Impact of supercritical CO2 saturation temperature on anthracite microstructures: Implications for CO2 sequestration

Abstract

CO2 sequestration in coal seams is one of promising methods to achieve the goal of carbon neutrality, which has attracted the worldwide attention. Reservoir temperature plays a vital role in supercritical CO2 (ScCO2)-coal interactions and it even serves as a crucial factor in determining the CO2 trapping capacity and the feasibility of CO2 geological storage in these coal formations. The impact of saturation temperature on the interactions between coal and ScCO2 at geo-storage conditions is rarely reported in the literature, which directly affects the recovery of CO2-enhanced coalbed methane and the sealing integrity of structural trap provided by the caprocks. The microstructural changes of coal is also essential in prevention of upwards migration and CO2 leakages. In this paper, changes in coal microstructures including functional groups, mineral compositions and multiscale pore distribution at the saturation pressure of 10 MPa and various temperatures of 35–85 ℃ were comprehensively investigated using the combination of low-pressure CO2/N2 adsorption, X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The results show that micropores and mesopores are enhanced by 24.2–42.7% and 25.8–49.6%, respectively, while macropores are decreased by 4.9–19.5%. The carbonate minerals are most sensitive to ScCO2, and temperature rise accelerates the mineral dissolution to form more new pores within coal matrix. ScCO2 has strong extraction capacity for hydrocarbons, and approximately 80% of aliphatic structures are extracted from coal macromolecules. The higher saturation temperature, the more aliphatic groups are mobilized. Some isolated organic pores or closed pores are also extracted and damaged to become accessible for gas flow, which directly enhances the CO2 storage capacity in coal reservoirs. Once the temperature exceeds 65 ℃, the further temperature rise has less influence on coal microstructures. The results of this study may play a significant role in assessing the hydrocarbon recovery and the trapping of CO2 in coal reservoirs.