Channel-width dependent pressure-driven flow characteristics of shale gas in nanopores

Abstract

Understanding the flow characteristics of shale gas especially in nanopores is extremely important for the exploitation. Here, we perform molecular dynamics (MD) simulations to investigate the hydrodynamics of methane in nanometre-sized slit pores. Using equilibrium molecular dynamics (EMD), the static properties including density distribution and self-diffusion coefficient of the confined methane are firstly analyzed. For a 6 nm slit pore, it is found that methane molecules in the adsorbed layer diffuse more slowly than those in the bulk. Using nonequilibrium molecular dynamics (NEMD), the pressure-driven flow behavior of methane in nanopores is investigated. The results show that velocity profiles manifest an obvious dependence on the pore width and they translate from parabolic flow to plug flow when the width is decreased. In relatively large pores (6 – 10 nm), the parabolic flow can be described by the Navier-Stokes (NS) equation with appropriate boundary conditions because of its slip flow characteristic. Based on this equation, corresponding parameters such as viscosity and slip length are determined. Whereas, in small pores (∼ 2 nm), the velocity profile in the center exhibits a uniform tendency (plug flow) and that near the wall displays a linear increase due to the enhanced mechanism of surface diffusion. Furthermore, the profile is analyzed and fitted by a piecewise function. Under this condition, surface diffusion is found to be the root of this anomalous flow characteristic, which can be negligible in large pores. The essential tendency of our simulation results may be significant for revealing flow mechanisms at nanoscale and estimating the production accurately.

Mineral- and Ion-Specific Effects at Clay–Water Interfaces: Structure, Diffusion, and Hydrodynamics

“…Molecular simulations played a key role in addressing the first aspect with the finding that continuous hydrodynamics holds down to remarkably small length scales, 5 even though it breaks down beyond 1-2 nm in particular due to the layering of the fluid at the interface, or for pore sizes of the order of that of the confined fluid molecules. 6 They also provide molecular scale information on the local viscosity of the confined fluid [7][8][9][10] as well as the relevant boundary conditions (stick or slip) which should be applied for a given interface, depending on the nature of both the solid wall and the confined fluid, with the quantification of the slip length [11][12][13] and its correlation with other properties such as the contact angle. 14 Due to their importance in the above-mentioned geological applications, the dynamics of fluids in nanopores of clay and related natural minerals [15][16][17][18][19][20] have attracted a lot of attention from the experimental and modelling point of view.…”

Effect of Surface Type on the Flow Characteristics in Shale Nanopores

“…Furthermore, to have an accurate understanding of gas transport characteristics in nanoporous/microporous media, the molecular dynamic (MD) simulation is also widely applied to understand the gas flow behaviors, as summarized and depicted in one recent review by Yu et al [39,40]. The nanochannels are generally used to mimic the fluids flow confined in shale nanoporous media in these MD simulations [22,41,42]. Since the shale organic matter is mainly composed of carbon atoms and the pore wall is hydrophobic; thus, the graphene sheets, carbon nanotubes, and arrayed carbons are abstracted as organic pore wall materials in numerous previous work [39].…”

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

“…In particular, the establishment of operational pressure controls to improve recovery from shale nanopores is a key strategy that can be applied without significant and costly changes to existing well infrastructure. Although it may take more time for operational pressure changes to propagate into nanopores, the existence of microfracture networks in low permeability shales will enhance propagation into the matrix 5 , and is an impetus for many existing studies on pressure-dependent methane transport in shale nanopores [6][7][8][9] .…”