Study On The Prediction Model Of Wax Deposition For Waxy Crude Oil In Pipeline

Waxy crude oil is a complex mixture which contains a large number of hydrocarbon and non-hydrocarbon components. In the process of pipeline transportation, if the temperature and pressure conditions are changed, especially when the temperature is below the wax precipitation temperature, the wax molecules dissolved in the crude oil will deposit on the pipe wall in the form of crystallization, which will lead to the wax deposition phenomenon. The wax deposition of crude oil will reduce the effective inner diameter of pipeline, increase the transportation pressure, reduce the transportation efficiency, increase the pigging frequency and change the rheological properties of the oil, and even cause the potential blockage accident, which will bring large economic loss to the oil companies. Since the wax deposition process is quite complicated, the current research for the wax deposition theory and prediction model of waxy crude oil is still in constantly evolving. The wax deposition in the pipeline has become the flow assurance issue needed to be addressed in the petroleum industry.Aimed on the problem that the mole fraction distribution and physical properties for each component of heavy fraction in the waxy crude oil cannot be easily determined, an improved heavy fraction splitting model was established, and different basic physical properties calculation correlations for the single carbon number component were evaluated to obtain the suitable method for the waxy crude oil system; the predictive ability of the two-parameter cubic equation of state (EoS) for the phase behavior of waxy crude oil system was improved through modifying its temperature function; considering the influence of pressure on the wax precipitation process, a thermodynamic prediction model was established to calculate the wax precipitation characteristic parameters of waxy crude oil based on the fluid thermodynamic phase equilibrium theory, thereby, the thermodynamic process of wax deposition can be accurately described; based on the the wax molecular deposition mechanisms of the oil flow in the pipeline, a wax deposition kinetic prediction model was established for the waxy crude oil pipeline transportation. The model not only can accurately calculate the wax deposition thickness and the wax content of the deposition layer in the experimental flow loop, but also can describe the wax deposition process in the actual waxy crude oil pipeline, thus, it provides the theoretical basis for the pipeline operation and reasonable pigging plan.The specific research achievements are as follows:(1) The accurate determination of the component molar fraction distribution in the waxy crude oil, and the reliability of the value for the basic physical properties of each component will have significant impact on the thermodynamic phase equilibrium calculation results, therefore, study on the characterization method of the heavy fraction is the basis and premise of establishing a wax precipitation thermodynamic prediction model. According to the rule of the mole fraction of each component with the change of the single carbon number, and based on the two-parameter model which was proposed mainly for the condensate oil/gas system by Ahmed et al., an improved six-parameter model was presented to extend the Cn+ component in the waxy crude oil to improve the prediction accuracy for the molar fraction distribution of each single carbon number component in the heavy fraction and the molecular weight of the Cn+. According to the calculation results of waxy crude oil samples, a conclusion can be drawn that the splitting result of the proposed model is much more close to the measured value, and the error is less than the Katz method, the Pedersen method, the Ahmed method and the Hosein method. Based on the six-parameter heavy fraction splitting model, different basic physical properties correlations for the single carbon number component were compared and evaluated, and the application range was determined for each correlation, therefore, the most suitable calculation correlation for waxy crude oil was obtained.(2) When using the method based on the EoS to calculate the liquid-solid phase equilibrium of waxy crude oil, the adaptability of the selected EoS will directly affect the accuracy of the results. In this thesis, aimed on the problem that the commonly used two-parameter cubic EoSs were not suitable for calculating the phase behavior properties and physical parameters of the waxy crude oil systems which contain more heavy hydrocarbon components, a function correlated with the true boiling point temperature (Tb), the acentric factor (ω) and the reduced temperature (Tr) of heavy hydrocarbon components was introduced, so the temperature function a(T) in the attractive term of the original PR EoS was modified, meanwhile, a temperature function β(T) was added in the repulsive term to further improve the phase behavior predictive ability of the EoS for the polar, non-polar and heavy hydrocarbon, and then the MPRHJ EoS was proposed. After comparing the calculation results of the bubble point pressure and the density for the waxy crude oil samples by MPRHJ, MPR2, PRHJ, PR, PRT, PRSF, PRHF, PRTBD and SRK EoSs, conclusions can be drawn that the proposed MPRHJ EoS combining with the Hosseinifar-Jamshidi correlation can obtain the results which are more close to the experimental values, and the average values of the absolute relative error for the bubble point pressure and the density calculation results are 4.32% and 2.26%, respectively.(3) Using the established heavy fraction characterization model and the MPRHJ EoS, based on the fluid thermodynamic phase equilibrium theory and the liquid-solid phase equilibrium numerical calculation model for the wax molecules precipitation in the crude oil, a wax precipitation thermodynamic prediction model was proposed. In the presented model, the proposed modified EoS was adopted to compute the liquid phase thermodynamic parameters in the liquid-solid phase equilibrium, an improved P-UNIQUAC activity coefficient model was utilized to describe the non-ideal of solid phase, considering the influence of pressure on the determination of the thermodynamic parameters for the oil system, the Poynting correction term in the solid phase fugacity calculation formula was addressed. After predicting the wax appearance temperatures of 6 groups of waxy crude oil samples from North Sea oilfield reported in the literature and 6 groups of waxy crude oil samples from domestic Tahe oilfield in Xinjiang province and Shengli oilfield in Shandong province tested by experimental apparatus, conclusions can be drawn that the results predicted by the presented model are in good agreement with the experimental data, and the average absolute relative error are 1.08% and 8.12%. After predicting the wax appearance temperature of a waxy crude oil sample under different pressure conditions, it can be concluded that the average absolute relative error of the calculation results by the presented model is 2.36%; in addition, the proposed model can accurately predict the amount of wax precipitation of waxy crude oil samples under different temperature conditions.(4) The heat and mass transfer properties, wax crystallization process and fluid flow state of the crude oil often correlate with each other in the pipeline, which is a very complicated process. Based on the molecular diffusion mechanism under the laminar flow, oil flow shear stress under the turbulent flow and wax "aging" of the wax deposition layer in the waxy crude oil pipeline transportation process, a kinetic prediction model was proposed to predict the deposition layer thickness and the wax content in the deposition layer in the waxy crude oil pipeline. After comparing the calculation results with the experimental data of the small and pilot flow loop, it can be concluded that the presented kinetic model can more accurately describe the wax deposition process, comparing with the calculation model only considering the molecular diffusion mechanism; additionally, the presented model can also be used to simulate the wax deposition process of the production pipeline, and make a description for the wax deposition rule in the pipeline.