Cracks, fractures and flow in shales - Insights from microseismicity

Hydraulic fracture stimulation in shales has revolutionised the North American energy market. However, how oil and gas flows from the impermeable matrix to fractures is still poorly understood. Shales are characterised by horizontally aligned phyllosilicate minerals, which leads to a well-defined directional variation in seismic velocities, or seismic anisotropy. Petrofabric analysis can be used to constrain the intrinsic anisotropy due to this crystal preferred alignment, but the observed seismic anisotropy is often much larger, suggesting more extrinsic mechanisms such as cracks and fractures are at play. Microearthquakes induced by fracture stimulation provide excellent probes of the in-situ anisotropy through observations of shear wave birefringence or 'splitting'. The analysis of microseismic data in a number of shale gas settings suggest that the interplay between induced and pre-existing vertical fractures plays a key role in developing fracture networks. However, recent analysis suggests that sub-horizontal and compliant microcracks may play an equally significant role in promoting flow in highly anisotropic shales.