Rock mechanical properties under reconstructed deep in situ thermo–hydro–mechanical conditions: Implications for CO₂‐related injection scenarios

Deep formations are subject to long‐term interactions among high temperature, pore pressure, and stress, and rock mechanical properties are fundamentally governed by these coupled in situ thermo‐hydro‐mechanical (THM) fields. However, most porous‐medium models are developed for non‐in‐situ settings and do not capture the inherent equilibrium state of rocks in geological environments. This study investigates the mechanical properties of 3000 m‐depth sandstone under reconstructed in situ THM conditions and evaluates the implications for CO 2 injection scenarios. Within a coupled porous‐medium formulation, we establish an elastic porosity equation and decompose the contributions of in situ temperature, pore pressure, and stress to mechanical properties. Calibration using data from wells FS7 and SS2 sandstones in the Songliao Basin, combined with finite‐element simulations, enables quantification of THM‐driven changes in porosity, permeability, and elastic‐plastic parameters. The results show that restoring in situ temperature leads to matrix thermal expansion, while in situ stress induces volumetric compaction, thereby reducing porosity and permeability, and strengthening the material. In contrast, increasing pore pressure promotes pore dilation and mechanical weakening. In situ stress exerts the strongest control on mechanical properties, followed by temperature, with pore pressure effects being comparatively minor. The sensitivities of these responses depend on the initial porosity: the high‐porosity, low‐strength FS7 sandstone is more sensitive to THM perturbations, whereas the lower‐porosity, higher‐strength SS2 sandstone remains more stable. In addition, the model is applied to an illustrative near‐field CO 2 injection scenario to examine parameter evolution under reconstructed in situ THM conditions. The simulations are conducted within a partially coupled THM formulation, demonstrating coherent multi‐physics responses and consistent evolution of mechanical parameters. Overall, this study establishes a mechanically consistent in situ baseline framework for analyzing sandstone mechanical properties under reconstructed multi‐physics conditions.