borehole wall. We tested the gage by a laboratory experiment and examined how it could measure accurately borehole deformation caused by packer inflation. The rock specimen had two drilled borehole of 76 mm in diameter with hydraulically-induced axial fractures. The results show that the deformation measured by the gage located near by the fracture mouth increased linearly with the packer pressure initially and it deviated from the initial trend afterwards. The deviation from linearity of the deformation-packer pressure curve was associated with the fracture opening. Then we confirmed that the packer pressure at the beginning of deviation was close to the reopening pressure P_r estimated by the Kirsch equation, i.e. P_r = 3S_h - S_H, where S_H and S_h (S _H ≥ S_h) are the biaxial compressive loads externally applied to the specimen as the direction of S_h being normal to the fractures. This indicates that we can estimate the magnitude of S_H from the detected reopening pressure and the Kirsch equation on condition that the magnitude of S_h is estimated using other approaches.