A unified pressure-temperature stability diagram for assessing injection-induced fault reactivation potential

Cold fluid injections into hot geological formations drive coupled pore pressure and temperature perturbations that alter in-situ stress states and can trigger fault reactivation, a critical risk for geoenergy and geostorage operations such as CO 2 storage. Existing analytical approaches often treat pressure and temperature perturbations independently, rely on two-dimensional Mohr circle analysis under a fixed stress regime, and therefore cannot directly constrain coupled operational thresholds for fault reactivation. This study employs a poro-thermo-elastic framework to demonstrate the asymmetric stress responses to concurrent fluid pressurization and formation cooling across normal, strike-slip, and reverse stress regimes. This asymmetric response can drive stress regime transitions during injection, such that the fault orientation most susceptible to reactivation depends on the stress state at failure rather than initial conditions. To jointly constrain pressure and temperature thresholds under these coupled conditions, a unified pressure-temperature ( P - T ) stability diagram is proposed to transform fault reactivation analysis from three-dimensional stress space into operational P - T space. The approach integrates poro-thermo-elastic constitutive relations with Mohr-Coulomb failure criteria to derive linear P - T profiles for each principal stress pair. The resulting stability envelopes identify governing stress pairs and critical fault orientations across all coupled conditions. Application to conceptual CO 2 storage and geothermal scenarios illustrates how spatial P - T distributions map onto the stability diagram to identify critical distances, determine susceptible fault orientations, and explicitly inform the maximum injection-induced pressure and temperature changes sustainable without triggering reactivation. The approach provides an analytical framework for early-stage assessment across multiple sites, stress regimes, and operational scenarios.