A new laboratory method for accurate measurement of the methane diffusion coefficient and its influencing factors in the coal matrix

Abstract

Abstract The diffusion coefficient is a key parameter for evaluating the methane diffusion properties of a coal matrix. However, all of the current measurement technologies for methane diffusion coefficients, including the particle method, the steady state method and the inverse diffusion method, have certain limitations. For example, the pore structure of the sample is easily destroyed, and the test time is long. Moreover, the results from different methods lack comparability under the various test principles and conditions. In this work, a new laboratory method for measuring the methane diffusion coefficient in coal matrixes was proposed, where coal matrix flakes instead of coal particles were used as the samples to be measured, and the factors influencing methane diffusion coefficients such as gas pressure, coal rank and moisture content were also analyzed. The results indicate that this new method can keep the intrinsic space structure of the coal matrix and yields methane diffusion coefficients on the order of 10−11–10−9 m2/s in coal matrix, which are close to realistic reservoir values. Under the same temperature, methane diffusion coefficients increase with increasing gas pressure regardless of the water saturation and coal rank. Additionally, the adsorption capacity increases when the metamorphic degree of coal increases, while the methane diffusion coefficient in coal matrixes exhibit a trend of first dropping and then rising (“U” shape) with an increase in coal rank. Moreover, moisture has an important effect on the methane diffusion coefficient. Increasing moisture can reduce the matrix adsorption capacity for methane, which makes it difficult to form a larger concentration gradient because the diffusion coefficients of dry samples are higher than those of the saturated water samples.