Study On Wax Layer Removal And Transport Behaviors During Crude Oil Pipeline Pigging

Pig is widely used in pipelines for wax removal.However,pigging operation relies heavily on “rule-of-thumb.” There are high risks for stuck pig and wax plugging for pipeline pigging.Because of its complexity and lack of methods,the research in field of wax removal in pipelines by pig is rather limited worldwide.The wax removal mechanism is still poorly understood.Hence,further study for wax layer removal and transport behaviors during crude oil pipeline pigging is significantly necessary for revealing the wax removal mechanism fully.A good understanding for wax removal in pipeline is beneficial in reducing the pigging risks and making the optimal pigging schemes.This work aims at revealing the nature of wax layer breaking and thus promoting the understanding of wax removal physics in pipelines.A unique setup together with the experimental method is designed to perform the wax removal experiments.It can be used to simulate the pigging operation for wax removal in pipes at different conditions.In this paper,a typical pigging operation for wax removal in pipes can be divided into four stages: 1)wax layer compression phase,2)wax layer breaking phase,3)wax plug production phase,and 4)wax plug drainage phase.The wax breaking force is defined as the average value of the wax layer resistive force during the wax layer breaking phase.The wax removal experiments for short wax casting distances show that the wax breaking force definition is reasonable and scientific.The effect of pig velocity on wax breaking force measurement is studied with the help of wax sample yield stress measurement at different stress ramp rates.The result shows that pig velocity does not affect the wax breaking force in pig velocity ranging from 0.03 to 0.09 m/s.Yield stress,viscoelasticity,wax precipitation characteristic,and wax crystals morphology and structure of wax samples are measured to provide insights into wax sample nature.The experimental result shows that the wax samples prepared in the same experimental conditions have good repeatability in all the above properties.The aspect ratio of wax crystals in wax sample mainly varies in range of 1.03-2.29.The wax crystals in wax sample mainly shaped long rod.With the increase in wax content of wax sample,the aspect ratio of wax crystals decreases,the boundary fractal dimension of wax crystals increases,and thus the yield stress of wax sample increases.The relative solid wax content of wax sample and experimental temperature can be fitted as a linear relationship.The yield stress and solid wax content of wax sample can be fitted as an exponential relationship.The wax sample is more strong in elastic response than viscous response.The WAT and wax content of wax deposit in flow loop increase with the increase in the wall shear stress,decrease with the increase in the dissolved wax concentration gradient near the pipe wall.The effects of wax thickness,mixing ratio of wax deposit to oil,pipe wall temperature,shape and hardness of the scraping element in the pig on wax removal are investigated comprehesively by using the above experimental facility and the prepared wax sample.It is found that the wax failure stress and the wax yield stress can be well fitted as a linear relation for the specified pig.It is additionally concluded that the wax failure stress always exceeds the yield stress of the same wax deposit during the wax removal process.It becomes more significant with the increase in wax hardness.This new finding changes our traditional understanding,in which the wax will be broken as the wax breaking force exerted by pig reaches the wax yield stress.A wax removal model with an average relative error of 10.69% is developed based on these experiment findings.