Thermodynamic Method for Evaluating the Gas Adsorption-Induced Swelling of Confined Coal: Implication for CO2 Geological Sequestration

Geological storage of CO2 in coal seam is an effective way for carbon emission reduction. Evaluating the adsorption-induced swelling behavior of confined coal is essential for this carbon emission reduction strategy. Based on the thermodynamic theory and the Gibbs adsorption model, a thermodynamic method for evaluating the gas adsorption-induced swelling behavior of confined coal was established. The influences of factors such as stress, gas pressure, and the state of gas on the adsorption-induced swelling behavior of confined coal were discussed. The predicted swelling deformation from the thermodynamic method based on the ideal gas hypothesis was consistent with the experimental result only under the condition of low-pressure CO2 (<2 MPa). The predicted swelling deformation from that method was larger than the experimental result under the condition of high-pressure CO2 (>2 MPa). However, the method based on the real gas hypothesis always had better prediction results under both the low- and high-pressure CO2 conditions. From the perspective of phase equilibrium and transfer, in the process of CO2 adsorption by the confined coal, gas molecules transfer from the adsorption site of high chemical potential to the low chemical potential. Taking the real gas as ideal gas will result in the surface energy increase in the established model. Consequently, the prediction result will be larger. Therefore, for geological storage of CO2 in coal seam, it is necessary to take the real gas state to predict the adsorption-induced swelling behavior of the coal. In the process of CO2 adsorption by the confined coal, when its pressure is being closed to the critical pressure, capillary condensation phenomenon will occur on the pore surface of the confined coal. This can make an excessive adsorption of CO2 by the coal. With the increase in the applied stress, the adsorption capacity and adsorption-induced swelling deformation of the confined coal decrease. Compared to N2 with CO2, the coal by CO2 adsorption always shows swelling deformation under the simulated condition of ultra-high-pressure injection. However, the coal by N2 adsorption will shows shrinking deformation due to the pore pressure effect after the equilibrium pressure. Taking the difference in the adsorption-induced swelling behavior and pore compression effect, N2 can be mixed to improve the injectivity of CO2. This suggests that CO2 storage in the deep burial coal seam can be carried out by its intermittent injection under high-pressure condition along with mixed N2.