Understanding and controlling the flow of water confined in nanopores has tremendous implications in theoretical studies and industrial applications. Here, we propose a simple model for the confined water flow based on the concept of effective slip, which is a linear sum of true slip, depending on a contact angle, and apparent slip, caused by a spatial variation of the confined water viscosity as a function of wettability as well as the nanopore dimension. Results from this model show that the flow capacity of confined water is 10∼10 times that calculated by the no-slip Hagen-Poiseuille equation for nanopores with various contact angles and dimensions, in agreement with the majority of 53 different study cases from the literature. This work further sheds light on a controversy over an increase or decrease in flow capacity from molecular dynamics simulations and experiments.
Surface
diffusion plays a key role in gas mass transfer due to the majority
of adsorbed gas within abundant nanopores of organic matter in shale
gas reservoirs. Surface diffusion simulation is very complex as a
result of high reservoir pressure, surface heterogeneity, and nonisothermal
desorption in shale gas reservoirs. In this paper, a new model of
surface diffusion for adsorbed gas in shale gas reservoirs is established,
which is based on a Hwang model derived under a low pressure condition
and considers the effect of adsorbed gas coverage under high pressure.
Additionally, this new model considers the effects of surface heterogeneity,
isosteric sorption heat, and nonisothermal gas desorption. Results
show that (1) the surface diffusion coefficient increases with pressure
and temperature, while it decreases with activation energy and gas
molecular weight; (2) contributions of viscous flow, Knudsen diffusion,
and surface diffusion to the total gas mass transfer are varying during
the development of shale gas reservoirs, which are mainly controlled
by nanopore-scale and pressure; (3) in micropores (pore radius of <2
nm), the contribution of surface diffusion to the gas mass transfer
is dominant, up to 92.95%; in macropores (pore radius of >50 nm),
the contribution is less than 4.39%, which is negligible; in mesopores
(2 nm < pore radius < 50 nm), the contribution is between micropores
and macropores.
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