基于核磁共振的页岩油吸附/游离微观分布定量表征——以鄂尔多斯盆地三叠系延长组7段为例

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

刘飞 杜金良 孙林 郭睿良 郝博斐 刘鹏

LIU Fei DU Jinliang SUN Lin GUO Ruiliang HAO Bofei LIU Peng

1. 低渗透油气田勘探开发国家工程实验室, 西安 710018; 2. 中国石油 长庆油田公司 勘探开发研究院, 西安 710018; 3. 西安石油大学 地球科学与工程学院, 西安 710065; 4. 中国石油 长庆油田分公司 第二采气厂, 西安 710018; 5. 西安科技大学 安全科学与工程学院, 西安 710054

1. National Engineering Laboratory for Exploration and Development of Low-Permeability Oil & Gas Fields, Xi'an, Shaanxi 710018, China; 2. Exploration and Development Research Institute of PetroChina Changqing Oilfield Company, Xi'an, Shaanxi 710018, China; 3. School of Earth Sciences and Engineering, Xi'an Shiyou University, Xi'an, Shaanxi 710065, China; 4. No. 2 Gas Production Plant, PetroChina Changqing Oilfield, Xi'an, Shaanxi 710018, China; 5. College of Safety Science and Engineering, Xi'an University of Science and Technology, Xi'an, Shaanxi 710054, China

页岩油赋存状态及其可动性定量评价是当前页岩油地质研究的核心和难点问题。以鄂尔多斯盆地三叠系延长组7段(简称长7段)页岩为研究对象，采用饱和—离心—核磁共振实验方法，结合前人提出的页岩油吸附比例方程，对页岩油吸附/游离含量、比例、微观分布及可动性特征等开展了综合研究。在饱和正十二烷及20 ℃离心温度的条件下，长7段页岩游离/吸附油量平均分别为1.981 4 mg/g和1.548 1 mg/g，吸附油比例平均为0.430 7。页岩油吸附相/游离相平均密度比为1.171 3，吸附相平均密度为0.877 8 cm3/g，平均吸附层厚度为0.980 2 nm。吸附油主要赋存于微小孔(小于100 nm)，游离油在微小、中、大孔中的赋存含量依次减少。高有机质页岩由于生烃增压微裂缝的存在和较不发育的有机质孔使得总体具有较高的游离油量和较低的吸附油量。石英相关孔隙会明显增大孔隙比表面积进而为吸附油提供更多赋存点位，而黏土矿物含量的增加会显著减少游离油赋存的孔隙体积。游离油量(Qf)与中值离心力(ΔPL)的比值(Qf/ΔPL)是评价页岩油可动性的新型有效参数，该值越高，反映页岩油可动性越好。对于长7段页岩而言，Qf/ΔPL=1.339 4 mg/(g·MPa)是页岩油可动性发生显著变化的阈值，大于该值，页岩油可动性明显变好。通过吸附比例方程计算的游离油理论赋存孔径下限介于1.960 4~5.881 2 nm之间，具体大小与孔隙形态有关。

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.

中国石油长庆油田低渗透油气田勘探开发国家工程实验室开放课题(2023-9901)资助。

https://doi.org/10.11781/sysydz2025030645