Molecular dynamics simulation of methane transport in confined organic nanopores with high relative roughness

Abstract

Understanding and characterizing the transport of shale gas (methane) through the nanopores of kerogens are critical for the accurate prediction of shale gas recovery. However, the key factors that regulate shale gas transport through highly roughened nanopores of shale kerogens are not fully understood. In this work, methane transport in organic nanopores with a high relative roughness is characterized using equilibrium and nonequilibrium molecular dynamics methods. According to our results, the CH 4 mass flux has a linear relationship with the pressure gradient, consistent with previous studies, while the calculated slip lengths and gas fluxes varied with different roughness geometries in the order of sigmoidal ≥ triangular > rectangular. Surface slip flow can be a major contributor to the overall gas flux, but surprisingly, the relative contribution of surface slip flow is independent of the pressure gradient. In contrast, the contributions of both slip flow and the average gas fluxes vary strongly with pore diameters. Typical contributions of the adsorbed layer to the overall gas flux are in the range 20-40% but vary from as high as 74% in a 4-nm pore to as low as 6% in a 16-nm pore. Compared to smooth nanopores, we find that, in nanopores with realistically high degrees of relative roughness, methane confinement in cavities decouples slip flow from the flow in the pore interior, significantly reducing the overall flux.

A Molecular Dynamics Investigation on Methane Flow and Water Droplets Sliding in Organic Shale Pores with Nano-structured Roughness

“…In the center region, the methane density is uniform (He et al 2019;Yu et al 2020) and the same (within 0.8%) in smooth and rough pores. The density peaks at the substrate indicate the formation of an adsorbed layer of methane.…”

“…Previous MD-based research includes nanoscopic simulations of fluid flow, droplet wetting and droplet interactions with smooth and rough solid surfaces. The effect of roughness on the behavior of water nanodroplets (Yaghoubi and Foroutan 2018), wetting transition (Khan and Singh 2014;Koishi et al 2009) and gas flow characteristics (He et al 2019;Yu et al 2020) have been reported. Fang et al (2019) performed MD simulation to elaborate on the mechanisms of extracting hydrocarbon using CO 2 in a nanoscale groove constructed by silicon.…”

“…However, it is very challenging to measure the shale roughness whose length scale is less than 5 nm. However, molecular dynamics simulations have been used extensively as appropriate tools to understand the fundamentals of rock-fluid interaction in organic shale pore, for example (He et al 2019;Yu et al 2020) have studied the roughness (length scale is at 2 nanometers) effect on shale gas transport. Besides, structured surface (with close length scale in our simulation) has also widely been applied in related roughness studies via MD (Khan and Singh 2014;Koishi et al 2009;Yaghoubi and Foroutan 2018).…”

“…Given at its complex pore structure at the tiny length scale, it is very challenging to fully reproduce a real kerogen pore. Here in this work, we adopted the recent MD work on kerogen study (He et al 2019;Kazemi and Takbiri-Borujeni 2016a), in which the graphite slit nanopores with structured roughness have been used as an idealized representation of a kerogen nanopore system.…”

Reduced methane recovery at high pressure due to methane trapping in shale nanopores

“…While shale nanopores are both rough and irregular, smooth graphite slit pore models have been widely used as a simplified corollary for kerogen nanopores in shale [70][71][72] . Surface roughness affects both the flow dynamics and the adsorption capacity of methane in pores [73][74][75][76] . However, given that our primary goal is to qualitatively compare retention in our model with neutron scattering observations, it warrants the use of smooth surface.…”