Pore structure and gas content evaluation of coal-rock gas using well log data

The pore structure of coal significantly affects the adsorbability, desorption, and seepage behavior of coal-rock gas. This study firstly represents a comprehensive assessment of pore structure and fractal dimension characteristics for the low-medium rank coal of the Jurassic-Triassic, Kuqa Depression, Tarim Basin, China, with a focus on their implications for methane adsorption capacity. In addition, integration the laboratory and well log data (CMR_NG) are used to evaluate gas bearing property of coal seams. Coal samples of low-medium rank were analyzed using low-pressure CO 2 adsorption (LP-CO 2 GA), low-temperature N 2 adsorption (LT-N 2 GA), mercury intrusion porosimetry (MIP), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM). The adsorption-desorption isotherms of coal samples are predominantly Type H3 and H4, indicating parallel plate pores and ink-bottle or narrow slit pores, which promote coal-rock gas enrichment. Micropores, responsible for most specific surface area (SSA), dominate the pore structure, alongside macropores with two T 2 peaks. Fractal dimensions (D 1 , D 2 from LT-N 2 GA; D 3 , D 4 from NMR) reflect pore characteristics, where higher D 1 and D 2 correlate positively with SSA and total pore volume (TPV) of micropores and mesopores. Langmuir volume (V L ) correlates with D 3 , indicating greater adsorption capacity, while lower D 4 suggests better connectivity and permeability. Furthermore, industrial components and gas bearing property are evaluated by using conventional well log and CMR_NG log. The favorable coal reservoir exhibits high fixed carbon content, a broad T 1 spectrum, and bimodal T 2 distributions from CMR_NG, indicating both adsorbed and free gas with high coal-rock gas content. These insights could enhance understanding of low-medium rank coal reservoir pore structure and fractal dimension characteristics, as well as their influence on methane adsorption, gas bearing property and seepage capacity. • The full-scale pore structure and fractal dimensions of low-rank coal are comprehensive analyzed. • Relationships between fractal dimensions and methane adsorption and seepage capacity are discussed. • Pore structure and gas bearing property are analyzed by integrating geophysical well logs and experiment data.