Investigating the role of elastostatic stress transfer during hydraulic fracturing-induced fault activation

We investigate the physical processes that generate seismicity during hydraulic fracturing. Fluid processes (increases in pore pressure and poroelastic stress) are often considered to be the primary drivers. However, some recent studies have suggested that elastic stress interactions may significantly contribute to further seismicity. In this work we use a microseismic dataset acquired during hydraulic fracturing to calculate elastic stress transfer during a period of fault activation and induced seismicity. We find that elastic stress changes may have weakly promoted initial failure, but at later times stress changes generally acted to inhibit further slip. Sources from within tight clusters are found to be the most significant contributor to the cumulative elastic stress changes. Given the estimated in situ stress field, relatively large increases in pore pressure are required to reach the failure envelope for these faults – on the order of 10 MPa. This threshold is far greater than the reliable cumulative elastic stress changes found in this study, with the vast majority of events receiving no more than 0.1 MPa of positive ∆CFS, further indicating that elastic stress changes were not a significant driver, and that interaction with the pressurised fluid was required to initiate failure. Thus, cumulative stress transfer from small events near the injection well does not appear to play a significant role in the reactivation of nearby faults.