Insights into the charging behavior of tight oil under nanoscale confinement effects

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.