Abstract:

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.

Key words: saline aquifer storage, impure CO₂, migration behavior, occurrence forms, numerical simulation

摘要：

二氧化碳地质封存(GCS)是实现“双碳”战略目标的重要技术途径之一。由于工业源CO₂通常含有多种杂质气体，其提纯成本高，操作难度大，在实际工程中往往将杂质气体与CO₂共同注入地下。基于某CCS示范工程区石千峰组储层地质特征，构建二维地质模型，采用CMG-GEM组分模拟器开展非纯净CO₂的反应运移模拟。选取N₂和H₂S作为典型杂质组分，模拟非纯净CO₂在咸水层中的运移路径、赋存形态及其空间分布特征，系统分析结构封存、残余气封存和溶解封存在不同阶段对CO₂封存行为的主导性变化，探讨毛细管压力在非纯净CO₂地质封存过程中的作用。模拟结果表明，注入初期CO₂主要以超临界态聚集于储层顶部，随后束缚态和溶解态CO₂比例增加，封存稳定性与安全性显著提升；CO₂运移过程呈现明显的时空分异特征，注入气体在浮力驱动下迅速上升并聚集于盖层底部，随后在密度差和浓度梯度驱动下缓慢回流，促进溶解扩散，停注后气体沿盖层底部径向扩散，形成最大扩散距离约650 m的“舌形”前缘；杂质组分运移行为差异明显，N₂因溶解度低聚集于气液相驱替前缘，CO₂和H₂S由于溶解度较高在近井区形成溶解富集区，最高溶解度分别达到1.4 mol/kg-H₂O和0.53 mol/kg-H₂O；毛细管压力通过抑制气相迁移速率增强溶解封存效率，在注入后期诱发地层水反向渗吸，促使更多CO₂以残余气形式滞留于孔隙中，有效提升残余气封存比例。综合分析表明，非纯净CO₂在咸水层中的封存行为受杂质特性、赋存形态及毛细管压力等因素共同影响。研究成果为非纯净CO₂地质封存工程的注入方案优化提供科学依据，对保障CO₂长期封存安全性与提升封存效率具有重要指导意义。

关键词: 咸水层地质封存, 非纯净CO₂, 运移行为, 赋存形态, 数值模拟

CLC Number: 

• TE822

• X701