Study On Wax Deposition Law And Economic Pigging Cycles Of Submarine Mixed Pipelines

Waxy crude oil is easy to form wax deposits on the inner wall of pipeline during transportation.Especially for submarine pipelines,because of the low temperature of the seabed(the temperature of the deep seabed is about 4?),the temperature of crude oil drops rapidly in the pipeline transportation process,wax crystals in the oil are easy to precipitate and deposit on the inner wall of the pipeline,resulting in the decrease of the pipeline circulation area and the decrease of pipeline transportation capacity.In addition,wax deposition will lead to gelation of crude oil,which will increase the safety risks of-pipelines.In serious cases,even the pipeline will be blocked,causing huge economic losses to production management.Therefore,in the pipeline transportation process,how to predict and control wax deposition has always been a research hotspot in the petroleum industry at home and abroad.Studying the wax deposition regularity of submarine mixed transportation pipeline,predicting the wax deposition distribution and formulating reasonable and reliable economic pigging cycles are conducive to ensuring the safe operation of pipelines,saving cost and providing reference value for guiding production practice.Firstly,based on the experimental data in references,OLGA was used to simulate the oil-gas-water three-phase wax deposition in indoor loop.The relative errors between OLGA simulation results and experimental results were analyzed by comparing the average wax deposition thickness under different gas and liquid conversion rates and different water content.It was proved that OLGA simulation of oil-gas-water three-phase wax deposition was feasible.Then,according to the pliysical properties of crude oil,including real boiling point data,viscosity,wax content and wax precipitation point,Multiflash software is used to transform crude oil from black oil lnodel to component model.Wax module in PVTsim is used to adjust the wax precipitation characteristics of-crude oil,including wax precipitation point,wax content and so on,and a fluid package is generated to prepare for OLGA simulation.The model of M-N submarine pipeline was established by OLGA,and the effects of single and multiple factors on wax deposition were analyzed.The results of single factor analysis show that the lower the inlet temperature,the smaller the diameter of the pipe,and the larger the oil delivery,the larger the average wax deposition thickness;the outlet pressure has little effect on the wax cleposition process;the higher the water content and the larger the gas-oil ratio,the smaller the average wax deposition thickness.According to the analysis of multi-factor orthogonal experiment,it is found that among the five factors studied,the importance of each factor to the deposition of oil-gas-water three-phase wax is as follows:oil delivery>water content>inlet temperature>gas-oil ratio>outlet pressure.Then,according to the results of multi-factor analysis,the main influencing factors of oil-gas-water three-phase wax deposition were screened out.On the basis of single-phase wax deposition model,considering the effects of gas shear and water content on wax deposition rate,a three-phase wax deposition model of oil,gas and water was established.Based on the wax deposition data of M-N submarine pipeline under different working conditions,a three-phase wax deposition model for oil,gas and water in M-N submarine pipeline was established by using non-linear regression method.The average relative error between the predicted results of this model and OLGA predicted results is 8.71%,which proves that the regression wax deposition model is reliable.Finally,by analyzing the variation of thermal cost,power cost and one-time pigging cost per unit length with wax deposition thickness,a pigging cycle model is established with the eost per ton of crude oil transported Per unit length as the objective fimction,and a method for determining the economic pigging cycle is proposed.The wax deposition thickness distribution and economic pigging period of M-N submarine pipeline under different working conditions are calculated by MATLAB software programming.The results show that with the increase of oil transportation,the wax deposition rate will increase and the economic pigging cycle will be shortened,while with the increase of gas-oil ratio or water content,the wax deposition rate will decrease and the economic pigging cycle will be prolonged.According to the production allocation data of M-N submarine pipeline in the next five years,the economic pigging cycle of the pipeline in the next five years is formulated.