Abstract
Hydrocarbon recovery from unconventional reservoirs (shale gas) is debated due to its environmental impact and uncertainties on its predictability. But a lack of scientific knowledge impedes the proposal of reliable alternatives. The requirement of hydrofracking, fast recovery decay and ultra-low permeability—inherent to their nanoporosity—are specificities of these reservoirs, which challenge existing frameworks. Here we use molecular simulation and statistical models to show that recovery is hampered by interfacial effects at the wet kerogen surface. Recovery is shown to be thermally activated with an energy barrier modelled from the interface wetting properties. We build a statistical model of the recovery kinetics with a two-regime decline that is consistent with published data: a short time decay, consistent with Darcy description, followed by a fast algebraic decay resulting from increasingly unreachable energy barriers. Replacing water by CO2 or propane eliminates the barriers, therefore raising hopes for clean/efficient recovery.
Highlights
Hydrocarbon recovery from unconventional reservoirs is debated due to its environmental impact and uncertainties on its predictability
Using advanced molecular simulation techniques, we show that, in the presence of the pressure-transmitting water, methane desorption is an activated process dominated by interfacial effects, with a wettability-dependent free-energy barrier
In the following we will focus on the ordered system, consisting of a hydrophobic nanomembrane represented here as an array of carbon nanotubes (CNTs) of radius r
Summary
Hydrocarbon recovery from unconventional reservoirs (shale gas) is debated due to its environmental impact and uncertainties on its predictability. These nanoporous kerogen reservoirs connect to the macroscopic fracture network and release their hydrocarbon content as the pressure in the fracking fluid is decreased This picture strongly differs from standard oil recovery from conventional reservoirs, which is usually described within the framework of fluid dynamics in porous media, involving a combination of Darcy’s law and percolation models accounting for the disordered nature of the fluid pathways through the rocks[8]. While such a formulation accounts for experimental data on gas flow in low-permeability shales[11], the molecular origin of slippage corrections in this context is not evident owing to the strong attractive molecular interactions between methane and kerogen Beyond such pitfalls, the dispersed texture of kerogen within the mineral matrix raises the question of the unexplored role of interfacial and wettability effects at their boundaries on hydrocarbon desorption and long-time recovery. As a practical implication of the present results, we show that such a multiscale approach involving retarded interfacial transport allows us to explain the unexpectedly fast decline and variable production rates observed in shale gas wells
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