Role of Negative Pore Pressure in Injection-Induced Post-Slip of Rock Fractures

ABSTRACT: Negative pore pressure induced during unconventional energy production may provide new insights into assessing and mitigating injection-induced post-slip of rock fractures. Using triaxial experimental setup in the laboratory, we sheared sawcut, filled, and natural fractures in granite under zero, positive, and negative pore pressures and measured the evolution of fracture permeability. The results suggest that negative pore pressure increases the peak strength in the sawcut and filled fractures, while reducing their permeability and post-peak stressing rate. The reduction in post-peak destressing rate is most closely related to negative pore pressure in the sawcut fracture with the lowest fracture permeability, whereas in the natural fracture, the reduction in post-peak destressing rate is associated with increased hydraulic aperture following consecutive shear tests. Moreover, in the sawcut fracture, the decay in post-peak stressing rate is more rapid under negative pore pressure, corresponding to a rapid reduction in post-slip frequency during injection-induced seismicity. This rapid decay inhibits post-slip events after the shut-in of fluid injection. 1. INTRODUCTION Injection-induced post-slip is commonly observed during unconventional energy production and is driven by pore pressure diffusion, shear stress redistribution, and slip-stress transfer (Cebry and McLaskey, 2020; Kim and Avouac, 2023; Guglielmi et al., 2015). Post-slip events are characterized by a declining frequency over time since the peak strength, resulting from an exponential reduction in post-peak stressing rate after the mainshock perturbation (Dieterich, 1994). A rapid decay represents swift re-sealing of fractures after the peak strength (Miller, 2020). According to available data of post-slip frequency versus time since peak strength (Table 1), the decay rate exponent from natural earthquakes varies more widely and is generally lower than that from induced seismicity of lower seismic magnitudes. The normalized post-peak stressing rate, calculated as the normalized post-slip frequency (Dieterich, 1994), drops more rapidly to nearly zero for induced earthquakes. Field observations indicate that the decay rate exponent increases with deeper post-slip events (Shebalin and Clément, 2017), contrasting with the higher exponents of the shallower induced seismicity. Fluid extraction during energy production has been demonstrated to result in fracture compression and inhibit post-slip events (Segall, 1985), which potentially increases the decay rate exponent. Thus, understanding the post-peak stressing rate during unconventional energy production may uncover the rapid decay of injection-induced post-slip events.