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The three-dimensional network structure, which is formed by the precipitated wax crystals, is generally considered as the direct cause for gelling of waxy crude oil. Through interaction with wax molecules, the two polar components, namely the resin and asphaltene,participate in the wax crystallization and subsequently alter the wax precipitation properties. This makes it rather complicated to explore the gelling behavior of crude oil. The micro-dynamic behaviors among resin, asphaltene and wax molecules have a direct bearing on the morphology of mesoscopic wax crystals, and then play a decisive role in the macro-rheological property of waxy crude oil multiphase system. Macroscopically, the effect of resin and asphaltene on the rheology of waxy crude oil and its characterization methods were summarized based on the rheological experiments. Mesoscopically, microscopic experiments and molecular simulation were adopted to analyze the effect of resin and asphaltene on the morphology and particle size of wax crystal, as well as the synergistic mechanism among resin, asphaltene and wax crystals. Microscopically, the molecular coalescence and interphase interaction were introduced through the molecular dynamics simulation. Meanwhile, the effect of resin and asphaltene molecules on the gelling properties of waxy crude oil was discussed. Additionally, prospect was made to the application of molecular dynamics simulation in the field of waxy crude oil gelling, which is expected to lay a theoretical foundation for the development of waxy oil pipeline transportation technology and provide reference to the research, development and application of pour point depressant.

Predicting the decompression behavior and controlling the fracture of CO2 pipelines are crucial research areas for the safe transport of CO2. Existing decompression wave prediction models have problems such as large errors in calculating the sound speed of the gas-liquid two-phase flow and difficulties in convergence near the critical point. The local sound speed calculation method based on isochoric thermodynamics was improved and a new decompression wave prediction model was established. The effects of gas compositions, initial phase state, initial pressure, and initial temperature on decompression wave curves and pressure drop curves were calculated and analyzed. Additionally, the decompression behavior of CO2 pipeline leaks in practical engineering scenarios was predicted. The results show that compared to post-combustion capture or pre-combustion capture, the initial decompression wave speed under gas by oxy-fuel combustion capture is reduced by approximately 26%, and the decompression plateau pressure increases by 22%. During the decompression process, the temperature and pressure operating point of medium inside the pipeline no longer move along the gas-liquid equilibrium line but directly enter the gas-liquid two-phase region. Compared to the dense phase state, the saturation pressure corresponding to the intersection of the pressure drop curve and the CO2 bubble point line is about 26% higher in the supercritical state, and the decompression plateau pressure is higher. Lower operating temperatures and higher operating pressures can effectively reduce the decompression plateau pressure. Due to the differences in operating parameters along the pipeline, the risk of pipeline fracture decreases sequentially from the starting point of the pipeline, with the front section of the pipeline being the high-risk section. This study provides a theoretical basis for CO2 pipeline design and engineering applications. [ABSTRACT FROM AUTHOR]