Measurement of the normal/tangential fracture compliance ratio (ZN/ZT) during hydraulic fracture stimulation using S-wave splitting data

We develop a method to invert S-wave splitting (SWS) observations, measured on microseismic event data, for the ratio of normal to tangential compliance (ZN/ZT) of sets of aligned fractures. We demonstrate this method by inverting for ZN/ZT using SWS measurements made during hydraulic fracture stimulation of the Cotton Valley tight gas reservoir, Texas. When the full SWS data set is inverted, we find that ZN/ZT= 0.74 +/- 0.04. Windowing the data by time, we were able to observe variations in ZN/ZT as the fracture stimulation progresses. Most notably, we observe an increase in ZN/ZT contemporaneous with proppant injection. Rock physics models and laboratory observations have shown that ZN/ZT can be sensitive to (1) the stiffness of the fluid filling the fracture, (2) the extent to which this fluid can flow in and out of the fracture during the passage of a seismic wave and (3) the internal architecture of the fracture, including the roughness of the fracture surfaces, the number and size of any asperities and the presence of material filling the fracture. These factors have direct implications for modelling the fluid-flow properties of fractures. Consequently, the ability to image ZN/ZT using SWS will provide useful information about fractured rocks and allow additional constraints to be placed on reservoir behaviour.