Adsorbed and Free Oil in Lacustrine Nanoporous Shale: A Theoretical Model and a Case Study

摘要

Free oil, rather than adsorbed oil, is potentially the most producible component of tight nanoporous shale reservoirs using existing technologies. To date, a variety of geochemical parameters, mainly related to the retention, migration, and expulsion of generated oil, have been developed to estimate the free oil in shale. However, there is still a lack of theoretical models accounting for the mechanism of oil accumulation in nanoporous shale, which would help evaluate adsorbed and free oil. In this paper, models were developed to estimate the amount of oil confined in nanoporous shale and that is mainly in adsorbed and free states to describe the characteristics of oil accumulation. The models were established for an adsorption saturation situation, and simultaneously considered the multilayer adsorption of an oil mixture and the microstructure of the shale pore system (including pore-throat-fracture). As a case study, adsorbed and free oil in lacustrine shales with different lithofacies, obtained from the Dongying sag of the southeastern Bohai Bay Basin of China, were evaluated using the models. Amounts of adsorbed and free oil obtained from the models agree well with those obtained from the previously established oil saturation index method. Microscopically, the amounts of adsorbed and free oil accumulating in slit-shaped pores were comprehensively impacted by the microstructure (size and volume) of pores and the state of the oil (densities and adsorption thickness). The models may be appropriate for single or mixed liquids confined in nanoporous shales, which develop a large number of slit-shaped pores (i.e., fracture) and few spherical pores with small dimension (such as about 10 nm in this study), and can help target areas favorable for shale oil exploitation. This study provides significant insight into the microscopic mechanism of oil accumulation in nanoporous shale.