Effects of Pre-CO₂ Soaking and High Temperatures on Permeability Evolution in Tight Sandstones

Depleted oil and gas reservoirs are potential sites for CO2 utilization and geological sequestration, where permeability plays a crucial role in storage stability. The mechanisms whereby coupled chemical-mechanical processes influence heterogeneity and drive permeability evolution during CO2 injection and storage remain poorly understood. As the reservoir depth increases, the increased temperatures and effective stress further alter reservoir permeability. This study developed a CO2-sandstone reaction system and permeability testing apparatus to investigate the permeability evolution of tight reservoir sandstone exposed to 7-day pre-CO2 soaking under varying temperatures (25–160 °C) and effective stresses (2–30 MPa). A theoretical model was proposed to quantify the permeability evolution based on the two-part Hooke’s hypothesis. The intrinsic microevolutionary mechanism of permeability change was characterized using scanning electron microscopy (SEM). The results indicated that pre-CO2 soaking had a more pronounced effect on the permeability of the hard part, whereas the effect decreased at higher temperatures (>100 °C). The permeability of the soft part decreased exponentially before CO2 soaking and decreased linearly after soaking, owing to CO2-induced dissolution and precipitation effects, which transformed portions of the soft part into hard components. SEM observations revealed that CO2 soaking increased the fractal dimension of micropores and exacerbated permeability reduction. The findings provide novel insights into the effect of pre-CO2 on the permeability evolution in tight sandstone reservoirs, highlighting the complex coupling mechanisms involving high temperature, effective stress, and CO2 interactions. This study provides an experimental basis and theoretical model for evaluating and optimizing reservoir permeability in CO2 geological storage.