Transport of Shale Gas in Microporous/Nanoporous Media: Molecular to Pore-Scale Simulations

As
the typical unconventional reservoir, shale gas is believed
to be the most promising alternative for the conventional resources
in future energy patterns, attracting more and more attention throughout
the world. Generally, the majority of shale gas is trapped within
the tight shale rock with ultralow porosity (<10%) and ultrasmall
pore size (as less as several nanometers). Thus, the accurate understanding
of gas transport characteristic and its underlying mechanism through
these microporous/nanoporous media is critical for the effective exploitation
of shale reservoir. In this context, we present a comprehensive review
on the current advances of multiscale transport simulations of shale
gas in microporous/nanoporous media from molecular to pore-scale.
For the gas transport in shale nanopores using molecular dynamics
(MD) simulations, the structure and force parameters of various nanopore
models, including organic models (graphene, carbon nanotubes, and
kerogen) and inorganic models (clays, carbonate, and quartz), and
flow simulation strategies (such as nonequilibrium molecular dynamics
(NEMD) and Grand Canonical Monte Carlo simulations) are systematically
introduced and clarified. The significant MD simulation results about
gas transport characteristic in shale nanopores then are elaborated
respectively for different factors, including pore size, ambient pressure,
nanopore type, atomistic roughness, and pore structure, as well as
multicomponent. Besides, the two-phase transport characteristic of
gas and water is also discussed, considering the ubiquity of water
in shale formation. For the lattice Boltzmann method (LBM) and pore
network model (PNM) approaches to conduct pore-scale simulations,
we briefly review its origins, modifications, and applications for
gas transport simulations in a microporous/nanoporous shale matrix.
Particularly, the upscaling methods to incorporate MD simulation into
LBM and PNM frameworks are emphatically expounded in the light of
recent attempts of MD-based pore-scale simulations. It is hoped that
this Review would be helpful for the readers to build a systematical
overview on the transport characteristic of shale gas in microporous/nanoporous
media and subsequently accelerate the development of the shale industry.