Study On Wax Removal Mechanism In Crude Oil Pipeline Pigging

Wax deposition is an intractable flow assurance challenge for waxy crude oil transportation,and pipeline pigging is the most commonly used wax remediation technique in field.However,due to the elusiveness in wax removal mechanism,field pigging operations still depend heavily on thumb rules,which causes pig stalling and wax blockage every now and then.Investigating wax removal mechanism for accurately predicting wax breaking force and wax removal efficiency,therefore,becomes significantly important for pigging program scheduling and risk reduction.In response,this work firmly focuses on wax removal mechanism and aims to model the wax breaking force and wax removal efficiency in pigging.In model development,the traditional coaxial compression and simple shear mechanisms are abandoned.The whole wax area is divided into three parts:undamaged wax,plastic deformation zone and wax chip.It is proved that the slip-line field within the plastic deformation zone is composed of two clusters of orthogonal straight slip-lines.Incorporating wax elasticity and scraping element deformation,wax breaking models for predicting the wax breaking force and wax removal efficiency of disk and cup pigs are established by analyzing the wax-pig interaction and the amount of scarped-off wax.For verification,wax-casting pigging experiments were performed,and a facility that allows pigging experiments with naturally deposited wax was designed.Experimental determination principles for wax breaking force and wax removal efficiency are given.The wax-scraper frictional angle is determined to be 13°.Experimental results confirm that a typical wax removal process can be divided into four distinct phases,namely,wax compression,wax breakage,plug formation and plug discharge.In wax-casting pigging experiments,the average relative deviation for wax breaking force is 14.20%,and that of wax removal efficiency is 5.40%.For flow loop pigging experiments,they are 11.14%and 4.36%,respectively.In last part of this paper,the impacts of multiple pig geometries on breaking force and wax removal efficiency of cup and disk pigs,including scraping element thickness,clamping rate,original rake angle,straight part length and scraping element assembly,are explicitly investigated.Targeted to minimize wax breaking force and improve wax removal efficiency,suggestions on pig design are also given.