Using S-wave splitting to measure fracture compliance and bn/bt during hydraulic fracture stimulation

The effect of fractures on seismic waves is controlled by the normal and tangential compliances of the fractures (BN and BT). Rock physics models and laboratory experiments have indicated that BN/BT will be influenced by (1) the bulk modulus of the fluid filling the fracture, (2) the degree of connectivity between the fracture and equant pore space, and (3) the internal architecture of the fracture, such as the roughness of fracture faces, or the presence of detrital or diagenetic infilling material. Therefore, measurements of BN/BT will provide useful information during hydraulic fracture stimulation. We develop a method to invert S-wave splitting (SWS) data, measured on microseismic events recorded on downhole geophone arrays, for the ratio of normal to tangential compliance (BN/BT) of sets of aligned fractures. We demonstrate this method by inverting for BN/BT using SWS measurements made during hydraulic fracture stimulation of the Cotton Valley tight gas reservoir, Texas. When the full SWS dataset is inverted, we find that BN/BT=0.74±0.04. Windowing the data by time, we have been able to observe temporal variations, finding that BN/BT varies as the stimulation progresses, and most notably when proppant is injected.