Estimating Wax Plug Transportation Force in Crude Oil Pipeline Pigging

Wax plug growth in pipeline pigging significantly impacts the crude oil transportation, even resulting in a wax blockage accident of pipeline. A pigging facility was constructed to conduct the pigging experiments. A physical model was proposed to illustrate the mechanism of wax plug transportation. Furthermore, a theoretical model was developed to estimate the wax plug transportation force per unit contact area. It is found that the wax plug transportation force per unit contact area is independent of the wax plug length and radius, while correlated to the wax yield stress and the interfacial lubrication coefficient. These predicted correlativities match with the experimental results. Additionally, the experimental results show that the wax plug transportation force per unit contact area is independent of the pig geometry and hardness. With the experimental results, the interfacial lubrication coefficient for wax sample prepared from crude oil and diesel oil were calculated to be 0.0749 and 0.0442, respectively. It is indicated that the extruded diesel oil at the contact surface provides a better lubrication effect for wax plug transportation than the extruded crude oil. Experiments with different pig geometries and hardnesses were performed to verify the modeling results. The cases of two field wax blockage accidents were used for validation. With the results from the developed model, an engineering implication was presented accounting for formation condition of wax blockage and emergency operation scheduling as well as selection of an appropriate pig diameter.