Diffusion characteristics of methane in various rank coals and the control mechanism

The methane diffusion behavior is one of the key factors that influence the production potential of coalbed methane (CBM) reservoirs. In this study, low temperature nitrogen, carbon dioxide adsorption and methane isotherm adsorption experiments for various rank coal samples were carried out to investigate the characteristics of pore size distribution and gas adsorption-diffusion behavior. A mathematical model similar to the Langmuir equation was established to describe the methane diffusion in coals. The variation law of the Langmuir effective diffusion coefficient (Dr) with coal rank and its influencing factors, control mechanism were analyzed. With the increase of coal rank, the development degree of micropore rises, and both the specific pore volume and specific surface area first decrease and then increase. The diffusion coefficient of methane in coals generally rises with the rise of pressure. The Langmuir volume rises with the rise of R-o,R-m (maximum vitrinite reflectance with oil). The influence of moisture and coal molecular structure on gas diffusion varies with coal rank. The moisture content has a more significant influence on the gas diffusion in the medium-rank coals than in the low- and high-rank coals. The outcomes may deepen our understanding of gas transport in coals and benefit the exploration of CBM.