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

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.