Nanofluids are considered promising agents for enhanced oil recovery in low-permeability reservoirs, but their application is often restricted by poor thermal and saline resistance and high costs. Moreover, limited studies have addressed the imbibition depth and oil migration processes during nanofluid imbibition in low-permeability reservoirs. In this work, a magnetic core–shell structured nanoparticle Fe₃O₄–TiO₂ was synthesized using inexpensive Fe₃O₄ nanoparticles and tetrabutyl titanate. The synthesized nanoparticles exhibited excellent thermal and saline resistance as well as recyclability. Their structure and functional properties were characterized. The nuclear magnetic resonance technology was applied to investigate the imbibition depth and the oil migration process during magnetic nanofluid imbibition. Results showed that the magnetic nanofluid possessed interfacial activity, wettability alteration capability, and strong thermal and saline resistance. At 80 ℃, the imbibition recovery of magnetic nanofluid reached 32.19%, 3.59% higher than that of SiO₂ nanofluid. The recycle rate of magnetic nanofluid was 81.31%, effectively reducing operational costs. The final imbibition depth of magnetic nanofluid reached 18.82 mm, with an average imbibition rate of 3.14 mm/d, which is 21.97% higher than that of the SiO₂ nanofluid and 39.10% higher than that of the simulated formation water. The imbibition process of magnetic nanofluid was dominated by capillary forces, with oil in micropores displaced into macropores. We expect that this study can contribute to the effective development of low-permeability reservoirs and provide theoretical guidance for field applications.

Slickwater fracturing fluids are widely used in the development of unconventional oil and gas resources due to the advantages of low cost, low formation damage and high drag reduction performance. However, their performance is severely affected at high temperatures. Drag reducing agent is the key to determine the drag reducing performance of slickwater. In this work, in order to further improve the temperature resistance of slickwater, a temperature-resistant polymeric drag reducing agent (PDRA) was synthesized and used as the basis for preparing the temperature-resistant slickwater. The slickwater system was prepared with the compositions of 0.2 wt% PDRA, 0.05 wt% drainage aid nonylphenol polyoxyethylene ether phosphate (NPEP) and 0.5 wt% anti-expansion agent polyepichlorohydrin-dimethylamine (PDM). The drag reduction ability, rheology properties, temperature and shear resistance ability, and core damage property of slickwater were systematically studied and evaluated. In contrast to on-site drag reducing agent (DRA) and HPAM, the temperature-resistant slickwater demonstrates enhanced drag reduction efficacy at 90 ℃, exhibiting superior temperature and shear resistance ability. Notably, the drag reduction retention rate for the slickwater achieved an impressive 90.52% after a 30-min shearing period. Additionally, the core damage is only 5.53%. We expect that this study can broaden the application of slickwater in high-temperature reservoirs and provide a theoretical basis for field applications.

Fracturing fluid property play a critical role in developing unconventional reservoirs. Deep eutectic solvents (DESs) show fascinating potential for property improvement of clean fracturing fluids (CFFs) due to their low-price, low-toxicity, chemical stability and flexible designability. In this work, DESs were synthesized by mixing hydrogen bond acceptors (HBAs) and a given hydrogen bond donor (HBD) to explore their underlying influence on CFF properties based on the intermolecular interactions. The hydrogen-bonding, van der Waals and electrostatic interactions between DES components and surfactants improved the CFF properties by promoting the arrangement of surfactants at interface and enhancing the micelle network strength. The HBD enhanced the resistance of CFF for Ca²⁺ due to coordination-bonding interaction. The DESs composed of choline chloride (ChCl) and malonic acid show great enhancement for surface, rheology, temperature resistance, salt tolerance, drag reduction, and gel-breaking performance of CFFs. The DESs also improved the gel-breaking CFF-oil interactions, increasing the imbibition efficiencies to 44.2% in 74 h. Adjusting HBAs can effectively strengthen the intermolecular interactions (e.g., HBA-surfactant and HBD-surfactant interactions) to improve CFF properties. The DESs developed in this study provide a novel strategy to intensify CFF properties.

The membrane method based on adaptive wettability shows great advantages in oil-water separation. At present, researches focus on the excellent application performance of the membrane material, while the quantitative analysis of interactions in oil-water separation is rarely recognized. Herein, we constructed an adaptable wettability membrane with multiple polymer networks by polydopamine (PDA) and mussel-inspired amphiphilic polymer. Based on the Owens three-probe liquid method, the surface energy of the modified membrane was verified to meet the adaptive wettability conditions, with surface energies (γ_S) of 147.6 mJ m⁻² (superhydrophilic/underwater superoleophobic) and 49.87 mJ m⁻² (superhydrophobic/superoleophobic), respectively. The adhesion or repulsion of the membrane to the oil phase under different conditions during the separation process was quantified by the chemical probe AFM technique. In addition, the oil-water selective separation mechanism was further analyzed in a simplified membrane microchannel model. The results show that the different wetting produces capillary additional pressure in opposite directions, resulting in different energies to be overcome when the oil or water passes through the microchannels, thus achieving selective separation.