A molecular dynamics study of methane/water diffusion and water-blocking effects in coalbed methane

L Lin and H Lu and LG Huang and PY Luo, FUEL, 386, 134234 (2025).

DOI: 10.1016/j.fuel.2024.134234

Coalbed methane is a valuable non-traditional resource, but its extraction efficiency is affected by various factors. Fracturing fluid injected during coal seam mining and indigenous liquid water in the coal seam can impede the diffusion and exploitation of coalbed methane, leading to the water-blocking effects (WBE). The cause, mechanism, and mitigation methods of the WBE are still not completely clear, and are also the topics of current research. To address this, a molecular model of coal seam pores was constructed in this study, which accurately reflects the chemical structure of coal seam. Using molecular dynamics simulations, the two-phase flow of methane and water in coal seam was studied at the micro level, and the effects of initial water film thickness, pore size, reservoir pressure difference, and porosity on the strength of WBE were quantitatively calculated. The results show that the presence of water greatly hinders the diffusion of methane, leading to WBE. Pore size and flow pressure difference in the coal seam also affect the two-phase flow rate of methane and water. Larger pore sizes, higher pressure differences, and high porosity are more conducive to the flow of methane and water. The mechanism of WBE was further investigated, with the study finding that a thick water film and large aperture can completely seal the pores in the coal seam, leading to WBE. However, a thinner water film or smaller pore size can allow the water phase to break into multiple clusters and mix with the methane gas phase, thereby alleviating WBE. Additionally, the larger porosity of the coal seam enables better flow of gas and liquid, with the gas-liquid velocity distribution following classical fluid theory. This study's findings provide insight into how to mitigate WBE and improve the efficiency of coalbed methane extraction.

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