Experimental Study on Mechanisms of Wax Removal During Pipeline Pigging

Pigging is widely used in pipelines for wax removal. However, pigging operation relies heavily on “rule-of-thumb.” Because of its complexity and lack of methods, the wax removal mechanism is still poorly understood. This work aims to reveal the nature of the wax breaking process and thus promote understanding of wax removal physics. A unique experimental setup is designed and constructed to perform a series of wax removal experiments. The test section consisting of a 50-mm-ID, 600-mm-long, horizontal pipe has an accurate temperature control for wax casting and removal process. Different types of polyurethane pigs without oversize against the test section were used for the experiments to eliminate the influence of friction force between the pig and the pipe wall. Mixtures of crude oil and field wax deposit were cast inside the test section to carry out the experiments. In particular, a novel yield stress measurement was introduced into this study to characterize the wax hardness. The effects of wax thickness, mixing ratio of wax deposit to oil, pipe wall temperature, shape and aggressiveness of polyurethane pig on wax breaking force were investigated comprehensively. In particular, the dependences of wax breaking force on the pig aggressiveness and pipe wall temperature are clarified for the first time. In addition, it is found that the relationship between wax failure stress and yield stress can be well fitted as a linear relation. Furthermore, the dependences of wax breaking force on the wax hardness and pig design being used are revealed quantitatively. Subsequently, it is concluded that there is an overwhelming effect between wax failure stress and yield stress, where the wax failure stress exceeds the yield stress of the same wax deposit during the wax removal process. The findings of this paper are beneficial in determining the optimal de-waxing frequency and evaluating the pigging risks.