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.
Highlights
Conventional hydrocarbon energy such as petroleum and coal has been sharply consumed with the social development in past few decades
To investigate the hydrodynamic behavior of methane confined in nanopores, nonequilibrium molecular dynamics (NEMD) simulations are carried out
The value of bulk density obtained from our equilibrium molecular dynamics (EMD) simulations agrees well with that obtained from the National Institute of Standards and Technology (NIST), which can validate the accuracy of our simulation methods.[41]
Summary
Conventional hydrocarbon energy such as petroleum and coal has been sharply consumed with the social development in past few decades. The results indicated that Knudsen diffusion occupies an important proportion in the shale gas transport.[24] Zhang et al developed a pore network model to simulate the gas flow regimes. The velocity profiles in different pores with widths ranging from 2 to 10 nm were plotted and they translated from plug flow to parabolic flow as the pore width increases.[27] Jin and Firoozabadi employed MD simulations to study methane flow in different nanopores They pointed out that the velocity profiles didn’t conform to the prediction based on conventional theory and the flow of adsorbed methane significantly enhanced the total flux.[28] some qualitative and suggestive conclusions have been obtained in these works, the knowledge on flow characteristic of shale gas at nanoscale is still rather lacking.
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