Influencing Factors and Mechanism of CO2 Trapping and Storage in Tight Sandstone Reservoirs Based on Fractal Characteristics of Pore Structure

To mitigate global warming induced by excessive carbon dioxide emissions, carbon dioxide displacement technology for carbon utilization and storage has attracted growing attention. In this paper, laboratory displacement experiments combined with Nuclear Magnetic Resonance (NMR), constant-rate mercury intrusion, and X-ray diffraction (XRD) tests are adopted to investigate the residual storage characteristics of carbon dioxide in tight sandstone reservoirs. This study evaluates the effect of depletion pressure on carbon dioxide storage efficiency. Combined with the pore-throat fractal dimension (Df) obtained from constant-rate mercury intrusion and the capillary tortuosity fractal dimension (DT) calculated via models, the relevant controlling mechanisms are illustrated. The results show that the residual storage efficiency can exceed 62.00% when the depletion pressure is higher than the supercritical pressure of carbon dioxide. Storage efficiency exhibits a significant correlation with the average pore-throat ratio, indicating that pore-throat matching characteristics play a vital role in carbon dioxide retention. Both the pore-throat fractal dimension Df and capillary tortuosity fractal dimension DT are positively correlated with the average pore-throat ratio and negatively correlated with the pore-throat radius, reflecting the impacts of pore structure complexity and fluid channel tortuosity on carbon dioxide migration and storage processes. X-ray diffraction test results further verify that quartz and clay minerals indirectly affect carbon dioxide storage performance by altering the preservation status and connectivity of pore throats. The innovation of this study lies in establishing a coupled analysis system integrating pore-throat heterogeneity, fluid channel complexity, and carbon dioxide phase evolution, which reveals the residual storage mechanism of carbon dioxide in tight sandstone reservoirs. Relevant research findings provide new insights for studies on carbon dioxide storage laws at the pore scale, and offer theoretical support for optimizing geological carbon dioxide storage schemes in tight reservoirs.