Damage characteristics of coal microstructure with liquid CO2 freezing-thawing

Abstract

Enhanced coalbed methane (ECBM) recovery is significantly affected by the pore structure and cleat network of coal during liquid CO2 (LCO2) injection into unmineable coal seams. However, the changes in the microstructure after coal samples are treated with LCO2 freezing-thawing are poorly understood. Therefore, a combination of mercury intrusion porosimetry (MIP) and X-ray micro-computed tomography (micro-CT) methods was employed to investigate the evolution of the pore structure and cleat network in coal samples treated with LCO2 under reservoir conditions (0.5 MPa, −50 °C, for 6 h). The results indicated that average increase rates of the total pore volume, total pore area, and average pore diameter are 18.60, 3.24, and 29.90%, respectively, for the treated coal specimens. The increase rates of the specific surface area and incremental pore volume were higher for seepage pores than for adsorption pores after the coal samples were treated with LCO2. Numerous new pores are generated and the seepage capacity of the pores is increased as a result of LCO2 injection for ECBM processes. Comparison of the fractal dimension D1′ of the treated specimens (average value of 2.82) with that of raw coal samples (D1, average value of 2.71) revealed that the treated specimens had more evident fractal characteristics, indicating their more complex morphology and stronger seepage capacity. Micro-CT analyses indicated a significant amount of newly generated fractures and showed that many original cleats expanded after the coal samples underwent LCO2 freezing-thawing. The connectivity of the cleat network in the coal slices was poor, resulting in slightly increased permeability of the treated specimens, which indicateds that the degree of LCO2-induced damage to the coal pore structure was greater than that to the cleat network under the laboratory conditions.