Experimental Study and Modeling of Size-Dependent Light Oil Sorption and the Kinetics in Shales: Insights into the Interaction Mechanism of Shale Oil.

Understanding pore-scale shale oil adsorption is fundamental for in situ resource assessment and CO2-enhanced recovery. Particle size critically controls the accessible pore network and specific surface area, thereby governing the adsorption behavior. In this work, shale samples with different particle sizes were comprehensively characterized via scanning electron microscopy, low-temperature nitrogen adsorption, and Fourier-transform infrared spectroscopy to elucidate pore structure and oxygen-containing functional group distribution. Then, using n-hexane (C6H14) as a light shale oil proxy, we systematically investigated the adsorption isotherms and kinetic behavior across varying particle sizes. Results showed that C6H14 adsorption characteristics followed type II isotherm characteristics, with particle size variations not altering the isotherm shape. The microscopic volume and specific surface area largely determined the adsorption capacity. Adsorption kinetic analysis and diffusion coefficient calculations revealed that increasing the particle size extended pore channels and enhanced diffusion resistance, thereby inhibiting adsorption. Notably, the effective diffusion coefficient decreased with an increase in particle size. Similar to water sorption on porous media, shale oil adsorption was classified into two mechanisms: attachment to oxygen-containing functional groups and pore filling. Based on these findings, a modified Dent model was proposed to describe C6H14 vapor adsorption isotherms across different particle sizes. This model successfully fitted the isotherms and enabled differentiation between primary/secondary surface adsorption and micropore filling. In addition, adsorption kinetic data were well-described by the classical double-exponential model. That model reveals two different stages of diffusion: an initial rapid stage controlled by external diffusion followed by a slower stage dominated by intraparticle diffusion. These investigations provide novel theoretical insights into shale oil adsorption mechanisms with important implications for CO2-enhanced shale oil recovery.