Laboratory and Modeling Study on Gas Diffusion with Pore Structures in Different-Rank Chinese Coals

Abstract

The coalbed methane production rate is mainly controlled by two parameters: diffusivity in the coal matrix and permeability in the cleat system. As one key parameter, diffusion plays an important role in coalbed methane producing process. However, till now there is no systematic study on the relationship of diffusivity, pore size and coal ranks. In this work, three Chinese coal samples which belong to low, middle and high rank, respectively, were studied using experimental and modeling methods. Four gases, H2, N2, CH4 and CO2, are used to study the diffuse characters at four different pressures steps. The experimental results showed that the adsorption balance time varies with different testing gas type and it is closely related with coal rank. Balance time testing with CO2 is the longest, and then is CH4, while He is the slowest. Moreover, high rank coal takes the longest time to reach balance, while low rank coal takes the least. Generally, the sorption balance time for JCC-01 is about 150–350 S 0.5, it is about 250–400 S 0.5 for CZ-1, while it is about 900–1200 S 0.5 for JCC-01. Modeling results showed that for all these three ranked Chinese coals, the bidisperse model can be used to model the diffuse process. The β value, which is the ratio of macropore adsorption/desorption to the total adsorption/desorption, increases with the increasing of pore pressure, except for sample JCC-01 when measured using CO2. There is no regular law for both micropore and macropore diffusion coefficient. In order to study the relationship of diffusivity, pore structure and coal ranks, the experiments of mercury injection test and low-temperature liquid nitrogen experiment were done to analyse the relationship. The results show that, for the low rank coal sample TCG-1, the mesopore takes the majority while the macropore also takes part of the pore distribution, there are few even no micropore. For middle rank sample CZ-1, the mesopore also takes the majority and the macorpore accounts for a small percentage. While for high rank coal sample JCC-01, micropore takes the majority, and mesopore and macropore take small part of the pore structure distribution. The conclusion drawn from these testing results can be used to explain the adsorption and diffusion laws found before.