Monitoring the evolution of fracture compliances during hydraulic stimulation using passive seismic data

Seismic anisotropy is a useful attribute for the detection and characterization of aligned fracture sets in petroleum reservoirs. While many techniques to estimate anisotropy have been successful in inferring fracture density and orientation, they generally provide little constraint on the ability of the fractures to facilitate fluid flow. A potentially useful property to provide insight into this is the ratio of the normal to tangential fracture compliance (ZN/ZT). ZN/ZT is sensitive to many properties including: the stiffness of the infilling fluid, fracture connectivity and permeability, and the internal architecture of the fracture. Here we demonstrate a method to infer ZN/ZT using shear wave splitting measurements on two microseismic datasets from hydraulic stimulations. Both examples show apparent increases in ZN/ZT during the stimulation process. We suggest that this may be produced by the development of new, clean fractures that have a greater normal compliance than their natural counterparts, combined with increases in fracture connectivity and permeability. The ability to monitor ZN/ZT during stimulations provides a means to gain insight into the evolving flow properties of the induced fracture network, and may be beneficial for assessing the effectiveness of stimulation strategies.