Direct and Microseismic Observations of Hydraulic Fracturing in Barre Granite and Opalinus Clayshale

While hydraulic fracturing is a well-established and widely implemented process, there are still some aspects that can be better understood. Specifically, researchers do not fully understand the relation between the hydraulic fracture network and the microseismic events induced during hydraulic stimulation. In this laboratory study, we address this knowledge gap by conducting hydraulic fracturing experiments where we directly observe microstructural changes using a high-speed camera attached to a 5X magnification lens and analyze the images using a digital image correlation code. These data are compared to simultaneously acquired acoustic emissions, from which we infer the location and focal mechanisms of induced microseismic events. Experiments were conducted on Opalinus clayshale and Barre granite, at injection rates of 0.019 and 0.39 ml/s for each rock. The simultaneous recording of microseismic and visual observations is unique and provides significant insight into the details of hydraulic fracturing. Our results show several interesting differences between granite and shale, and between injection rates. For example, we find that while the creation of hydraulic fractures is generally considered as tensile at the field scale, evidence of shearing can be observed to varying degrees at the microstructural scale. Specifically, we see that microstructural shearing is more evident in granite in the form of distinct en echelon microcracks. On the effect of the injection rate, we find that a reduced injection rate tends to create a more complex network of microcracks, along with a lower proportion of double-couple (shear) focal mechanisms in the microseismic data.