Dynamic fluid states in organic-inorganic nanocomposite: Implications for shale gas recovery and CO2 sequestration

Abstract

Despite wide research on organic-inorganic composites in various fields, the molecular-scale partitioning of fluids in nanoporous organic-inorganic composites remains poorly understood. The microscopic characteristics of fluid states in nanopores of shale composites during gas production are key to the enhancement of shale gas recovery, which remain unexplored. Here we conduct a microscopic modeling for shale nanocomposite, which realizes separate force field parameterization and controllable pore network optimization. A shale clay-kerogen nanocomposite with heterogeneous pore structure is thereafter built to investigate the dynamic characteristics of fluid states during pressure depletion and CO2 sequestration using molecular simulations for the first time. In this study, the different gas recovery mechanisms for pressure depletion and CO2 sequestration are revealed: pressure depletion mainly produces CH4 free state, while CO2 injection predominately extracts CH4 dissolved state. We propose CH4 adsorbed state as the potential target for enhanced shale gas recovery in the late production stage due to its minimal recovery. Three distribution forms of water molecules and two developing trends of water clusters are distinguished in the nanocomposite. Increasing water content can reduce the CH4 recovery during pressure depletion, but improve the CH4 recovery during CO2 injection. We suggest low water content may optimize the shale gas recovery efficiency. The deformation of the kerogen structure is observed to enhance the gas solubility in the kerogen matrix. In the CO2 injection stage, multi-components can promote the recovery of the CH4 adsorbed state. In a broader perspective, the microscopic modeling is widely applicable to other natural and synthetic organic-inorganic nanocomposites interacting with fluids.