Analysis of diffusion properties of methane in low rank coal

Diffusion properties of methane in coal is an important reservoir parameter affecting CBM production. Using the lignite samples from the Xiaolongtan Formation of the Miocene Neogene in southeastern Yunnan Province, the isothermal adsorption experiments of methane in low rank coal were carried out. Based on the relationship between adsorption amount and time obtained from isothermal adsorption experiment, the diffusion coefficient of methane in coal was calculated by using an unipore model of unsteady gas diffusion. The diffusion law of methane in coal and its control mechanism were revealed. The results show that gas diffusion law in coal obeys Langmuir equation; methane effective diffusion coefficient and diffusion coefficient in coal increase with the increase of pressure; adsorption time constant decreases with the increase of pressure and obeys the law of negative exponential function. Langmuir effective diffusion coefficients and diffusion coefficients of four experimental coal samples are(1.71-5.46)×10^-4 s^-1 and(2.17-6.91)×10^-12 m²/s respectively. Langmuir pressure is 0.63~1.97 MPa. Under the same pressure and temperature, the effective diffusivity and diffusivity of dry coal samples are larger than that of equilibrium moisture coal samples. With the increase of temperature, the diffusivity of coal increases, and the effective diffusivity and diffusivity increase with the increase of temperature.