Quantitative characterization of adsorbed and free shale oil microscopic distribution based on nuclear magnetic resonance: a case study of Chang 7 member of Triassic Yanchang Formation in Ordos Basin

Abstract

Quantitative evaluation of shale oil occurrence states and mobility is a key and challenging issue in current shale oil geological research. Taking the shale from the seventh member of the Triassic Yanchang Formation (Chang 7 member) in the Ordos Basin as the research object, this study combined nuclear magnetic resonance (NMR) experiments with saturation-centrifugal tests, using the shale oil adsorption ratio equation proposed by previous research. It conducted a comprehensive study on the adsorbed and free oil amount, ratio, microscopic distribution, and mobility characteristics of shale oil. The results showed that under conditions of saturation with n-dodecane and centrifugation at 20 ℃, the average amounts of free oil and adsorbed oil in shale of the Chang 7 member were 1.981 4 mg/g and 1.548 1 mg/g, respectively. The average proportion of adsorbed oil was 0.430 7. The average density ratio between the adsorbed and free phases of shale oil was 1.171 3, the average density of adsorption phase was 0.877 8 cm3/g, and the average thickness of the adsorption layer was 0.980 2 nm. Adsorbed oil mainly exist in micropores (< 100 nm), and the amount of free oil in micropores, mesopores, and macropores sequentially decreases. Organic-matter-rich shale generally contain higher amount of free oil and lower amount of adsorbed oil due to the existence of hydrocarbon generation-induced microfractures and less developed organic pores. Quartz-related pores significantly increase the specific surface area of pores, thus providing more occurrence sites for adsorbed oil, whereas an increase in clay mineral content significantly reduces the pore volume available for free oil. The ratio of free oil amount (Qf) to median centrifugal force (ΔPL) is identified as a new and effective parameter for evaluating shale oil mobility. Higher ratio indicates better shale oil mobility. For shale in Chang 7 member, Qf/ΔPL=1.339 4 mg/(g·MPa) represents the threshold at which shale oil mobility undergoes a significant change. Above this threshold, the shale oil mobility significantly improves. The lower limit of theoretical pore size of free oil, calculated using the adsorption ratio equation, is between 1.960 4 nm and 5.881 2 nm, and the specific size is related to pore morphology.

Keywords

• nuclear magnetic resonance
• adsorbed and free state
• shale oil
• microscopic distribution
• yanchang formation
• ordos basin