Research On The Influence Of Asphaltenes And Resins On Wax Deposition Of Waxy Model Oils

Wax deposition is one of the important flow assurance problems of waxy crude pipelines.Asphaltenes and resins are polar nonhydrocarbon components with the largest content in waxy crudes,however,the influences of asphaltenes and resins on wax deposition of waxy oils have not been well-studied.Therefore,this work carried out wax deposition experiments with waxy model oils containing asphaltenes or resins utilizing a homely-made Taylor-Couette cold finger device.The mass,rheological and compositional properties of waxy deposits generated with waxy model oils were analyzed,and the chemical properties of asphaltenes or resins present in the waxy deposits and oils were also contrasted.The main observations are as follows.First of all,the wax deposition experiments were performed with waxy model oils containing asphaltenes,and the influences of asphaltenes on the formation,growth and composition of waxy deposits were then studied.It was observed that the wax deposition rate decreases while the wax content of waxy deposits increases in the presence of asphaltenes,which is related to the gelation temperature depression of model oils caused by asphaltenes.The asphaltenes with higher concentration in model oils are more effective in accelerating the aging process of wax deposition.Detailly,the waxy deposits are more enriched in the heavy n-paraffin constituents and less enriched in the light n-paraffin constituents;and the critical carbon number(CCN)of the deposits was observed to increase slightly from C₂₆ to C₂₇.These phenomena aggregate as the gelation temperature depression by asphaltenes becomes greater.Furthermore,the mass fraction,molecular structure and chemical properties of asphaltenes as a function of deposition time were characterized to uncover the mechanism of asphaltene enrichment.It was found that the asphaltene content of the deposit constantly increases as time elapses,which can be defined as asphaltene enrichment and was not found to have obvious connection with the content of asphaltene in model oils.The asphaltene content can reach nearly 8 times over that of the original oil for the wax deposition test of 24h.The asphaltene fraction entrained in the deposits presents similar function groups as those present in the asphaltene fraction contained by the waxy model oils.Compared with the original asphaltenes present in the waxy model oils,the asphaltenes extracted from the deposits have higher molecular weight and polarity but lower polydispersity index(PDI).It can be implied that the asphaltene enrichment is selective and the asphaltenes of higher molecular weight or polarity are prior to entering into the deposit layer.The limitation of effective diffusion of asphaltenes into waxy deposits is achieved by the decrease of porosity of waxy deposits caused by the increment of wax content rather than the difference of asphaltene particles in the dispersion degree.Besides,the decrease of conductivity of waxy deposits caused by the less polar asphaltenes might be one of the main factors affecting the enrichment of asphaltene into the deposit layer.It was inferred that Brownian diffusion and electrokinetic effect can be plausible mechanisms of asphaltene enrichment.In addition,the mechanical characteristic of the deposits was analyzed and the rheological mechanism of deposit sloughing was studied.The deposit sloughing is observed to be accelerated by the asphaltenes at a concentration higher than 0.20 wt%in model oils and high shear stress applied to the surface of deposit layers.This is respectively because the structure strength of the waxy deposits is weakened upon the addition of asphaltenes with higher concentration;and the increasing shear of fluid flow accelerates the creep of deposit gels and thereby accelerates the process of structural failure.The newly formed deposits present lower wax appearance temperature,wax content and asphaltene content than aged deposits.Besides,the carbon number distribution(CND)of the newly formed deposits post-sloughing shifted towards lower carbon number in comparison with the aged deposits.Eventually,the influences of resins on the rheological properties and wax deposition characteristics of waxy model oils were investigated.It was indicated that the resins at experimental concentrations of 0?3.0 wt%decrease the WAT by nearly 2?,the depression of which is not found to vary significantly with the resin concentration of model oils.The resins decrease the viscosity,gelation temperature and yield stress of model oils,and the modification effect becomes greater with the increasing resin content.The influences of resins on the mass,rheological and compositional properties of deposits are in accordance with those of asphaltenes.The difference is that the CCN for deposits formed with resin-containing waxy model oils can reach C₂₈,higher than that formed with asphaltene-containing waxy model oils of C₂₇.Furthermore,the rate of resin enrichment is rather slower than that of asphaltene enrichment.For instance,the resin content of the deposits is observed to increase to approximately 3 times that of the original oil after 24h under the same experimental conditions of oil shear and operating temperature.In addition,the deposit sloughing was not observed in the presence of resins,which is propably because of the compact network structure of the deposits formed with the resin-containing model oils.Compared to model oils free of resins,the wax crystals of deposits formed in presence of resins have greater number density and smaller aspect ratio,which vary slowly as the resin concentration of model oils increases.