Discrimination of critically stressed faults and safety-optimized hydraulic fracturing design: Insights from meter-scale physical modeling

This study presents an integrated approach combining metre-scale physical experiments, numerical simulations and theoretical modelling to systematically investigate the mechanisms and controlling factors of fault slip induced by hydraulic fracturing, with the ultimate goal of establishing fundamental design principles for fault slip mitigation. First, we quantify the influence of fault geometry and mechanical properties on stress concentration by deriving a quantitative stress concentration equation through multivariate regression analysis. Second, a novel three-dimensional (3D) Coulomb failure stress (CFS) expression incorporating stress concentration coefficients is proposed to overcome the limitations of conventional regional stress analysis. Third, the theoretical stress transfer model is validated against experimental data, showing strong agreement between predicted and measured CFS changes, with a relative error of less than 10 %. Our results demonstrate that hydraulically isolated faults are primarily controlled by regional stress states during slip initiation. Critically stressed faults exhibit significant slip near injection points, while non-critical faults remain stable. The proposed 3D CFS expression successfully discriminates between stress-transfer induced PNR-1z and pore-pressure driven PNR-2 seismic events. Finally, faults are classified into four distinct types, each associated with tailored hydraulic fracturing design protocols: type I faults require mandatory avoidance; type II-III faults require controlled injection parameters; and type IV faults permit cost-optimised operations. These findings provide theoretical advances and practical guidelines for mitigating induced seismicity in hydraulic fracturing.