Stress rates induced by hydraulic fracturing control seismic nucleation

High-pressure hydraulic fracturing can enhance dense shale gas production, but it may reactivate faults through a decrease in frictional strength driven by both proximal and projected poroelastic stress, posing the risk of seismicity and even project shutdown. Hazard assessment is important for shale gas development. We explore the impact of poroelastic stresses on earthquake nucleation by deriving the critical characteristic slip length and nucleation length of a fault that rigorously incorporates both the magnitude and the rate of change in effective stress. We use this as an index to estimate the projected evolution from quiescent to seismic response of faults reactivated both during and after hydraulic fracturing operations. This work shows that increasing injection rate imposes dynamic loading on faults, reducing critical characteristic slip lengths and enhancing the potential to nucleate earthquakes. Conversely, reducing injection rates increases critical characteristic slip lengths and diminishes immediate seismic risk—allowing continued accumulation of tectonic stresses and the potential of larger delayed seismic events in the future. For sub-vertical faults, small events occur at shallow depths during injection with larger events on the deep fault post-shut-in. We interpret the seismicity observations from the Luxian shale gas field in the Sichuan Basin based on this assessment method. Pre-conditioning fracturing is suggested as a method to diminish the hazard of induced earthquakes, a risk that is also mitigated by greater well-to-fault separations and reduced injection rates. This work provides insights into fault slip behavior and seismic nucleation induced by hydraulic fracturing and guidance for its mitigation.