Temporal Variations of Near‐Surface Anisotropy Induced by Hydraulic Fracturing at a Shale Play Site in Southwest China

Knowledge of the geometric properties of fractures and cracks in a petroleum reservoir is important to reservoir exploitation. When aligned and partially connected, fractures and cracks can act as conduits for fluid flow and thus can significantly increase the permeability of the reservoir. The aligned fractures and cracks, on the other hand, are an effective means to generate seismic anisotropy. In this study, we utilize the seismic data recorded by a vertical array installed in a shallow borehole at a shale play site in southwest China. By applying seismic interferometry to the ambient noise data recorded by 12 three-component geophones, we extract P and S waves propagating vertically along the borehole. The S waves show up to 20% velocity variations with respect to their polarization directions. Such large S wave anisotropy can be explained by the horizontal transverse isotropic model and is likely caused by natural fractures that are widely present in the area and align approximately in the NE-SW direction. During the 13-day period of hydraulic fracking treatment, we also observe large and systematic temporal variations in S wave velocity, degree of S wave polarization anisotropy, and fast polarization direction. By comparing our observations with normal strain changes calculated with a half-space elastic model, we speculate that strain changes induced by hydraulic injection and fracturing are likely to be responsible for the observed temporal variations in seismic anisotropy. As such, seismic interferometry with shallow borehole acquisition might provide an alternative means to monitor hydraulic fracturing and wastewater injection in the future.