Insights into fractures and fabric of shales from microseismicity

Shales typically have low permeabilities due to their fine grain size, unless fractures are present to provide fluid conduits. Both aligned fractures and the preferred orientation of phyllosilicate minerals in shales are effective mechanisms to produce seismic anisotropy. Microseismicity recorded during hydraulic fracture stimulation can be used to provide excellent estimates of anisotropy through the observation of shear wave splitting. Here I illustrate through examples how these measurements can be used to infer the development and evolution of vertical fracture parameters during stimulation as well as provide improved estimates of the in situ rock fabric. A key challenge is untangling intrinsic anisotropy due to mineral alignment from extrinsic anisotropy due to cracks and pores aligned parallel to the dominate petrofabric of the shales. This may be improved by incorporating petrofabric and microstructural analyses of shale samples from the reservoir to better constrain the orientations and proportions of the constituent minerals.