The low permeability of shale matrices necessitates overcoming a threshold pressure gradient to initiate hydrocarbon flow, which poses a major constraint on recovery efficiency. However, the microscopic mechanisms underlying the threshold pressure gradient, particularly the roles of interfacial interactions and pore confinement, remain unclear. A comprehensive understanding of the threshold pressure gradient is essential for enhancing recovery strategies and improving shale oil extraction efficiency. This study provides a comprehensive analysis of the interfacial and size effects on the threshold pressure gradient within kerogen, quartz, and portlandite pores using molecular dynamics simulations. A method for assessing molecular thermal motion and quantifying the threshold pressure gradient was developed using molecular dynamics simulations. The results indicate that the threshold pressure gradient decreases in the order of kerogen, quartz, and portlandite pores. The adsorption characteristics of shale oil components at the interface were clarified through density distribution and molecular behavior analysis, and the factors contributing to the threshold pressure gradient were identified. It was found that the threshold pressure gradient is significantly influenced by the strength of interfacial interactions between the polar shale oil components and the solid matrix. Additionally, an analytical model was proposed to predict the correlation between the threshold pressure gradient and the pore size, which can extend the prediction of the threshold pressure gradient to a larger scale of thousands of nanometers. These findings offer insights into shale oil recoverability in nanopores and provide theoretical guidance for its extraction.

Carbon-based nanomaterials have received heightened global interest by petroleum researchers because of their abundant stocks of necessary raw materials, ease of size control, readiness for modification, and high stability. In light of the practical demand for oil development, this study reviews the recent progress in the research of enhancing oil recovery using carbon-based nanomaterials of various dimensions, including carbon dots, carbon nanotubes, carbon nanofibers, and graphene and its derivatives. Moreover, the study elaborates on the application of these materials in high-efficiency oil displacement, profile control and water shutoff, as well as the fracturing process. The related challenges and solutions in practical oil exploration and development are analyzed, and the application prospects of these materials in future oil reservoirs and oilfields are predicted. This review provides valuable theoretical and experimental references for the large-scale application of carbon-based nanomaterials.

Interactions at the oil/brine/rock interfaces play a pivotal role in the mobility of crude oil within reservoir matrices. Unraveling the microscopic mechanisms of these interactions is crucial for ion-engineered water flooding in secondary and tertiary oil recovery. In this study, the occurrence and transport behavior of crude oil in kaolinite nanopores covered with thin brine films was investigated by molecular dynamics simulation. There is an apparent interface layered phenomenon for the liquid molecules in slit pores and the polar oil components primarily concentrate at the oil/brine interfacial region and form various binding connections with ions. The interfacial interactions between the polar oil components and brine ions exhibit an inhibitory effect on the transport of crude oil through nanopores. The interaction mechanism between acetic acid molecules and hydrated ions was elucidated by interaction modes and interaction intensity, which was proved to illustrate the flow difference in different brine film systems. Moreover, a strategy of exchanging the binding sites of divalent cations with acetic acid molecules by monovalent cations with a higher concentration was proposed. The cation exchange scheme was further validated, demonstrating an enhancement in the oil mobility within nanopores. These findings deepen our understanding of oil/brine/rock interfacial interactions and provide a significant molecular perspective on ion-engineered water flooding for enhanced oil recovery.

A deep understanding of the geometric impacts of fracture on fracturing fluid flowback efficiency is essential for unconventional oil development. Using nuclear magnetic resonance and 2.5-dimensional matrix-fracture visualization microfluidic models, qualitative and quantitative descriptions of the influences of connectivity between primary fracture and secondary fracture on flowback were given from core scale to pore network scale. The flow patterns of oil-gel breaking fluid two-phase flow during flowback under different fracture connectivity were analyzed. We found some counterintuitive results that non-connected secondary fracture (NCSF, not connect with artificial primary fracture and embedded in the matrix) is detrimental to flowbackefficiency. The NCSF accelerates the formation of oil channeling during flowback, resulting in a large amount of fracturing fluid trapped in the matrix, which is not beneficial for flowback. Whereas the connected secondary fracture (CSF, connected with the artificial primary fracture) is conducive to flowback. The walls of CSF become part of primary fracture, which expands the drainage area with low resistance, and delays the formation of the oil flow channel. Thus, CSF increases the high-speed flowback stage duration, thereby enhancing the flowback efficiency. The fracturing fluid flowback efficiency investigated here follows the sequence of the connected secondary fracture model (72%) > the matrix model (66%) > the non-connected secondary fracture model (38%). Our results contribute to hydraulic fracturing design and the prediction of flowback efficiency.

Understanding molecular interactions between oil and reservoir matrix is crucial to develop a productive strategy for enhanced oil recovery. Molecular dynamics simulation has become an important method for analyzing microscopic mechanisms of some static properties and dynamic processes. However, molecular modeling of shale oil and reservoir matrix is still challenging, due to their complex features. Wettability, which is the measurement of oil-matrix interactions, requires in-depth understanding from the microscopic perspective. In this study, the density, interfacial tension and viscosity of eleven common components in shale oil are calculated using molecular dynamics simulations. Then a molecular model of Gulong shale oil is built, based on the reported experimental results and simulations. Compared with the variation in hydrocarbon content, the change in polar component content leads to more significant variations in the physical properties of shale oil. This molecular model is also employed to investigate the wettability of shale-oil nanodroplets on minerals and organic matter, with or without the surrounding aqueous phase. This work suggests fresh ideas for studying the oil-matrix interactions on the nanoscale and provides theoretical guidance for shale oil exploitation.