Stress Dependence of Cooling-Induced Acoustic Emissions in Westerly Granite

Thermally induced stress changes in rock lead to creation and reactivation of microcracks with accompanying acoustic emissions (AEs). Therefore, monitoring AEs during cooling (or heating) of rock potentially allows us to determine the rock’s original stress state. This approach would require predicting time-dependent changes in the thermally induced stress and applying an appropriate AE-based failure criterion, so that the in situ stress can be determined by subtracting the induced stress from the failure stress. The ultimate goal of this research is to develop a technology to determine the stress state in geothermal reservoirs by detecting the differences in the onset of microscale rock failure induced by local cooling of the borehole circumference at discrete azimuths. As the first step to achieve this goal, we examined spatio-temporal evolution of AEs which were thermally induced in Westerly Granite blocks by abrupt local cooling. Four sets of experiments were conducted with different sample and cooling geometry, stress state, and pre-existing damage (cracks). The preliminary results indicate that the onset of AE activity can be correlated best to the thermal front propagation in rock samples and to the applied stress. The conditions that lead to the onset of AE activity and its relation to the ultimate tensile strength of rocks—a parameter which is determined from conventional rock testing—are still poorly understood, which requires further investigation.