Molecular Insights into Adsorption/Free Gas Storage and Transport in Nanopores of Coal

Abstract

Accurately predicting the content of adsorption gas and free gas in nanopore structures, as well as the dynamic production of adsorption gas and free gas, is of great importance for the production of coalbed methane wells. However, the storage and transport of adsorption gas and free gas in different nanopores are still unclear. In this study, the molecular adsorption simulation results and pore size distribution data were combined to calculate the amounts of adsorption gas and free gas in different pore sizes of the coal sample. The molecular mechanism for the adsorption gas and free gas in tunnels of different sizes during the transport behaviors is proposed. The results indicate that in pore sizes smaller than 1 nm, the amount of adsorption gas is 18.85 cm³/g, which is up to 74.31% of the total amount of adsorption gas, hardly any free gas is present. At 1–2 nm pores, the amount of adsorption gas is 4.14 cm³/g, and then the amount of free gas is 1.20 cm³/g, which provides about 71.59% of the total amount of free gas. In other types of pores (2–5, 5–10, and >10 nm), the amounts of adsorption gas are 0.30, 0.23, and 0.17 cm³/g and the amounts of free gas are 0.05, 0.07, and 0.35 cm³/g, respectively. By transport simulation, a molecular mechanism for the adsorption gas and free gas in tunnels of different sizes during the transport behaviors was provided. In the first stage, with more free methane molecules stored in the macropores and fractures expelled from the coalbed, the gas production from coalbed methane wells has risen sharply at the beginning of the stage. Subsequently, the dramatic depletion of free methane molecules results in the desorption and migration of adsorption methane molecules. In the second stage, more adsorption gas is desorbed and migrates into the micropores, thereby providing a stable increase in gas production. In the last stage, only a small amount of free gas is initially stored in the macropores and fractures and the adsorption gas desorbs from the micropores and begins to slowly migrate through different size tunnels; the gas production in coalbed methane wells will gradually decrease.