Microscopic pore structure changes in coal induced by a CO2-H2O reaction system

Geological carbon dioxide (CO2) sequestration in deep coal beds is a potential technique for enhancing coal bed methane recovery. Since porosity and connectivity between pores or fractures are the most important parameters affecting mineral dissolution and percolation of coal bed methane, it is crucial to understand the pore structure changes in coal induced by CO2 injection. To investigate the role that mineral reactions play in coal pore structure changes, three groups of CO2-H2O-coal interaction experiments were carried out in parallel at 40 °C and 5 MPa. Based on X-ray diffraction (XRD) analysis, the mineral compositions of coal samples after interaction exhibit significant changes, notably a marked decrease in calcite and dolomite. The pore type, pore size distribution, effective porosity, and spatial configuration of coal samples before and after interaction were studied through combined low-temperature N2 adsorption and desorption (LTNAD), nuclear magnetic resonance spectroscopy (NMR), and X-ray computed tomography scanning (CT). The results show that new types of pores were formed and pore shapes became more complex due to mineral reactions. The number of macropores and fractures increased significantly, with the average pore diameter also increasing. Overall, the experiments show that CO2-H2O-coal interaction plays a positive role in pore structure modification, which can effectively enhance the coal bed methane recovery.