Infering Dynamic Rock Properties During Hydraulic Fracturing from Microseismicity

Being able to estimate the dynamic properties of rock failure in shale from microseismic data allows for the characterization of reservoir rock at resolutions that are unattainable from conventional 3D seismic imaging and allows for additional insight into the fracturing process that will help constrain different models for the fracture generation. Using events generated by hydraulic fracturing, we demonstrate a technique for estimating the Vp/Vs ratios throughout a hydraulic fracture from seismic moment tensors. Downhole recording microseismic events allow us to estimate the seismic moment tensor of shear-tensile events. Using a set of well-located, high quality events with similarly oriented fracture planes, we can invert for Vp/Vs, the ratio of the compressional to shear wave velocities, of the volume around the fracture. While traditionally this parameter is used as a proxy for reservoir rock properties, decreases in this ratio likely indicates a reduction in the rock strength due to fracturing. From sets of events on nearly vertical fractures, we observe decreasing Vp/Vs ratios throughout the stage. Lowering Vp/Vs is also observed in previously fractured areas. We also estimate the variations in apparent Vp/Vs as a function of the dip of the fracture plane to constrain anisotropic responses.