Simulation of migration behavior and occurrence forms of impure CO₂ in saline aquifer geological storage

Geological carbon storage (GCS) is one of the key technologies to achieve Carbon peak and carbon neutrality goals. Since industrial-source CO₂ usually contains various impurity gases, and its purification is costly and technically challenging, these impurities are often injected into the subsurface together with CO₂ in practical engineering applications. Based on the geological characteristics of the Shiqianfeng Formation in a CCS demonstration project area, a two-dimensional geological model was established, and reactive transport simulations of impure CO₂ were conducted using the CMG-GEM compositional simulator. N₂ and H₂S were selected as representative impurity components to investigate the migration pathways, occurrence forms, and spatial distribution characteristics of impure CO₂ in the saline aquifer. The study systematically analyzed the dominant trapping mechanisms, including structural trapping, residual gas trapping, and solubility trapping, at different storage stages, and explored the role of capillary pressure in the geological storage process of impure CO₂. The simulation results indicate that during the early injection stage, CO₂ predominantly accumulates in the supercritical state at the top of the reservoir. Over time, the capillary trapping effect gradually emerges, more CO₂ is retained in the reservoir pores, promoting the continuous conversion of supercritical CO₂ into bound and dissolved forms, thereby significantly enhancing the overall storage stability and safety. CO₂ migration exhibits pronounced spatiotemporal heterogeneity: the injected gas rapidly rises under buoyancy and accumulates beneath the caprock, then gradually migrates downward under density and concentration gradients, promoting dissolution. After injection ceases, the gas spreads laterally along the base of the caprock, forming a tongue-shaped migration front with a maximum diffusion distance of approximately 650 m. The transport behaviors of the impurity components differ significantly. Due to its low solubility, N₂ tends to accumulate at the leading edge of the gas-liquid displacement front, whereas CO₂ and H₂S, with higher solubility, form dissolution-enriched zones near the injection well, reaching peak solubilities of 1.4 mol/kg-H₂O and 0.53 mol/kg-H₂O, respectively. Capillary pressure plays a crucial role by suppressing the rate of gas-phase migration, enhancing dissolution trapping efficiency, and inducing reverse imbibition of formation water during the post-injection stage, thereby promoting greater retention of CO₂ in pore spaces in the form of residual gas and effectively increasing the proportion of residual trapping. Comprehensive analysis demonstrates that the storage behavior of impure CO₂ in saline aquifers is jointly governed by impurity properties, trapping mechanisms, and capillary pressure effects. The findings provide scientific support for optimizing injection strategies in impure CO₂ geological storage projects and offer important guidance for ensuring long-term storage security and improving storage efficiency.