Effect of supercritical-CO2 interaction time on the alterations in coal pore structure

The supercritical CO2 (S-CO2) interaction time is a vital parameter in CO2 geo-sequestration in coal seams, which is under-evaluated up to date. Through this study, we provide an experimental insight into this aspect by evaluating the effect of S-CO2 interaction time on the coal pore structural alterations. The results revealed that the S-CO2 pre-exposure has a notable impact on the adsorption capacity of coal, in which short-term (order of hours) and long-term (order of weeks) S-CO2-treated coal show less and higher adsorption capacities, respectively, compared to untreated coal. The pore characterisation results infer that both surface area and pore volume are decreased by about 20% after short-term S-CO2 treatment and then show an approximate linear increment with further longer treatment time. Based on the experimental observations, we propose a conceptual model to explain the temporal alterations, in which we elucidate that the complex temporal alterations in coal pore morphology occur due to the combined effect of multiple factors: 1) S-CO2 adsorption-induced coal matrix swelling and consequent pore shrinkage, that causes physical constriction of pores, reducing the surface area and the pore volume, 2) S-CO2-induced plasticization that extracts and mobilizes the low-molecular-weight hydrocarbons, which are entrapped in pore mouths or necks, increasing the number of accessible pores. The resultant overall alteration at a given S-CO2 pre-exposure time depends on the competitive behaviour and the significance of the multiple processes. The proposed model is supported by 1) measuring the S-CO2 adsorption-induced volumetric swelling of the coal matrix, which concludes that 70% of volumetric swelling has already been completed within 24 h of coal-S-CO2 interaction, causing the pore shrinkage and, 2) analysing the FT-IR spectrums, which confirms the S-CO2-induced bond dissociation and substitution, confirming the re-arrangement of coal macromolecular structure that possibly leads to hydrocarbon mobilization and pore structural alterations.