Pigging is widely used in pipelines for wax removal. However, pigging operation relies heavily on "rules-of-thumb." Because of its complexity, rather limited pigging models were presented to predict the wax removal mechanics in past decades. This work aims to develop a pigging model for wax removal in pipelines. A unique experimental facility was designed and constructed for simulating pigging operation for wax removal in pipelines. This facility comprises five main parts: an experiment system, a wax casting system, a motor and control system, a measurement and data acquisition system, and a special designed pig system. The mixture of crude oil and field wax deposit was cast inside the test section to carry out the pigging experiments with disc and cup pigs. It was found that hardness of the scraping element in pig has a profound effect on wax removal, and this effect depends on the wax thickness on the pipe wall tightly. A pigging model, which could well explain the effects of wax thickness, wax hardness, pipe diameter, pig geometry as well as hardness of the scraping element in pig, was established based on the experimental findings. To the best of our knowledge, this is the first time to incorporate the effect of hardness of the scraping element in pig on wax removal into a pigging model. 17 sets of pigging experiments were used to verify the developed pigging model with an average relative error of 10.69%. The pigging model developed in this work could be a practical tool in designing economic and safe pigging programs.
Summary
Pigging is widely used in pipelines for wax removal. However, pigging operation relies heavily on “rules of thumb.” Because of its complexity, rather-limited pigging models were presented to predict the wax-removal mechanics in past decades. This work aims to develop a pigging model for wax removal in pipelines. A unique experimental facility was designed and constructed for simulating pigging operation for wax removal in pipelines. This facility is composed of five main parts: an experiment system, a wax-casting system, a motor and control system, a measurement and data-acquisition system, and a specially designed pig system. The mixture of a crude-oil and a field-wax deposit was cast inside the test section to carry out the pigging experiments with disk-and-cup pigs. It was found that hardness of the scraping element in the pig has a profound effect on wax removal, and this effect depends closely on the wax thickness on the pipe wall. A pigging model, which could well explain the effects of wax thickness, wax hardness, pipe diameter, pig geometry, and hardness of the scraping element in the pig, was established on the basis of the experimental findings. To the best of our knowledge, this is the first time to incorporate into a pigging model the effect of the hardness of the scraping element in the pig on wax removal. Seventeen sets of pigging experiments were used to verify the developed pigging model, with an average relative error of 10.69%. The pigging model developed in this work could be a practical tool in designing economic and safe pigging programs.
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.
Wax deposition is a severe flow assurance challenge that threatens waxy crude oil production and transportation. For wax remediation, pipeline pigging is the most widely used technique. However, the elusiveness of wax removal mechanism and the lack of reliable methods to evaluate wax breaking force and wax removal efficiency easily trigger pig stalling and wax blockage in field pigging operations. Modeling wax breaking force and wax removal efficiency, therefore, promotes the pigging confidence. This Review seeks to clarify the current picture of wax removal research in crude oil pipeline pigging. Relevant wax deposit properties including wax layer thickness and strength are discussed. Wax removal mechanisms are summarized from perspectives of wax–pig interaction, macroscopic force response, and scenarios with oil flow. Prediction models of wax breaking force and wax removal efficiency are analyzed comprehensively. Pig geometry optimization using this model is given. In addition, the key roles of wax deposit strength, viscoelasticity and thixotropy, foam pig investigation, and wax plug prediction are highlighted for guiding future endeavors in this area.
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