As conventional oil dwindles, tight oil gains importance for its vast potential. The physical property limit refers to the minimum pore size at which reservoir fluid can be charged under a defined accumulation overpressure, and it is crucial for the accurate assessment of tight oil reservoirs. Micro- and nano-scale pores are the primary storage spaces in tight formations, and the size effect, arising from strong fluid-solid interactions, plays a significant role in influencing the charging behavior of tight oil. However, understanding of tight oil charging in confined space is still limited. In this work, a typical tight oil from the Fengcheng Formation in the Junggar Basin was chosen as the researched objective. Molecular dynamics simulations were employed to investigate the charging process of tight oil into nanoslits pre-occupied by formation water. The critical pressures for tight oil injection into nanoslits with varying sizes were calculated to determine the physical property limit of tight oil charging. Under critical charging conditions, the capillary force acts as the dominant resistance and approximated the threshold charging pressure. The simulated threshold charging pressure was significantly higher than the capillary force predicted by the classic Young-Laplace equation without considering size effects. This result suggests that conventional fluid mechanics theories in confined space overestimate the physical property limit, leading to an inflated assessment of tight oil accumulation. Simulation results show that, as the nanoslit size decreases, interfacial tension and contact angle increase, while water film thickness decreases. By accounting for these size effects, the modified capillary force closely matched the simulated threshold charging pressure. Using the modified capillary force, the physical property limit of tight oil in the Fengcheng Formation, under an accumulation overpressure of 15 MPa, was examined to be 7.2 nm. Furthermore, the effect of mineral types on threshold charging pressure was investigated and give order of illite > calcite > orthoclase > quartz. Additionally, a comparison between the calculated charging pressures with experimentally measured values was conducted, and their strong consistency confirms the validity of the revised Young-Laplace model. This study enhances our understanding of the tight oil charging mechanisms, highlights the importance of size effects, and provides significant insights for the accumulation assessment of tight oil reservoirs.

CO₂ diffusion can promote the expansion and reduce viscosity of crude oil, which plays an important role in improving the effect of CO₂ flooding. However, the effect of water slug on mass transfer and diffusion of CO₂ in water-crude oil system is not considered in the current study in CO₂ water-alternating-gas flooding. To solve this problem, the diffusion experiments of CO₂ in water-crude oil system were carried out. The dynamic compression factor of CO₂ considering supercritical state was proposed, and a new model for CO₂ diffusion coefficient calculation was established. The diffusion law of CO₂ in water-crude oil system was studied also, and the influence of water slug, different starting time and convection on the diffusion law of CO₂ in water-crude oil system were analyzed. The results show that the revised diffusion coefficients of CO₂ in crude oil and formation water are 1.17×10^-9 m²/s and 0.44×10^-9 m²/s respectively. The diffusion of CO₂ in the water-crude oil system can be divided into crude oil diffusion and water slug diffusion. In the crude oil diffusion, at the initial stage of molecular diffusion, it is affected by convection in a short time, resulting in a high pressure drop rate. After that it is mainly affected by concentration diffusion, resulting in a slow pressure drop rate. In water slug, the molecular diffusion is not affected by convection, the difference of CO₂ concentration is smaller, and the solubility of CO₂ in water is smaller than that in crude oil. So the pressure drops slowly and the pressure change rate becomes smaller. At different starting time, the pressure drop rate is different and the diffusion mass transfer rate of CO₂ is different accordingly. To eliminate the influence of convection on the diffusion and mass transfer of CO₂ in the initial diffusion stage, a reasonable start time should be selected.