In this study, the mechanism of physical–chemical viscosity reduction of different heavy oils under ultrasonic wave is explored. Experiments of viscosity reduction and viscosity recovery of different heavy oils under ultrasonic excitation were carried out, and the optimal ultrasonic parameters, ultrasonic physical disturbance, and cavitation viscosity reduction extent of different oil samples were determined. Based on the element analysis methods, four-component analysis, gas chromatography analysis, and formation water pH value test, the micro-mechanism of the oil chemical structure change and water samples under ultrasonic wave was analyzed. The results show that the water content, temperature, and initial viscosity of heavy oil are the key to reduce the viscosity of heavy oil. The higher viscosity of the initial oil sample, the higher water content, and the temperature were needed. Compared with the lower viscosity oil sample, the higher viscosity oil sample has higher extent of cavitation viscosity reduction and lower extent of physical disturbance viscosity reduction under ultrasonic wave. After ultrasonic treatment, the contents of heteroatoms, resins, and asphaltenes in heavy oil samples with high viscosity decreased significantly, and the conversion extent of high carbon chain to low carbon chain was greater. In addition, the pH of water in heavy oils decreased after ultrasonic treatment, and the pH of water in high viscosity heavy oil decreased more significantly after ultrasonic treatment.

The high dosage of surfactant terribly restrains the extensive application of viscoelastic surfactant (VES) fracturing fluid. In this study, a novel gemini surfactant (GLO) with long hydrophobic tails and double bonds was prepared and a VES fracturing fluid with a low concentration of GLO was developed. Because of the long tails bending near the double bonds, there is a significant improvement of the surfactant aggregate architecture, which realized the favorable viscosity of the VES fluid at a more economical concentration than the conventional VES fracturing fluids. Fourier transform infrared spectrometry (FT-IR), nuclear magnetic resonance spectrometry (¹H NMR, ¹³C NMR), and high-resolution mass spectrometry (HRMS) were employed to study the formation of the product and the structure of GLO. The designed GLO was produced according to the results of the structure characterizations. The formula of the VES fracturing fluid was optimized to be 2.0 wt % GLO + 0.4 wt % sodium salicylate (NaSal) + 1.0 wt % KCl based on the measurements of the viscosity. The viscosity of the VES fluid decreased from 405.5 to 98.7 mPa·s as the temperature increased from 18 to 80 °C and reached equilibrium at about 70.2 mPa·s. The VES fluid showed a typical elastic pseudoplastic fluid with a yield stress of 0.5 Pa in the rheological tests. It realized a proppant setting velocity as low as 0.08 g/min in the dynamic proppant transport test carried by GLO-based VES fracturing fluid. Compared to the formation water, the filtrate of the VES fracturing fluid decreased the water contact angle (CA) from 56.2 to 45.4° and decreased the water/oil interfacial tension (IFT) from 19.5 to 1.6 mN/m. Finally, the VES fracturing fluid induced a low permeability loss rate of 10.4% and a low conductivity loss rate of 5.4% for the oil phase in the experiments of formation damage evaluation.

To investigate the influence of pressure difference changes on the micro start-up and percolation of heavy oil, a micro visualization displacement device was used to characterize the start-up time and oil–water percolation state of heavy oil. The mechanism of different pressure differences, as well as the frequency and amplitude of pressure difference changes, on the start-up and percolation balance of heavy oil was clarified. The results indicate that high-pressure difference and pressure difference changes can reduce the start-up time of heavy oil. A reasonable frequency of pressure difference changes effectively promotes the balance between positive and negative pressure shear and fluid–solid response. Large pressure difference changes can effectively break the viscous and adsorption resistance during heavy oil start-up; reasonable pressure difference can exert the synergistic effect of pressure difference and infiltration, achieving a balance between the water wave and the initial water film thickening process as well as the continuous percolation process of wire drawing, oil droplets, and oil columns during the medium-to-high water content period; a reasonable frequency of pressure difference variation during the high water content period can promote the superposition of inertia effects at the oil–water interface and break the balance of the oil–water interface. A large amplitude of pressure difference variation is beneficial for the strong deformation of the oil–water interface and the shear dislocation peeling of the oil–solid interface. Therefore, a relatively high amplitude of pressure difference variation and a reasonable frequency of pressure difference variation, as well as the synergistic effect of pressure difference and infiltration, are the keys to effectively start heavy oil and improving oil recovery during the ultrahigh water-cut period.