CO2 interaction induced mechanical characteristics alterations in coal: A review

Abstract

Abstract Coal geo-mechanical alterations caused by CO2 interaction should be critically evaluated to enhance the efficiency of CO2-ECBM and CO2 sequestration processes, while mitigating associated hazards. We provide a detailed overview on the CO2 interaction induced mechanical alterations in coal, by comprehensively reviewing the influential parameters and causative factors. Although studies provide persuasive evidence to conclude the general fact that CO2 interaction causes severe mechanical degradation in coal, the degree of strength reduction is influenced by numerous parameters: 1) stress state - elevated in-situ stresses diminish the CO2-induced coal strength reduction, due to reduced CO2 adsorption potential and the hindered swelling effect, 2) CO2 adsorption pressure and phase - magnitude of strength reduction increases with increasing CO2 adsorption pressure and CO2 phase transition into supercritical state causes a noticeable strength reduction, because of the higher adsorption and polymerization capacities of supercritical CO2, 3) CO2 interaction time - due to rapid CO2 adsorption process, significant mechanical degradation occurs in the short-term, whereas additional CO2 exposure continuous to reduce the strength, but at a slower rate, 4) coal seam characteristics - high-rank coal with a well-formed cleat system exhibits greater strength reductions, compared to low-rank coal, because coal cleat system facilitates easy invasion of CO2 and acts as a locus for CO2 adsorption, expanding the coal matrix along the cleat walls. Moreover, we highlight that there are multiple factors that may contribute to the overall mechanical strength alteration: 1) CO2 adsorption induced surface energy reduction, 2) CO2-induced plasticization of coal polymer-structure, 3) formation of micro-cracks due to shrinkage, free- and differential-swelling, and 4) dissolution of cleat and pore filling minerals, causing bond energy reduction, influencing grain-to-grain contact and forming secondary porosity systems in coal. However, we infer that the mechanical degradation depends on the heterogeneity and anisotropy of the coal reservoir. Therefore, the degradation should be specifically assessed by considering the targeted reservoir characteristics, prior to the implementation of CO2-ECBM and CO2 sequestration field projects.