The potential for low-grade metamorphism to facilitate fault instability in a geothermal reservoir

Swarms of earthquakes during shale gas exploitation in the Changning area of the Sichuan Basin indicate that hydraulic fracturing induces seismicity both within the target reservoir formation but also to depths of several kilometers below the horizontal well. These remote earthquakes are possibly triggered by total stress perturbations resulting from the hydraulic fracturing. We use a dislocation-based analytical model to simulate multistage hydraulic fracturing of three horizontal wells at a single well pad to explore the spatiotemporal evolution of the total stress perturbations. Results show that the number and distribution of fracturing stages affect both the distribution and magnitude of stress changes and that the stress change diminishes with distance. The undrained injection-induced stress change is less than 10-3 MPa at distances ≥1 km for single-stage fracturing but reach 10-1 MPa for multistage fracturing of 30 stages in all three wells. Undrained stress changes scale linearly with the magnitude of fluid leak-off into the formation – halving the effective fracture width halves the induced stress magnitudes and with an identical distribution – limiting the potential for fault reactivation. Scaling analysis for pressure diffusion distal from the reservoir indicates that the short-term (days to weeks) impact is indeed essentially undrained. Estimates for long-term reservoir depletion (months to years) identify a similar induced stress signal of opposite sign but with similar Coulomb potential for reactivation in the long term. Such magnitudes of Coulomb stress changes suggest the possibility of fault reactivation on deep critically-stressed faults at kilometer separation from the injection both in the short term due to stimulation and in the long term as a result of reservoir depletion