Effects of veins on mechanical deformation and fracturing behavior of shale rocks under triaxial compression stress states: Insights from local strain measurements, P-wave velocities and acoustic emission activity

Veins featured by heterogeneous mineral components compared to the host rock are thought to give rise to the heterogeneous stress and strain during rock deformation, thereby impacting rock fracture behavior. To study the influence of veins on the mechanical and fracturing behavior of shale reservoir rocks, a series of triaxial compression tests were performed on different shale samples at room temperature and a constant confining pressure of 30 MPa. Samples contained either carbonate-rich veins or were vein-free. For the characterization of local strain within veins and host rock and the evolution of micro-fracturing during bulk sample deformation, we employed local strain gauge measurements, ultrasonic P-wave velocities, and acoustic emission monitoring. The peak stresses of bulk samples containing veins are generally lower, compared to their vein-free counterparts. For the samples with a vein, the spatiotemporal distribution of AE activity shows that fracturing was initiated in the vein, consistent with a pronounced decrease in the trace of P-wave velocity traveling through the vein. The final trajectory of fracture was either confined within or through the vein. We attributed this contrasting behavior to the varying vein geometry and the mechanical contrast of elastic moduli between the vein and host rock. This study underscores the role of veins in determining shale rock mechanical properties and fracturing behavior, which is important for the treatment of unconventional reservoirs and other relevant rock engineering projects.