Multiphase fluid-rock interactions and flow behaviors in shale nanopores: A comprehensive review

The complicated flow behaviors of multiphase fluids in shale reservoirs are significantly influenced by fluid-fluid and fluid-rock interactions due to the non-negligible intermolecular forces at the nanoscale, which is crucial for the effective development and efficient extraction of shale oil. The complexity of multiphase fluid distribution and flow behaviors in shale reservoirs is further increased by low porosity, low permeability, poor connectivity, high inhomogeneity, and multi-component minerals, making the development process more challenging. Molecular dynamics simulation is widely to precisely capture the intermolecular forces and effectively explain the complex distribution and flow behaviors of these fluids under fluid-fluid and fluid-rock interaction forces. In this review, the characteristics of mineral composition, pore structure, porosity, permeability, and fluid types are first elaborated to illustrate the particularity of shale reservoirs and fluids compared to conventional scale reservoirs. The results show that shale minerals are composed of inorganic and organic matter with extremely low porosity and permeability, and nanoscale pore size, in which the complicated oil-water-CO2 multiphase fluid types are caused by the primary underground water, fracturing water and injected CO2. The research progress of molecular simulation on the fluid-fluid and fluid-rock interaction mechanisms and on multiphase shale fluids flow behaviors are then reviewed in detail. The strong intermolecular interaction forces can result in the different occurrence states of fluids, the fluid-fluid interfacial slip, the fluid-rock boundary slip and heterogeneous fluid viscosity/density, significantly exacerbating the complexity of fluids flow. Meanwhile, the injected CO2 in the formation becomes a supercritical state with high diffusivity and strong solubility, and causes oil expansion, density and viscosity reduction, interfacial tension reduction, wettability alteration and molecular diffusion, which effectively replaces adsorbed hydrocarbon components by competitive adsorption behaviors, and promotes oil flow. The challenges and outlook of molecular simulation research and upscaling applications are finally discussed. This review aims to provide a microscopic understanding of the distribution characteristics and flow behaviors of multiphase shale fluids in nanoconfined space for both unconventional oil and gas researchers and industry professionals.