Hysteresis mechanism of supercritical CO₂ desorption in coal and its implication for carbon geo-sequestration

Sequestration of CO₂ in the unmineable coal seams is not only one of the most ideal options for reducing greenhouse gas effects, but also the only way for the coal industry to reduce CO₂ emissions and achieve low carbonization sustainable development. However, the key unresolved issues regarding the CO₂ geo-sequestration in coal seams is: “how long does CO₂ injected into a coal seam remain in the seam?”. In this regard, on the basis of clarifying the hysteresis law of CO₂ desorption in coals, this paper reveals the mechanism of supercritical CO₂ desorption hysteresis, establishes a quantitative model for the geological storage of CO₂, and explores the use of desorption hysteresis to achieve a long-term safe storage of CO₂ in coal seams. The study results shows that the degree of desorption hysteresis of supercritical CO₂ in coal is greater than that of subcritical CO₂, and a stable hysteresis characteristic similar to a “parallel line” in the supercritical phase is formed between the adsorption and desorption isotherm. The fundamental reason for the desorption hysteresis is that the micro and nano sized pores in coal form curved surfaces due to their hydrophilicity, which generate strong capillary pressure following the Laplace’s equation, absorb liquid water, truncate and fix the supercritical CO₂ fluid, and ultimately form CO₂ residual trapping. For example, the cylindrical inorganic pores with a diameter of 40–10 nm in coal can generate a capillary pressure of 7.30–29.12 MPa, which is sufficient to block supercritical CO₂. Taking the desorption isotherm of Jiulishan coal as an example, using the quantitative model for the geological storage of CO₂ established in this study, it has been estimated that the total trapping capacity of the No.21 coal seam at depths of 900–1 500 m is stable at 35–37 m³/t. Among them, the adsorption trapping capacity accounts for about 80%, residual trapping capacity accounts for about 15%, and structural trapping capacity only accounts for 5%. Desorption hysteresis suggests that some measures should be taken to increase the proportion of CO₂ residual trapping in coal seams as much as possible, the reason is that the residual CO₂ sealed by capillary blockage is safer and has no risk of leakage compared to the free and adsorbed CO₂ sealed by surrounding rock. The physical parameters such as ash content, moisture content, pore size, and morphology of coal seams are the main factors affecting the residual trapping efficiency.