Abstract
The underlying mechanism of shale gas migration behavior is of great importance to understanding the flow behavior and the prediction of shale gas flux. The slippage of the methane molecules on the surface is generally emphasized in nanopores in most predicted methods currently. In this work, we use molecular dynamic (MD) simulations to study the methane flow behavior in organic (graphene) and inorganic (quartz) nanopores with various pore size. It is observed that the slippage is obvious only on the graphene nanopores and disappeared on the quartz surface. Compared with the traditional Navier-Stokes equation combined with the no-slip boundary, the enhancement of the gas flux is nonnegligible in the graphene nanopores and could be neglected in the quartz nanopores. In addition, the flux contribution ratios of the adsorption layer, Knudsen layer, and the bulk gas are analyzed. In quartz nanopores, the contributions of the adsorption layer and the Knudsen layer are slight when the pore size is larger than 10 nm. It is also noted that even if the Knudsen number is the same, the flow mode may be various with the effect of the pore surface type. Our work should give molecular insights into gas migration mechanisms in organic and inorganic nanopores and provide important reference to the prediction of the gas flow in various types of shale nanopores.
Highlights
With the depletion of conventional energy resources and the expanding energy demands, the unconventional energy resources such as the shale oil and gas have attracted more and more attention [1,2,3,4,5,6,7]
The second density peak is 0.393 g/cm3. Such density values of the first peak and the second peak could be 5.52 and 1.54 times than that of bulk, ρbulk. Such layer structures near the graphene surface are independent on the pore size as long as the reservoir pressure and temperature conditions are the same, which is in agreement with the methane confined in various graphene nanopore [22] and alkane confined in the nanopores [12, 45, 47]
In this work, based on molecular dynamic simulation, we presented the density distributions and the flow performance of methane in organic graphene nanopores and inorganic quartz nanopores
Summary
With the depletion of conventional energy resources and the expanding energy demands, the unconventional energy resources such as the shale oil and gas have attracted more and more attention [1,2,3,4,5,6,7]. According to the value of pore diameter, d, the pores could be divided as micropore (d < 2 nm), mesopore (2 nm ≤ d ≤ 50 nm), and macropore (d > 50 nm) suggested by the International Union of Pure and Applied Chemistry (IUPAC) [1, 2, 12] In one aspect, such amounts of nanopores provide large internal surface areas and result in great adsorption of shale gas; the adsorbed gas occupies a significant percentage of the gas-in-place in shale [13–. As the mean free path of methane molecules changes to be comparable to the pore size in such tiny pores [18,19,20], the traditional Darcy Law may be invalidated and make the prediction of shale gas flow be challenging [6, 21, 22]
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