Relationship between sealing faults, pressure domains and fluid-induced seismicity

Sealing faults are nearly impermeable barriers that can form boundaries between subsurface pore-pressure domains. In the case of hydrocarbon systems, sealing faults commonly form part of a structural trap; they are thus likely to be important elements for future storage of CO2, hydrogen, and other gases in depleted reservoirs. The Triassic Montney Formation in British Columbia, Canada hosts low-permeability gas reservoirs containing sealing faults that have previously been assumed to compartmentalize pressure domains within a regional transition from overpressured to normally pressured parts of the basin. In this study, we show that the distribution of induced seismicity associated with hydraulic fracturing (HF) exhibits a statistically significant spatial correlation with zones of high lateral gradient in pore pressure. These high-gradient zones are interpreted as sealing fault systems. The largest induced seismicity sequence, including a 4.5 ML mainshock on 2018/11/30, occurred during HF treatments in two horizontal wells, between which there is an exceptionally large contrast (~ 10 MPa) in measured pore pressure. Numerical simulation of a simplified model of a hydraulic fracture intersecting a nearby vertical fault, followed by fault rupture using a rate-state friction rheology, generates results that are in good agreement with observed strike-slip faulting near one of the HF wells. Our study demonstrates that sealing faults exhibit previously unrecognized behaviour that may be important for understanding induced seismicity risk.