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.
Highlights
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
To account for the scaling of permeance with alkane length and fluid density, we propose a molecular description in which transport arises from a combination of slip-like friction of the hydrocarbons with the matrix and a free volume term
Details about the models and simulations can be found in the Supplementary Discussion and Methods; see Supplementary Table 1. This will serve as the basis of a theoretical scaling model of transport, based on the analysis of the fluctuations of microscopic variables via the fluctuation dissipation theorem (FDT)[27]
Summary
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. Attempts have been made to palliate for the breakdown of Darcy approach by including slippage in gas flow, via, for example, the Klinkenberg effect[12,13], such empirical corrections cannot capture the complex adsorption and transport behaviour of hydrocarbon in ultra-confining porous materials.
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