Structural Behavior Of Gelled Waxy Crude Oil And Its Impact On Start-up Flow Of Pipelines

For the poor fluidity at normal temperature,waxy crude oil often need to be heated to improve its fluidity before being pumped into pipeline.After prolonged shutdown,which is unavoidable during production,crude oil in the pipeline may form gel structure because of the temperature reduction.The restart of gelled waxy crude oil pipeline is one of the maior concerns of oil pipeline flow assurance.Engineering practice witnesses that a transitory startup of the pump during the pipeline shutdown period(known as transitory restart),which causes the nearly gelled oil to flow,can reduce the minimum pressure difference required for successful restart so as to lower the pipeline blocking risk.The rheology and non-newtonian fluid mechanism of transitory restart is the influence of pre-shear on the structural behavior of gelled waxy crude oil and start-up flow in pipelines.With this background,the main achievements of the present work are as follows:The effect of the quasi-steady assumption on the numerical computation of start-up flow of structured fluid in pipeline was clarified,which ensures the rigorousness of numerical analysis of start-up flow.For the start-up flow of compressible Herschel-Bulkley fluid in pipelines,scale analysis was used to theoretically analyze the momentum equation.A dimensionless number Ut was defined,which is proportion to the aspect ratio of pipeline,Reynolds number and flow index,and/or inversely proportion to the compressibility coefficient of the fluid and Bingham number.Numerical studies show that the effect of these five parameters on the applicability of the quasi-steady assumption depends only on the value of Ut.The smaller the value of Ut,the less the quasi-steady assumption affects the simulation results.On this basis,Ut=0.065 was recommended as the critical value for determining whether the quasi-steady assumption is applicable.When Ut<0.065,compared with the results computed without quasi-steady assumption,the error of the total restart time computed with quasi-steady assumption is below 2%,and the error of the outlet velocity during its recovery process computed with quasi-steady assumption is below 1%.According to structural behavior of gelled waxy crude oil,the Dullaert-Mewis elasto-viscoplastic thixotropic model was modified to better describe the elasto-viscoplastic thixotropic behavior of gelled waxy crude oil.Based on the modified Dullaert-Mewis model,a numerical algorithm was established to deal with the start-up flow of compressible gelled waxy crude oil in pipelines.The results show that the viscoelasticity of gelled crude oil can reduce the minimum pressure difference required for successful restart,increase the steady-state velocity and save the total restart time.A high plastic viscosity ratio,strong shear thinning behavior,a low structure break down rate and a high structure build up rate will all reduce the steady-state velocity,increase the minimum pressure difference required for successful restar and the total restart time under given restart pressure.In addition,high compressibility of gelled oil will reduce the minimum pressure difference required for successful restart,and increase the steady-state velocity and total restart time under given restart pressure.The understanding about the incompletely recoverable behavior of gelled waxy crude oil was deepened.By means of a rheometer,structural behavior of crude oil after being pre-sheared with or without subsequent cooling was studied.The results show that the gel structure is not able to completely recover once being broken,whether in isothermal holding or in subsequent cooling condition.According to the numerical computation on the basis of the structure breakdown experiments,quantitative results of the effect of transitory restart were obtained.That is,transitory restart can significantly reduce the minimum pressure difference required for successful restart,increase the steady-state velocity and save the total restart time.The present work provides a theoretical basis for the safe operation of waxy crude oil pipeline.