Stress Measurements in Crystalline Rock: Comparison of Overcoring, Hydraulic Fracturing and Induced Seismicity Results

In preparation of a decameter-scale fault stimulation experiment at the Grimsel Test Site, Switzerland, a comprehensive rock stress characterization survey was conducted. The survey combines overcoring of CSIRO-HI and USBM probes with hydrofracture measurements with concomitant monitoring of the induced microseismicity. Impression packer surveys were run following the hydrofracture tests to determine the orientation of the induced fracture at the wellbore. The orientation of the fracture away from the wellbore was determined from the pattern of the microseismicity. The use of a transverse isotropic model for inverting the strains measured during overcoring was essential to obtain stress solutions that were consistent with the hydrofracture and microseismicity results. The indicate that the minimum principal stress, s₃, is sub-horizontal and oriented north-south. The maximum principal stress, s₁, dips at 30-40° to the east and has a magnitude of 13-17 MPa. The magnitude of s₂, is similar to s₃, and lies in the range 8-10 MPa.