Molecular simulation of the constant composition expansion experiment in shale multi-scale systems

The phase behavior of hydrocarbons in shale reservoirs has garnered increasing attention in the petroleum industry. Significant differences in the phase behavior of petroleum fluids between conventional reservoirs and shale reservoirs have been observed. Because of the existence of nano-scale porous media in shale reservoirs, there are substantial surface fluid interactions that can lead to a heterogeneous distribution of molecules and an alteration of the fluid phase behavior. In this work, for the first time, we use molecular simulations to investigate the confinement effect on the phase behavior of reservoir fluids in a multi-scale (nano-scale + macro-scale) media. The model has a confined region that mimics the nano-scale porous media and a bulk region that represents the macro-scale porous media (macropores and fractures) in shale reservoirs. For the first time, we use the Gibbs ensemble Monte Carlo (GEMC) simulation at imposed pressures to simulate the constant composition expansion (CCE) experiment for multi-component hydrocarbon mixtures in a multi-scale porous media. Our results show that because of the confinement effect, the composition of the bulk and the confined regions differ significantly. In our molecular simulations, the composition of the lighter components in the bulk region increases, while heavier components are mainly trapped in the confined region. These changes in the composition cause a significant shift or disappearance of the saturation pressure in the bulk region of the multi-component multi-scale hydrocarbon systems. (C) 2019 Published by Elsevier B.V.