Comparative Study on Differences in Adsorption Layersof Different Minerals and Fluid Compositions in Shale

Abstract Defining the lower limit of fluid mobility is essential for shale oil development. The lower limit in unconventional reservoirs is controlled by the adsorption layer thickness in shale nanopores; however, accurate determination of this thickness remains challenging. Taking shale samples from the Da’anzhai section of the Sichuan Basin as the research object, chromatographic analysis combined with molecular dynamics (MD) simulation was applied to identify the composition of adsorbed shale oil and to clarify the mechanisms responsible for the differences in adsorption layer thickness for oil and gas on different minerals. MD simulations were performed using ClayFF and OPLS-AA force fields under 300 K and 80 MPa, with 50 ns NPT equilibrium and 30 ns NEMD simulation. An experimental method for adsorption layer thickness measurement was established on the basis of mass conservation, using a 250 nm radius quartz nanocapillary bundle model with wettability modification to reproduce rock-core analogs, and experiments were also carried out under formation conditions (80 °C, 30 MPa). In addition, existing simulation-based experimental approaches for the adsorption boundary layer were compared with the method proposed in this study. The results show that shale oil in MD simulations can be represented by a mixture of C8H18, C15H32, and C24H50. Variations in adsorption layer thickness among minerals and fluids are primarily governed by crystal structure, molecular chain length, and molecular weight, together with the synergistic interactions of surface functional groups at the solid–liquid interface. Experimental observations further demonstrate that an adsorption layer exists irrespective of pore lipophilicity, leading to negative slip. A flow velocity prediction expression for a nanocapillary bundle model was developed with slip length incorporated.