Adsorption Behavior and Storage Capacity Prediction of CO₂ Injection into High-Rank Coal

CO₂ geological sequestration (CCUS) serves as a critical technological pathway to achieve the strategic goal of the "dual-carbon" initiative. Coal seams characterized by dual porosity represent a favorable geological structure for storing carbon dioxide. The adsorption characteristics and storage capacity of CO₂ in the coal seam are the core indexes to evaluate the feasibility of CO₂ injection and storage. Anthracite samples from the south of the Qinshui basin were selected for multi-physical field-coupled CO₂ isothermal adsorption experiments. This study investigates the adsorption behavior of high-rank coal samples toward CO₂, compares the applicability of various theoretical adsorption models, selects the optimal one to build a storage capacity prediction model, and evaluates the CO₂ storage potential of high-rank coal seams. The results indicate that pressure has a positive effect on the adsorption of CO₂ (before reaching the supercritical state), while temperature inhibits the adsorption of CO₂ by coal samples. With the increase in water saturation, the adsorption capacity of coal samples for CO₂ gradually decreases. The theoretical model of micropore filling (the D-A model) has good applicability for predicting the adsorption of CO₂ in coal samples. On the basis of experimental results, estimation models for CO₂ content under different occurrence states and a predictive model for total storage capacity were established. The content of each CO₂ occurrence state is influenced by factors such as formation temperature, pressure, and water saturation. CO₂ is mainly stored in coal reservoirs in the form of adsorption. With increasing burial depth, the proportion of free-state CO₂ shows an increasing trend, while the content of dissolved-state CO₂ exhibits a decreasing trend. The synergistic evolution of three states determines the vertical zoning of the CO₂ storage capacity. These findings advance the understanding of storage mechanisms following CO₂ injection into high-rank coal reservoirs and provide a quantitative method for assessing the feasibility and storage potential of such injections.