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-CO 2 ) 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 CO 2 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-CO 2 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-CO 2 treatment. Results show that SC-CO 2 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-CO 2 modifies pore network structures in coal samples with bedding dip angles of 0°, 30°, 45°, and 90°. For the 60° coal samples, SC-CO 2 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-CO 2 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-CO 2 treatment, offering insights into optimizing CBM production and CO 2 sequestration strategies.