Effect of Shale Reservoir Characteristics on Shale Oil Movability in the Lower Third Member of the Shahejie Formation, Zhanhua Sag

Abstract

To reveal the effect of shale reservoir characteristics on the movability of shale oil and its action mechanism in the lower third member of the Shahejie Formation (Es3l), samples with different features were selected and analyzed using N2 adsorption, high‐pressure mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR), high‐speed centrifugation, and displacement image techniques. The results show that shale pore structure characteristics control shale oil movability directly. Movable oil saturation has a positive relationship with pore volume for radius > 2 μm, as larger pores often have higher movable oil saturation, indicating that movable oil is present in relatively larger pores. The main reasons for this are as follows. The relatively smaller pores often have oil‐wetting properties because of organic matter, which has an unfavorable effect on the flow of oil, while the relatively larger pores are often wetted by water, which is helpful to shale oil movability. The rich surface provided by the relatively smaller pores is beneficial to the adsorption of immovable oil. Meanwhile, the relatively larger pores create significant pore volume for movable oil. Moreover, the larger pores often have good pore connectivity. Pores and fractures are interconnected to form a complex fracture network, which provides a good permeability channel for shale oil flow. The smaller pores are mostly distributed separately; thus, they are not conducive to the flow of shale oil. The mineral composition and fabric macroscopically affect the movability of shale oil. Calcite plays an active role in shale oil movability by increasing the brittleness of shale and is more likely to form micro‐cracks under the same stress background. Clay does not utilize shale oil flow because of its large specific surface area and its block effect. The bedding structure increases the large‐scale storage space and improves the connectivity of pores at different scales, which is conducive to the movability of shale oil.

Oil Retention in Shales: A Review of the Mechanism, Controls and Assessment

“…The maximum adsorption capacity of calcite for extracted shale oil-asphaltene reaches only 2.16 mg/g (Mohammadi and Sedighi, 2013), and the adsorption capacity is relatively low, which may be related to the fact that calcite (a neutral mineral) generally exhibits no electric charge (Zhang et al, 2015). Among inorganic mineral components, clay minerals provide the main adsorption surfaces for shale oil (Li et al, 2016;Ning et al, 2020).Due to the difference in cation substitution mechanism, charge and layer charge among clay minerals, the adsorption capacity of various clay minerals or even different surfaces within the same clay minerals for oil is different: 1) Al 3+ in the montmorillonite crystal layer is replaced by divalent cations to generate a negative charge, which can attract Ca 2+ or Na + , while Si 4+ ions in the illite crystal layer are located on the lattice surface, and the negative charge due to Al 3+ substitution generates a greater attraction force to K + so that illite can adsorb more negative organic macromolecules (Zhang et al, 2015); 2) kaolinite with an octahedral-tetrahedral structure, which is different from that of other clay minerals, contains oil-wet silicate surfaces and water-wet alumina surfaces, and the adsorption capacity of oil components per unit area is 2.47 and 1.44 mg/m 2 , respectively (Tian et al, 2018). Moreover, different types of clay minerals have various specific surface areas, which may also affect adsorbed oil content (Zhu et al, 2012).…”

“…Mineral particles and composition play specific roles in controlling movable shale oil. Overall, although intergranular pores can be formed among clay minerals, tiny intergranular pores and the high surface adsorption capacity of clay minerals hinder the storage of movable liquid hydrocarbons (Su et al, 2018;Ning et al, 2020). The adsorption affinity of quartz and carbonate minerals for oil is low (Raji et al, 2015;Li et al, 2016), and the increase in their contents promotes the development of microcracks (Ougier-Simonin et al, 2016) and the preservation of macropores (Zhang P. L. et al, 2021), which facilitate the occurrence of mobile liquid hydrocarbons.…”

“…Bao (2018) speculated that shales within the oil generation window and a porosity higher than 6.5% promoted the storage of movable oil through the relationship between the OSI and porosity for the Paleogene shale in the Dongying Sag. The shale pore structure is a crucial factor controlling the shale oil mobility (Davudov et al, 2020;Jiang et al, 2020;Liu et al, 2020;Ning et al, 2020). Zou et al, 2015 found that oil cannot flow through pores below the lower limit of the flow threshold (20 nm) through simulations of a nanoporous template with a controllable pore diameter.…”

“…The pore size of shale reservoirs should exceed the flow threshold, which is a necessary condition for movable shale oil. The proportion of movable oil gradually increases with increasing pore size (Zhang P. F. et al, 2020) because the pore connectivity of macropores is higher and provides a considerable pore volume for movable oil (Li T. W. et al, 2017;Jiang et al, 2020;Ning et al, 2020), and the diffusion coefficient of oil increases with increasing pore diameter Zhang et al, 2019). When the pore throat radius is large, the diffusion motion of FIGURE 9 | Determination of shale movable oil and adsorbed oil according to T 1 -T 2 spectrum (modified from Li J.…”

Exploring the coupling relationship between hydrocarbon generation of continental shale and nanopore structure evolution—A case study of Shahejie formation in Bohai Bay Basin

“…A large number of studies show that the mobility of shale oil is positively correlated with the total pore volume, total effective pore volume and average pore diameter of reservoir. (Huang et al 2018;Li et al 2019;Ning et al 2020).…”