Superdiffusive gas recovery from nanopores

Abstract

Understanding the recovery of gas from reservoirs featuring pervasive nanopores is essential for effective shale gas extraction. Classical theories cannot accurately predict such gas recovery and many experimental observations are not well understood. Here we report molecular simulations of the recovery of gas from single nanopores, explicitly taking into account molecular gas-wall interactions. We show that, in very narrow pores, the strong gas-wall interactions are essential in determining the gas recovery behavior both quantitatively and qualitatively. These interactions cause the total diffusion coefficients of the gas molecules in nanopores to be smaller than those predicted by kinetic theories, hence slowing down the rate of gas recovery. These interactions also lead to significant adsorption of gas molecules on the pore walls. Because of the desorption of these gas molecules during gas recovery, the gas recovery from the nanopore does not exhibit the usual diffusive scaling law (i.e., the accumulative recovery scales as R ∼ t 1/2 but follows a super-diffusive scaling law R ∼ t n (n > 0.5), which is similar to that observed in some field experiments. For the system studied here, the super-diffusive gas recovery scaling law can be captured well by continuum models in which the gas adsorption and desorption from pore walls are taken into account using the Langmuir model. a Article accepted by Phys. Rev. Fluids †

Recovery of Multicomponent Shale Gas from Single Nanopores

“…At earlier time ( t̃ < 0.01), the fluxes of both methane and ethane follow a superdiffusive scaling law Q ( t̃ ) ∼ t̃ –α with 0.4 < α < 0.5, which differs from the Q ( t̃ ) ∼ t̃ –0.5 for purely diffusive gas recovery. Similar deviation from the purely diffusive scaling law has been reported for the recovery of pure gas from nanopores, and is caused by the delayed removal of gas molecules adsorbed on the pore walls . At t̃ ≳ 0.04 ( t̃ ≳ 0.01), the production of methane and ethane from the 2 nm-wide (4 nm-wide) pore starts to deviate from the power law scaling behavior.…”

“…Molecular simulations of gas recovery are limited to systems with pores many orders of magnitude shorter than in real shale formations, and this necessarily introduces some undesirable features such as an extremely large pressure gradient along pore length and from the pore opening to the gas bath during gas recovery. Nevertheless, the fact that the present and our earlier MD simulations capture the scaling law of gas recovery rate reported in field studies and continuum simulations , suggests that these undesired features do not introduce significant artifacts into the simulation results. Therefore, MD simulations not only can be used to understand the transport and adsorption properties of gases in nanopores as demonstrated extensively in the past years, but can also be used as a powerful tool for exploring the essential physics of gas recovery process.…”

“…To gain generalized insight into the kinetics of gas recovery using our MD simulations, we nondimensionalize the time using a characteristic time t c . Since gas recovery from nanopore is generally considered as a diffusive process, following previous works, , t c is chosen as where L is the nanopore length, D m 0 and Kn 0 are the reference molecular diffusion coefficient and Knudsen number inside the pore prior to gas recovery. D m 0 and Kn 0 depend on the molecular properties of the gas species and the pore size.…”

“…To gain generalized insight into the kinetics of gas recovery using our MD simulations, we nondimensionalize the time using a characteristic time t c . Since gas recovery from nanopore is generally considered as a diffusive process, following previous works, 26,37 t c is chosen as…”

Molecular Modeling of Subsurface Phenomena Related to Petroleum Engineering

“…Besides, they also observed more adsorption for smooth surfaces than rough ones and for flexible matrix compared to a rigid one. Wu et al , observed that the gas recovery from nanopores follows a superdiffusive scaling law for single and multicomponent fluids. Furthermore, He et al studied the methane transport in clay nanopores where they doubted the applicability of the Knudsen diffusion model to estimate the methane transport under confinement.…”