Research on Pig Movement and Wax Transport during the Wax Removal Process of Oil Pipelines

Research on the process of wax transport during pipeline pigging is of great significance for the maintenance and safe operation of pipelines. To elucidate the transportation mechanism of oil–wax mixtures during pigging and its impact on pig velocity, this study integrates pig dynamics with a computational fluid dynamics (CFD) model for oil–wax mixture transport, utilizing the moving reference frame (MRF) method. The transient interactions between the oil–wax two-phase flow and the pig are systematically investigated under varying bypass rates and wax layer thicknesses. A 6-inch loop experiment was conducted to comparatively analyze wax transport behavior and pig motion. Experimental results reveal that the wax migration process is in good agreement with CFD simulations, and the predicted pig velocities are consistent with experimental measurements within the uncertainty range of the experimental parameters, thereby validating the reliability of the CFD model. Simulation results indicate that wax transport downstream of the pig proceeds through three distinct stages: formation of the oil–wax mixed slug, formation of oil–wax agglomerates, and stable transport of the agglomerates. During the slug formation stage, significant fluctuations in pig velocity are observed, whereas after the development of agglomerates, the pig velocity stabilizes and exhibits a positive correlation with the viscosity of the upstream oil–wax mixture. Jet flow is shown to effectively promote downstream wax transport, thereby reducing wax accumulation at the pig front. The velocity of oil–wax mass transport ranges from 0.6 to 1.3 times the pipeline flow velocity. Maintaining the pig velocity below 0.6 times the pipeline flow velocity can further mitigate the risk of wax blockage. These findings provide essential insights for the optimization of intelligent pig design and the reduction of operational risks associated with crude oil pipelines.