Mineralogical control on the dissolution induced microscopic pore alternations of high-rank coal during CO2 sequestration

Abstract

The microstructure of deep coal seam plays a crucial role in determining the CO2 geological sequestrations. To investigate the effects of pore and mineral distribution on the changes and mechanisms of coal microstructure under supercritical CO2 (SC–CO2) interactions, we employed low-temperature N2 (LTN2) adsorption, low-temperature CO2 (LTCO2) adsorption, and X-ray diffraction (XRD) techniques to explore the development of microstructures and mineral compositions of coal samples with distinct pore distribution characteristics under SC-CO2 exposure. The results revealed that mineral dissolution under SC-CO2 saturation led to an increase in both specific surface area (SSA) and pore volume (PV) of mesopores, with SSA increasing by 13.6%–332.31% and PV increasing by 4.59%–148.61%. Subsequently, SC-CO2 exposure induced the formation of numerous new pores with diameters smaller than 3 nm within the coal matrix, particularly an abundance of new ultra-micropores (r > 0.5 nm) and large micropores (r > 0.85 nm). This significantly altered the irregularity of pore structure and adsorption capacity, thereby playing a crucial role in CO2 storage. Micropores exhibited much larger SSA and PV than mesopores, underscoring their significant contribution to CO2 geological sequestration. The spatial mineral distribution in coal influences the alteration characteristics of pores, which explains the different microscopic pore structures formed after SC-CO2 exposure. The impact of spatial mineral distribution on SC-CO2-induced pore alteration is analysed, emphasising the intrinsic heterogeneity of minerals within coal as the fundamental factor behind the diversity observed in pore development during CO2 sequestration. Finally, we found that the changes in micropores and mesopores under SC-CO2 exposure are closely related to the initial structure of microscopic pores. The development of micropores under SC-CO2 exposure is associated with the average pore size and fractal dimension of micropores, with the increment in mesopore irregularity increasing with the increase in micropore volume fractal dimension. By studying the influence of mineral and pore distribution on the evolution of coal microstructures, this study provides new insights into pore development under SC-CO2 geological sequestration, which can be valuable for predicting reservoir development in CO2-ECBM.