Influence of surface roughness on methane flow in shale kerogen nano- slits

PY Huang and LM Shen and F Maggi and ZW Chen and ZJ Pan, JOURNAL OF NATURAL GAS SCIENCE AND ENGINEERING, 103, 104650 (2022).

DOI: 10.1016/j.jngse.2022.104650

Understanding the mechanism of methane transport in real kerogen nano- pores is critical to evaluate gas productivity in shale formations accurately. This work presents a novel numerical method to generate nano-slits with different surface roughness for Type II-D kerogen in molecular dynamics (MD) simulations. The methane adsorption and transport phenomenon in the kerogen nano-slits are then studied. A modified velocity model is proposed by considering the effect of surface roughness on the slip velocity and the effect of gas rarefaction on the methane viscosity. The ratio of the mass flow rate in the adsorption region to the total flow rate is between 57% and 78% at a channel width of 2.5 nm depending on the surface roughness and pressure, but it drops to between 6% and 16% at a channel width of 10 nm. Hence, two mathematical equations are derived to estimate the mass flow rate in the adoption region from either (1) the adsorption per apparent surface area, or (2) the product of the bulk density and the width of the adsorption region, respectively. Finally, a new flow enhancement model is derived by combining the modified velocity model and the equation for mass flow rate in the adsorption region. The flow enhancement increases with the Knudsen number but decreases with the increase of surface roughness. The proposed flow enhancement model can match the MD simulation well. The proposed model for methane flow enhancement covers the slip flow and transition flow regimes and can potentially be implemented to study the large-scale flow properties, such as permeability, of the shale matrix at various conditions to better evaluate the shale gas production performance.

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