A gel based on polyacrylamide, exhibiting delayed crosslinking characteristics, emerges as the preferred solution for mitigating degradation under conditions of high temperature and extended shear in ultralong wellbores. High viscosity/viscoelasticity of the fracturing fluid was required to maintain excellent proppant suspension properties before gelling. Taking into account both the cost and the potential damage to reservoirs, polymers with lower concentrations and molecular weights are generally preferred. In this work, the supramolecular action was integrated into the polymer, resulting in significant increases in the viscosity and viscoelasticity of the synthesized supramolecular polymer system. The double network gel, which is formed by the combination of the supramolecular polymer system and a small quantity of Zr-crosslinker, effectively resists temperature while minimizing permeability damage to the reservoir. The results indicate that the supramolecular polymer system with a molecular weight of (268–380) × 10⁴ g/mol can achieve the same viscosity and viscoelasticity at 0.4 wt% due to the supramolecular interaction between polymers, compared to the 0.6 wt% traditional polymer (hydrolyzed polyacrylamide, molecular weight of 1078 × 10⁴ g/mol). The supramolecular polymer system possessed excellent proppant suspension properties with a 0.55 cm/min sedimentation rate at 0.4 wt%, whereas the 0.6 wt% traditional polymer had a rate of 0.57 cm/min. In comparison to the traditional gel with a Zr-crosslinker concentration of 0.6 wt% and an elastic modulus of 7.77 Pa, the double network gel with a higher elastic modulus (9.00 Pa) could be formed only at 0.1 wt% Zr-crosslinker, which greatly reduced the amount of residue of the fluid after gel-breaking. The viscosity of the double network gel was 66 mPa s after 2 h shearing, whereas the traditional gel only reached 27 mPa s.

High content of asphaltenes and waxes leads to the high pour point and the poor flowability of heavy oil, which is adverse to its efficient development and its transportation in pipe. Understanding the interaction mechanism between asphaltene-wax is crucial to solve these problems, but it is still unclear. In this paper, molecular dynamics simulation was used to investigate the interaction between asphaltene-wax and its effects on the crystallization behavior of waxes in heavy oil. Results show that molecules in pure wax are arranged in a paralleled geometry. But wax molecules in heavy oil, which are close to the surface of asphaltene aggregates, are bent and arranged irregularly. When the mass fraction of asphaltenes in asphaltene-wax system (ω_asp) is 0–25 wt%, the attraction among wax molecules decreases and the bend degree of wax molecules increases with the increase of ω_asp. The ω_asp increases from 0 to 25 wt%, and the attraction between asphaltene-wax is stronger than that among waxes. This causes that the wax precipitation point changes from 353 to 333 K. While the ω_asp increases to 50 wt%, wax molecules are more dispersed owing to the steric hindrance of asphaltene aggregates, and the interaction among wax molecules transforms from attraction to repulsion. It causes that the ordered crystal structure of waxes can't be formed at normal temperature. Simultaneously, the asphaltene, with the higher molecular weight or the more hetero atoms, has more obvious inhibition to the formation of wax crystals. Besides, resins also have an obvious inhibition on the wax crystal due to the formation of asphaltene-resin aggregates with a larger radius. Our results reveal the interaction mechanism between asphaltene-wax, and provide useful guidelines for the development of heavy oil.

Carbon dots (CDs) show great potential as a new type of oil-displacing agent for unconventional oil and gas development. However, the instability and easy aggregation epitomize the challenges that accompany the application of CDs in high temperature and high salinity (HT/HS) reservoirs. In this research, novel benzene sulfonate-modified carbon dots (BS-CDs) with remarkable thermal stability and salt resistance were fabricated through an in-situ electrochemical exfoliation method. Molecular simulation verifies that the introduction of benzene sulfonate groups substantially strengthens the electrostatic repulsion between BS-CDs, leading to outstanding dispersibility and stability even at a temperature of 100 °C and salinity of 14 × 10⁴ mg/L. Core flooding tests show that 0.05 wt.% BS-CDs nanofluid can significantly reduce the water injection pressure by 50.00% and enhanced oil recovery (EOR) to 68.39% under HT/HS conditions. According to the atomic force microscopy (AFM) scanning results, the adhesion force between the core (after BS-CDs treatment) and oil decreased by 11.94 times, indicating that the hydrophilicity of the core surface was increased. In addition, the distribution of the adhesion force curve is more concentrated, which means that the micro-scale wettability of the core changes from oil-wet to more homogeneous water-wet. This study provides a feasible way for the development and application of good thermal stability and salt resistance CDs in unconventional reservoir development.