Diffusion Behavior of Methane in 3D Kerogen Models

As
global energy demand increases, natural gas recovery from source
rocks is attracting considerable attention since recent development
in shale extraction techniques has made the recovery process economically
viable. Kerogens are thought to play an important role in gas recovery;
however, the interactions between trapped shale gas and kerogens remain
poorly understood due to the complex, heterogeneous microporous structure
of kerogens. This study examines the diffusive behavior of methane
molecules in kerogen matrices of different types (Type I, II, and
II) and maturity levels (A to D for Type II kerogens) on a molecular
scale. Models of each kerogen type were developed using simulated
annealing. We employed grand canonical Monte Carlo simulations to
predict the methane loadings of the kerogen models and then used equilibrium
molecular dynamics simulations to compute the mean square displacement
of methane molecules within the kerogen matrices under reservoir-relevant
conditions, that is, 365 K and 275 bar. Our results show that methane
self-diffusivity exhibits some degree of anisotropy in all kerogen
types examined here except for Type I-A kerogens, where diffusion
is the fastest and isotropic diffusion is observed. Self-diffusivity
appears to correlate positively with pore volume for Type II kerogens,
where an increase in diffusivity is observed with increasing maturity.
Swelling of the kerogen matrix up to a 3% volume change is also observed
upon methane adsorption. The findings contribute to a better understanding
of hydrocarbon transport mechanisms in shale and may lead to further
development of extraction techniques, fracturing fluids, and recovery
predictions.