Qualitative and quantitative evaluation of the alteration of micro-fracture characteristics of supercritical CO2-interacted coal

CO2 geo-sequestration necessitates injection of CO2 into coal reservoirs, in which CO2 becomes supercritical due to high pressure/temperature conditions. Consequently, the coal-supercritical CO2 (S - CO2) interaction triggers micro-structural alterations, inferring the necessity of temporal and spatial analyses. In this study, three imaging techniques were used to evaluate the S - CO2-induced micro-structural alterations in heterogeneous coal specimens. Results from the qualitative analysis indicate that S - CO2 interaction causes new fracture formation and extension/widening of existing fractures in coal. Quantitative analysis concludes that fracture fraction and fracture-matrix interface area increase linearly with S - CO2 interaction time. Spatial disposition of mineral/maceral phases at specific arrangements significantly affects the induced fracture geometry, where most of the fractures form at mineral-maceral interfaces, parallel to mineral layering. Comparison of S - CO2 interaction under unconstrained and constrained conditions reveals that S - CO2-induced micro-fracture alterations have both similarities and dissimilarities, due to specific swelling mechanisms.