Coupled Effects of CO ₂ and Temperature on Tight Reservoir Sandstone: Mechanical Properties, Mechanisms, and Models

Tight sandstone reservoirs, characterized by low porosity and permeability, present substantial potential for CO2 utilization and sequestration. Understanding the mechanical behavior of tight sandstone under the influence of CO2 is critical for assessing geological CO2 sequestration and the CO2 fracturing capabilities of reservoirs. As the burial depth of the target reservoir increases, the formation temperature gradually rises, considerably altering the mechanical properties of reservoir sandstone, especially the interaction between CO2 and sandstone. However, few studies on the coupled effects are available. In this study, we built a high-temperature CO2 soaking system that allows CO2 injection across various formation temperatures (25 °C–160 °C). Uniaxial compression tests were conducted to explore the mechanical properties of the tight sandstone subjected to CO2 soaking at various temperatures. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive spectroscopy were used to quantitatively characterize the evolution of the mineral composition and micromorphology of tight sandstone after CO2 soaking at different temperatures. A stress–strain damage constitutive model was established to describe the behavior of tight sandstone under the coupled effects of temperature and CO2. The quantitative relationships between mineral dissolution, pore-throat evolution, and crack propagation revealed by SEM, XRD, and EDS analyses were integrated into the model to describe the prepeak damage evolution and deformation characteristics. The proposed model not only effectively characterized the macroscopic mechanical behavior of tight sandstone under coupled temperature–CO2 conditions but also provides a mechanistic explanation for the role of microstructural changes in controlling damage accumulation and strength degradation.