Reliable prediction of gas recovery from shale formations is essential for achieving the full potential of shale gas. A distinguishing feature of shales is that nanoscale pores dominate their porosity. Hence, confinement and fluid−wall interactions modulate shale gas storage, transport, and recovery, which are neglected in conventional gas recovery models. Because these nanoscale effects can be simulated using molecular dynamics (MD), related works have proliferated. Here, we review MD modeling of shale gas recovery at the pore scale. The design and simulation protocols of MD systems for studying gas recovery are surveyed first. Then, the gas recovery in three scenarios, i.e., recovery of single-component gas, recovery of multicomponent gas, and gas recovery involving multiphase flows, are reviewed. In particular, works exploring and elucidating new pore-scale phenomena and guiding and validating new pore-scale continuum models are highlighted. Finally, we recommend best practices in MD shale gas studies and suggest several research directions.
Molecular transport across liquid–vapor interfaces covered by surfactant monolayers plays a key role in applications such as fire suppression by foams. The molecular understanding of such transport, however, remains incomplete. This work uses molecular dynamics simulations to investigate the heptane transport across water–vapor interfaces populated with sodium dodecyl sulfate (SDS) surfactants. Heptane molecules’ potential of mean force (PMF) and local diffusivity profiles across SDS monolayers with different SDS densities are calculated to obtain heptane’s transport resistance. We show that a heptane molecule experiences a finite resistance as it crosses water–vapor interfaces covered by SDS. Such interfacial transport resistance is contributed significantly by heptane molecules’ high PMF in the SDS headgroup region and their slow diffusion there. This resistance increases linearly as the SDS density rises from zero but jumps as the density approaches saturation when its value is equivalent to that afforded by a 5 nm thick layer of bulk water. These results are understood by analyzing the micro-environment experienced by a heptane molecule crossing SDS monolayers and the local perturbation it brings to the monolayers. The implications of these findings for the design of surfactants to suppress heptane transport through water–vapor interfaces are discussed.
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