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For ensuring the safe and stable operation of waxy crude oil pipeline transportation, in this research, the molecular dynamics model was established to characterize the deposition and wall sticking behavior of waxy crude oil multiphase system pipeline transportation. The equal density interpolation fitting method was proposed to determine the wall contact angle of simulation results. Through verification, the error between the simulation results and the experimental results measured by Dos Santos et al. (2006) was less than 5%, which showed that the established model was accurate and reliable. Using the established model, the deposition and wall sticking behavior of waxy crude oil nucleated clusters was simulated. It was found that the nucleated clusters would first adhere to the wall surface to form the solidified oil layer. Then, the wax and asphaltene molecules would diffuse to the deposit layer, and the oil molecules in the solidified oil layer reverse diffused to the direction of the oil flow. With the adhering and spreading degree of clusters on the wall surface increasing, the deposit layer gradually aged, and the gelled deposit layer with a higher density and hardness would form. On this basis, the micro influence mechanism of the surface free energy was studied. It was found that the higher the surface free energy, the more hydrophilic the pipeline wall was, and the higher the adhesion degree would be. Moreover, based on the ABF sampling and the potential of mean force calculations, the selective deposition process of waxy crude oil deposited on the sedimentary layer was studied. The micro information on the deposition sites, the binding conformation, and the binding energy of different molecules in clusters deposited on different molecules in sedimentary layer were analyzed. The investigations in this study could provide theoretical support for paraffin removal and control, which could ensure the safe and stable operation of the waxy crude oil production system.

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The molecular dynamics model was established to characterize the homogeneous and heterogeneous nucleation processes for a waxy crude oil multiphase system at the nanoscale. Using the established model, the homogeneous and heterogeneous nucleation processes were numerically simulated, and the accuracy of the model was verified by comparing the simulation results with those of classic nucleation theory. On this basis, the micromechanism for the effect of different cooling rates on the nucleation process was studied, and it was determined that the homogeneous and heterogeneous nucleation would coexist and compete with each other in the initial stage and, at last, a large spherical cluster would form. Furthermore, the calculation model for the interfacial free energy was proposed, the interfacial free energy with nucleation centers of different specific area factors were numerically calculated, and the influence rules of the interface effects on the nucleation process were clarified. Besides, the heterogeneous nucleation process of different nucleation center materials was simulated, and the aggregation degree and element composition of nucleation clusters were analyzed. Finally, the influence mechanism of doping ions on the nucleation process was investigated. The investigations in this study provided a theoretical basis for ensuring a safe and economic operation of waxy crude oil production.

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Abstract:The basic theory of exergy was used to derive the formulae of physical and chemical exergy in the process of pipeline transportation, combined with the effect of wax deposition on the thermodynamic parameters including specific heat, density, chemical potential and concentration gradient. On the basis of this, the expression of various exergy losses were derived, and the exergy balance model was then built in the process. For the case study, a waxy crude oil pipeline in China was selected. The mechanism for how wax deposition affected the physical and chemical exergy loss was studied through analyzing the axial pipeline distribution of pressure, temperature, flow rate and thickness of insulation layer. Finally, under the design flow of 66 × 10 3 kg·h −1 , the orthogonal experimental analysis method was used for comparing the degree of specific factors which could influence the total exergy efficiency. The highest exergy efficiency combination of working conditions was then determined. This research could provide a theoretical basis for guiding safe and economic operation in the actual pipeline transportation process.