Apparent and True Diffusion Coefficients of Methane in Coal and Their Relationships with Methane Desorption Capacity

Abstract

The diffusion coefficient of methane in coal is a key parameter for the prediction of coalbed methane production. The apparent diffusion coefficient is different from the true diffusion coefficient, which would result in the deviation of methane production. In this study, the particle method using the unipore model and the counterdiffusion method is adopted to measure the methane diffusion coefficients. The results indicated that the true diffusion coefficient obtained by the counterdiffusion experiment decreases first and then increases with increasing methane pressure. The apparent diffusion coefficients obtained by the particle method with two different grain sizes are lower than the true diffusion coefficient. The relationship between the apparent diffusion coefficient and the true diffusion coefficient is analyzed, and the desorption capacity factor (DCF) is proposed to reflect the gap between them. The apparent diffusion coefficient is closer to the true diffusion coefficient when the DCF is small. When using the particle method to estimate the methane diffusion coefficient, experiments with large coal particles and a small methane concentration gradient should be adopted.

Experimental and Simulation Studies on Adsorption and Diffusion Characteristics of Coalbed Methane

“…We determined that this error rate occurred due to the small size of the sample and the assumptions of the model, such as homogeneous medium and isotropic permeability. The diffusion coefficient calculated by the history matching was 2.66 × 10 −10 cm 2 /s, which is reasonable compared with the typical value calculated from experiment studies [32][33][34]. The cleat properties, fracture permeability and fracture porosity, were adjusted from 4 mD to 23.85 mD, and from 0.005 to 0.15 because of the large number of cleats compared to the area of coals.…”

Gas Diffusion in Coal Powders is a Multi-rate Process

“…A common method of measuring gas diffusion coefficients is the so-called "particle method" (Dong et al 2017). This involves grinding coal into a powder and sieving out particles within a fixed diameter interval.…”

“…The vessel is then exposed to atmospheric pressure, and the volume of gas produced from the vessel is recorded with time. The diffusion coefficient of the coal is estimated by calibrating a mathematical model of gas diffusion to the observed gas production time-series (Guo et al 2016;Yue et al 2017;Dong et al 2017;Cheng-Wu et al 2018).…”

“…More recently, spherical diffusion models with transient diffusion coefficients have been adopted, whereby a spatially uniform diffusion coefficient is defined using a heuristic function that continually declines with time (Dong et al 2017;Yue et al 2017;Zhao et al 2017;Cheng-Wu et al 2018). Such an approach leads to simple to evaluate analytical solutions, which are straightforward to accurately calibrate against observed data.…”

Influence of Overlying Caprock on Coalbed Methane Migration in the Xutuan Coal Mine, Huaibei Coalfield, China: A Conceptional Analysis on Caprock Sealability

“…The diffusion coefficient is calculated through the counter diffusion method, which is derived from the diffusion concentration difference between both ends of the sample container. Following the gas diffusion in coal follows Fick's law; the diffusion coefficient can be fundamentally calculated as follows [39].…”