Molecular insight into the occurrence characteristics of confined shale oil with associated gas

Abstract

The occurrence state of shale oil in a nanoscale confined matrix of a shale reservoir significantly impacts reserve evaluation and exploitation. As an endogenous gas, associated gas markedly affects the distribution of adsorbed shale oil and the mobility of free phase oil, yet a clear quantitative analysis of its effect has not been carried out, and the underlying control mechanism remains unclear. In this work, employing a molecular dynamics simulation method, the occurrence characteristics of multiple-component shale oil with different ratios of associated gas were investigated. Simulation results demonstrate that shale oil can be classified into free and adsorbed states in shale nanoslits based on its adsorption density profile. Upon increasing the associated gas ratio, the proportion of adsorbed oil decreases remarkably, which is an effect originating from the competitive adsorption between associated gas and oil, and the different oil components exhibit distinct response performances. As for the free states of the shale oil, the dissolved associated gas decreases their viscosity, leading to higher mobility. In addition, the impact of the mineral type on the occurrence characteristics of shale oil was investigated. The proportions of the adsorption states for the three typical minerals follow the order quartz ≈ kaolinite > calcite, while their sensitivities to associated gas follow the order quartz > calcite > kaolinite. The interactions between shale oil, associated gas, and mineral surfaces were analyzed to reveal the underlying mechanisms behind these response patterns. Insights gained from this study may contribute to a better theoretical understanding of the micro-scale behaviors of associated gas in influencing shale oil occurrence in reservoir nanopores, providing foundational data and theoretical guidance for shale oil reserve evaluation and development.