Subcontinuum mass transport of condensed hydrocarbons in nanoporous media

Abstract

Although hydrocarbon production from unconventional reservoirs, the so-called shale gas, has exploded recently, reliable predictions of resource availability and extraction are missing because conventional tools fail to account for their ultra-low permeability and complexity. Here, we use molecular simulation and statistical mechanics to show that continuum description—Darcy's law—fails to predict transport in shales nanoporous matrix (kerogen). The non-Darcy behaviour arises from strong adsorption in kerogen and the breakdown of hydrodynamics at the nanoscale, which contradict the assumption of viscous flow. Despite this complexity, all permeances collapse on a master curve with an unexpected dependence on alkane length. We rationalize this non-hydrodynamic behaviour using a molecular description capturing the scaling of permeance with alkane length and density. These results, which stress the need for a change of paradigm from classical descriptions to nanofluidic transport, have implications for shale gas but more generally for transport in nanoporous media.

Abstract

Shale gas and oil are trapped in nanoscale porous networks of ultra-low permeability. Here, the authors develop a molecular model of alkane transport through nanoporous materials, showing that the mechanisms controlling flow at the nanoscale lead to a simple scaling of permeance with hydrocarbon size and density.