Tracking fracks: Evolution of fracture normal/tangential compliance during hydraulic fracture stimulation

S-wave splitting (SWS) from microseismic events may be used to estimate anisotropy in the region around hydraulic fracture stimulations. The anisotropy can then be used to characterize the distribution of fractures in a reservoir. In addition to fracture orientation, SWS can be used to estimate the ratio of normal to tangential compliance (ZN/ZT). ZN/ZT is sensitive to (1) the stiffness of the infilling fluid, (2) fracture connectivity and permeability, and (3) the internal architecture of the fracture (e.g. fracture roughness, degree of cementation). Here we demonstrate the use of SWS to infer the evolution of ZN/ZT from two hydraulic stimulation datasets from tight gas reservoirs. In both examples we observe an apparent increase in ZN/ZT as the stimulation progresses. We suggest that this increase 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 network 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 success of drilling and stimulation strategies.