Experimental study on diffusion property of methane gas in coal and its influencing factors

The diffusion property of methane gas in coal is one of the key parameters which would influence the production of coal-bed methane well. The diffusion property of methane gas under different coal ranks, confining pressure, injected gas pressure, temperature and moisture content are analyzed by the experiment in methane gas diffusion. Relationships and their models between diffusion coefficient and the factors, such as vitrinite reflectance, confining pressure, injected gas pressure, temperature and moisture content are established. Controlled mechanism of methane gas diffusion in coal is revealed. The results indicate that under the same temperature, gas pressure and confining pressure, the diffusion coefficient of methane gas in different rank coals exhibit a trend of first dropping rapidly and then rising slowly(asymmetric “U” shape) with the metamorphic degree of coal increases. At the same temperature and gas pressure, with the increase of confining pressure, diffusion coefficient of methane gas in coal dropped by the power function. The diffusion coefficient of methane gas in coal increases with the increase of injected gas pressure. With the increase of temperature, the diffusion coefficient of methane gas in coal increases logarithmically. The diffusion coefficient of methane gas in saturated coal is smaller than that of dry coal. These results effectively guide coal-bed methane (CBM) development, and it has theoretical and practical significance.