Understanding the mechanisms of light oil adsorption in shale: Insights from thermodynamics and kinetics

Adsorption is a critical process for oil storage and transport in shale formations, and a thorough understanding of such processes is essential for accurately assessing the adsorbed oil content and improving shale oil development efficiency. Despite the importance of adsorption in shale oil reservoirs, few studies have explored the adsorption mechanisms of oil in shale from both thermodynamic and kinetic perspectives in the past, even though these two factors significantly govern the adsorption process. To this end, this study selects n-heptane (C₇H₁₆) and shale as the adsorbate-adsorbent pair, and aims to fill this gap by integrating isothermal adsorption experiments with adsorption thermodynamic and kinetic models. The results show that the adsorption/desorption isotherms of light oil in shale follow a Type II curve and exhibits unclosed hysteresis loop due to strong oil-shale interaction and oil dissolution in organic matter. Compared to BET model, the Dent model provides the best fit for the adsorption isotherms, indicating that the light oil adsorption process involves multilayer adsorption at two distinct sites. Thermodynamic parameters, including ΔH (−3.98 kJ/mol), ΔG (−0.2385 kJ/mol), ΔS (−0.0126 kJ/mol·K), and q_st (11.72 kJ/mol), confirm that light oil adsorption is an exothermic, weakly to moderately spontaneous, entropy-reducing, and physical process. Kinetic analysis reveals that the double-exponential model best describes the adsorption kinetics (R² > 0.95, RMSE ≤ 0.05), indicating the light oil adsorption in shale is a two-stage process: a rapid adsorption stage driven by external diffusion and a slower stage controlled by intraparticle diffusion. These thermodynamic and kinetic characteristics provide abundant and novel information for deeply understanding the shale oil adsorption mechanisms.