Evolution Characteristics of Coal Microstructure and Permeability during the Liquid CO2-ECBM Process for Carbon Utilization

Abstract

The utilization of liquid CO2 (LCO2) for enhanced coalbed methane recovery holds significant promise for developing unconventional natural gas and controlling gas disasters. A comprehensive revelation of the evolution characteristics of coal microstructure under LCO2 stress damage and CO2 hydrate acid erosion is essential for understanding coalbed permeability enhancement. Using experimental and theoretical methods, this study revealed the multiscale evolution of coal’s chemical components, pore structure, and permeability under LCO2 treatment. With increasing LCO2 impact pressure and CO2 hydrate acid erosion duration, coal mineral content and pore-throat channels underwent dissolution, acid leaching, transformation, and precipitation. The major coal elements─Mg, Ca, and Na─exhibited distinct migration behaviors, with migration maximum rates of 22.73%, 29.12%, and 32.58%, respectively. Their concentration variations directly reflect dissolution, precipitation, transformation, and the reprecipitation of coal mineral components. Meanwhile, the coupled effects of LCO2 impact damage and CO2 hydrate acid erosion promoted aromatic polycondensation and aliphatic/aromatic oxidation, increasing carboxyl, carbonyl, and ether groups by 48.53%, 47.36%, and 51.16%, respectively. Cleavage of aliphatic chains, ether bonds, and −OH groups exposes more −OH on coal, raising polarity and weakening the macromolecular framework. This creates numerous dissolution pores, enhances pore connectivity, and improves throat openness. Long-term low-temperature freezing, transient phase-transition stress, and chemical acid erosion induced by LCO2 irreversible fatigue damage in coal, formed abundant new pores. Changes in proportions of micropores (xAW), mesopores (xCW), macropores (xBW), and pore water variation ratios (ηi) show that adsorption pores transform into seepage pores, optimizing gas flow. Porosity (Φ) and permeability (k) increase linearly, up to 108.96% and 128.57%, respectively. These findings confirm that physical impact from LCO2 combined with chemical erosion from CO2 hydrate acidification synergistically enhances coal permeability, promoting gas extraction.