Multimechanism of Gas Transportation in Micro–Nanoscale Pores of Coal Seam: An Advanced Model of Gas Adsorption and Diffusion

Abstract

Gas transport in coal reservoirs with rich micro–nanoscale pores is complex and critically important for coaled methane production. This paper proposes novel models for calculating the adsorption thickness and surface diffusion coefficients in multilayer adsorption. Coupling these two models with factors such as adsorption swelling, real-gas effect, and nanoconfinement effect, a novel multimechanism diffusion–seepage model for gas in the micro–nanoscale pores of coal is proposed for calculating the apparent permeability, considering three flow mechanisms (surface diffusion, Knudsen diffusion, and slip flow). The applicability of the proposed model is verified through molecular dynamics simulations and experimental results. In pores with radii smaller than 0.238 μm, methane transport is strongly influenced by the number of adsorption layers, and the apparent permeability of monolayer adsorption is higher. These simulation results are reversed for pores with radii smaller than 3 nm. Surface diffusion negligibly affects the apparent permeability when the pore radius exceeds 8 nm, indicating that the adsorption behavior influences methane transport mainly by changing the effective pore radius. When the pore radius exceeds 43.5 nm, Knudsen diffusion contributes less than 1% to the apparent permeability, and the pores can be considered as seepage pores. At pore pressures above 3.5 MPa, the apparent permeability decreases and then increases with an increasing pore radius because the permeability is more affected by surface diffusion of the adsorbed gas than by free gas transport. The inflection point pore radius is 3 nm or smaller and decreases with a decreasing pore pressure. The apparent permeability increases with a decrease in pore pressure. This trend is enhanced at low pressures, especially in smaller pores. Methane transport is negligibly affected by the real-gas effect and nanoconfinement effect at pore radii above 0.103 μm, by adsorption swelling at pore radii above 0.162 μm, and by adsorption at pore radii above 0.303 μm.