Sub-nanometre pore adsorption of methane in kerogen

Developing unconventional shale gas resource has increased rapidly in recent years. However, while methane adsorbed inside organic kerogen matter is a source of shale production, it is still not a fully understood process. Here, we use molecular simulations to investigate methane adsorption in local micropores that are less than 1 nm inside realistic kerogen samples. We find an exponential scaling law for the local pore adsorption capacity and rationalise the pore density with the effective pore diameter, reservoir pressure, and sample porosity. This scaling law is determined from four kerogen samples at different porosities, each taken from a different shale reservoir, which have been experimentally validated in previous work. We find that pores closer to methane's diameter are responsible for ~20% of the adsorption inside the sample and it is these small pores and lower pressures that dictate the largest adsorption capacity inside kerogen. Predictions of adsorption isotherms from properties of the kerogen structures are now possible using a proposed numerical percolation model by means of this scaling law. Adsorption predictions using our model show remarkably good agreement with molecular dynamics results in this work and isotherms in the literature, at a fraction of the computational cost. This work opens up a new route for determining adsorption isotherms of dense porous media from knowledge about their local pore structure, and can be scaled efficiently to support experimental campaigns, where molecular simulations would be intractable.