Mineral dissolution and pore alteration of coal induced by interactions with supercritical CO2

Abstract

CO2 injection into coal seams can not only enhance the recovery of coalbed methane, but also achieve the goal of CO2 geological sequestration, reducing greenhouse gas emissions. CO2 is likely to be stored in the form of supercritical state, when the burial depth of coal seams exceeds 800 m. However, the existing knowledge is still lacking regarding how supercritical CO2 (SC–CO2) reacts with coal. In this work, alterations of pore structure and mineral compositions induced by SC-CO2 interactions have been explored. The results show that coal pore volume and porosity are greatly increased after SC-CO2 interactions, which contributes to the increase of gas adsorption. A comparison of pore fractal dimension has been made between raw coal and treated samples, suggesting that SC-CO2 interaction can enhance pore roughness and structure complexity. XRD analysis indicates that mineral content within coal is remarkably changed after SC-CO2 saturation. Carbonate minerals are sensitive to SC-CO2 fluids, and mineral consumption plays a key role in pore alterations. There are multiple effects of SC-CO2 fluids on both mineral compositions and pore distribution. Mineral dissolution, differential swelling, hydrocarbon extraction and migration account for the variations of coal microstructures and mineral content. The increase in pore volume and structure complexity of coal seams after interaction is quite favorable for CO2 adsorption and storage. This result provides powerful support for CO2 geological sequestration and CO2-ECBM.