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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.

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.

Based on the theory of non-equilibrium thermodynamics, considering the dynamic effect of molecular diffusion and the change in thermodynamic parameters caused by wax precipitation, the phenomenological relations of different thermodynamic “force” and “flow” interactions were derived. The corresponding thermodynamic model of a waxy crude oil pipeline transportation system was built, and then, the excess entropy production expression was proposed. Furthermore, the stability criterion model of the pipeline transportation system was established on the basis of Lyapounovstability theory. Taking the oil pipeline in Daqing oilfield as an example, based on the four parameters of out-station temperature, out-station pressure, flow rate and water content, the stable and unstable regions of the system were divided, and the formation mechanisms of the two different regions were analyzed. The experimental loop device of wax deposition rate was designed, and then, the wax deposition rate under the four parameters was measured. The results showed that the stable region of the wax deposition rate fluctuation was basically in accordance with the stability region analyzed by the criterion model established in this paper, which proved that the stability criterion model was feasible for analyzing the stability of the waxy crude oil pipeline transportation process.

In order to explore the micro-dynamic principle of radial deposition and adhesion behavior and analyze the effect-principle of the features in construction of rough tube surfaces on each link, a molecular dynamic model was established to characterize this behavior. On this basis, the deposition and adhesion behavior of the homogeneous and heterogeneous nucleation clusters were simulated, respectively. Research showed that the waxy crude oil could settle on the inner tube, and after a series of actions of oil molecules adhering to the inner face of the tube to form the pure oil in its solidified state, the sediment layer contained the wax and asphaltene molecules owing to their diffusion process, the pure oil in its solidified state back diffused to the oil transport flow, the adhesion extent of the sediment on the inner face gradually strengthened, the sediment layer that adhered to the wall gradually aged, and the sediment gradually hardened with an increased density. Furthermore, the distribution and variation principle of different types of energy in the process of adhesion were analyzed. The effect-principle of features in construction (different wall morphologies, phase area fractions, and grooved column heights) on the adhesion strength level, wetting pattern, and hydrophobicity of clusters was studied.