Multiscale damage characteristics and energy evolution patterns of coal treated with supercritical carbon dioxide

Abstract

Coalbed methane (CBM) reservoirs in China are typically characterized by low permeability, requiring stimulation techniques to improve gas recovery. Supercritical carbon dioxide (SC-CO2) fracturing has emerged as a promising waterless stimulation method, offering advantages such as improved reservoir integrity, the enhancement of complex fracture networks, and potential for CO2 geological sequestration. As a sedimentary rock, coal exhibits a distinct bedding structure formed during diagenesis, critically affecting permeability enhancement and CBM extractability. To evaluate the effects of SC-CO2 exposure on coal containing bedding planes with different orientations, a series of experiments is conducted using coal samples with bedding dip angles of 0°, 30°, 45°, 60°, and 90°. A multi-technique approach, including Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), ultrasonic testing, and acoustic emission (AE) monitoring, is employed to characterize changes in functional groups, pore structure, mechanical properties, and acoustic properties before and after SC-CO2 treatment. Results show that SC-CO2 significantly reduces the intensity of infrared spectral peaks and alters the abundance of functional groups in coal samples, thereby facilitating the desorption of coalbed methane. Pore structure analysis reveals that SC-CO2 modifies pore network structures in coal samples with bedding dip angles of 0°, 30°, 45°, and 90°. For the 60° coal samples, SC-CO2 promotes the development of multiscale pores, thereby providing enhanced pathways for gas migration. Ultrasonic results indicate that P-wave velocity reduction increases with the bedding dip angle, while wavelet packet energy undergoes redistribution and concentration, with the most pronounced fluctuations in sub-bands 1 and 10. Uniaxial compression tests reveal a marked reduction in compressive strength, coupled with decreased energy absorption and increased energy dissipation capacity. AE analysis demonstrates b-value trends across different stress loading stages, reflecting distinct failure modes—progressive and sudden failure. Furthermore, SC-CO2 treatment increases the number and proportion of AE events concentrated in dominant frequency bands, indicating enhanced macro-crack propagation. This study provides a comprehensive understanding of the multiscale damage characteristics and energy evolution of coal under SC-CO2 treatment, offering insights into optimizing CBM production and CO2 sequestration strategies. FTIR is employed to characterize functional group alterations in the SC-CO2-treated coal. NMR multifractal and ultrasonic wavelet packet analyses are utilized to characterize pore evolution and pore complexity in the SC-CO2-treated coal. Mechanical tests and AE analyses are carried out to characterize energy transformation and crack propagation in the SC-CO2-treated coal under uniaxial loading. The multiscale damage characteristics and evolution of the coal treated by SC-CO2 are discussed and elucidated with the comprehensive consideration of FTIR, NMR, ultrasonic, mechanical, and AE tests. FTIR is employed to characterize functional group alterations in the SC-CO2-treated coal. NMR multifractal and ultrasonic wavelet packet analyses are utilized to characterize pore evolution and pore complexity in the SC-CO2-treated coal. Mechanical tests and AE analyses are carried out to characterize energy transformation and crack propagation in the SC-CO2-treated coal under uniaxial loading. The multiscale damage characteristics and evolution of the coal treated by SC-CO2 are discussed and elucidated with the comprehensive consideration of FTIR, NMR, ultrasonic, mechanical, and AE tests.