Damage effects on coal mechanical properties and micro-scale structures during liquid CO2-ECBM process

Abstract

The utilization of liquid CO2 for enhanced coalbed methane recovery has great environmental and commercial potential to sink greenhouse gas emissions and enchance CH4 extraction. A comprehensive understanding of mechanical properties and micro-scale changes to coal saturated with liquid CO2 would fill the knowledge gap regarding CO2 utilization and be greatly beneficial to industry. The present study employed extensive experimentation using uniaxial compression test, acoustic emission (AE) monitoring, Brazilian splitting test, scanning electron microscopy, and energy dispersive X-ray diffraction spectrometry to determine the damage features of coal microstructure, mechanical characteristics, failure modes, and minerals variation between original and liquid CO2-saturated coal samples. The pre-existing cracks were enlarged and more cracks were generated in the saturated coal samples than in the raw samples. The liquid CO2-saturated coal samples exhibited larger plastic deformation and lower compressive strength in unstable crack propagation stage and more AE counts in rupture stage than those of unsaturated samples. The average compressive strength and tensile strength of the saturated samples were 21% and 27% lower than that of the raw samples. More ragged failure cracks were formed in the liquid CO2-saturated coal specimens than in the unsaturated coal samples. The failure modes of the saturated coal samples were inclined to ductile fracture as opposed to brittle fracture in the unsaturated coal samples. The width and number of micro-cracks on the saturated samples were both increased, and the amount of partial minerals that filled in the micro-cracks reduced after liquid CO2 saturation, which indicated an interaction between minerals in coal and liquid CO2 during the saturation process.