posted on 2023-03-01, 18:37authored byRunxi Wang, Saikat Datta, Jun Li, Saad F. K. Al-Afnan, Livio Gibelli, Matthew K. Borg
Rapid declines in unconventional shale production arise
from the
poorly understood interplay between gas transport and adsorption processes
in microporous organic rock. Here, we use high-fidelity molecular
dynamics (MD) simulations to resolve the time-varying adsorption of
methane gas in realistic organic rock samples, known as kerogen. The
kerogen samples derive from various geological shale fields with porosities
ranging between 20% and 50%. We propose a kinetics sorption model
based on a generalized solution of diffusive transport inside a nanopore
to describe the adsorption kinetics in kerogen, which gives excellent
fits with all our MD results, and we demonstrate it scales with the
square of the length of kerogen. The MD adsorption time constants
for all samples are compared with a simplified theoretical model,
which we derive from the Langmuir isotherm for adsorption capacitance
and the free-volume theory for steady, highly confined bulk transport.
While the agreement with the MD results is qualitatively very good,
it reveals that, in the limit of low porosity, the diffusive transport
term dominates the characteristic time scale of adsorption, while
the adsorption capacitance becomes important for higher pressures.
This work provides the first data set for adsorption kinetics of methane
in kerogen, a validated model to accurately describe this process,
and a qualitative model that links adsorption capacitance and transport
with the adsorption kinetics. Furthermore, this work paves the way
to upscale interfacial adsorption processes to the next scale of gas
transport simulations in mesopores and macropores of shale reservoirs.