Quantitative assessment and case study of CO₂ geological storage in deep coal seams

Geological storage of CO₂ as an idea carbon reduction technology is expected to become an important means of mitigating the greenhouse effect. Therefore, quantitatively assessing the potential of geological storage of CO₂ in deep coal seams and investigating the interaction between supercritical CO₂ and deep coal rocks have become a hot research topic. Taking Jiulishan coal sample from Jiaozuo mining area in Henan, China as the experimental research object, we analyse the mechanism of supercritical CO₂ adsorption and sequestration in deep coal seams, carry out CO₂ isothermal adsorption experiments of the coal samples at 35 ℃ and 45 ℃, explain and correct the error of negative adsorption isotherms under high pressure, and obtain the actual adsorption amount of CO₂ of the coal sample at different temperatures. Here, we propose a new method for calculating CO₂ geological storage capacity, which can not only correct the storage capacity miscalculation caused by Gibbs adsorption, but also can accurately evaluate CO₂ theory and effective storage capacity in different burial depths of coal seams. results show that: ① In theory, adsorption saturation means that all adsorption sites have been occupied, the volume and density of the adsorption phase have stabilized, and the adsorption amount should no longer changes. However, all adsorption isotherms measured in the laboratory show that the adsorption amount decreases with the increase of pressure under high-pressure saturation stage, which does not conform to the Langmuir adsorption principle. Therefore, the adsorption isotherm measured in the laboratory must be corrected before it can be applied to the assessment of CO₂ storage capacity in deep coal seams; ② The CO₂ storage capacity in coal mainly consists of the adsorption and free CO₂ amount. The adsorption CO₂ amount needs to be calculated using the adsorption phase density and Gibbs adsorption amount, while the free CO₂ amount needs to know the pore volume occupied by the free phase in coal. It can only be calculated based on the total pore volume in coal minus the adsorption phase volume. Therefore, the adsorption phase is the decisive factor for accurately evaluating the adsorption and free CO₂ storage capacity; ③ Using a modified CO₂ geological storage quantification model and taking the 800-2000 m deep coal seam in the Xiuwu research area of Jiaozuo mining area as an example, it is calculated that The theoretical storage capacity of CO₂ per unit mass of coal is 1.52−2.16 mmol/g, The total effective storage capacity is 11.19×10⁹ m³, which is equivalent to 21.97 Mt. This case not only corrected the mass balance miscalculation of Gibbs adsorption data, but also considered the impact of adsorption phase occupying pore space on free storage capacity, and thus, it has important implication for improving the accuracy of predicting CO₂ geological storage capacity in deep coal seams.