Comparative Study of Enhanced CH₄ Desorption in Real-Reservoir Coal Containing Trace O₂ under Injection of Different...

To enhance the extraction efficiency of deep, low-permeability coalbed methane, gas-injection techniques have attracted attention for their combined effects of permeability enhancement and production stimulation. Previous studies, however, lack systematic comparisons among different injected gases (N₂, CO₂, H₂O) and often neglect trace O₂ in coal, leading to discrepancies with actual reservoir conditions, while thermal coupling effects of superheated gas injection are rarely considered. In this work, Xiaolongtan lignite from Yunnan containing trace O₂ was investigated via molecular dynamics simulations across a temperature range from 373.15 to 773.15 K (100-500 °C) to examine CH₄ desorption behavior. For the first time, the Grand Canonical Monte Carlo method was employed to determine the lowest-energy adsorption configurations of CH₄ and O₂. Results show that the enhancement of CH₄ desorption follows the order H₂O > CO₂ > N₂. The results revealed that within the vacuum region, H₂O injection produced the maximum CH₄ average relative concentration, the largest diffusion coefficient of 3.454 × 10^-20 m²·ps^-1, and exhibited a diffusion activation energy of 0.101 kcal/mol, the lowest, indicating a reduction in the diffusion barrier for CH₄. This study provides important parameters and a theoretical basis for the thermal-injection-enhanced CBM recovery.