Study On Wax Deposition Characteristics Of Waxy Crude Oils Treated With Pour Point Depressant

Most of crude oils produced in China are of high gel point and viscosity.Addition of pour point depressant(PPD)is commonly used to improve the flowability.Solid deposition is one of the important problems of waxy crude oils.However,the wax deposition characteristics of waxy crude oils added with PPD have not been well-studied.Hence,the waxy crude oils with different modification effects were obtained by changing the PPD dosage and treatment temperature.Wax deposition experiments were performed with a self-developed Taylor-Couette cold finger device.The formation,aging,mechanical properties and heterogeneous structure of waxy deposits were analyzed.The main findings are as follows.Initially,the influences of nanocomposite pour point depressant(NPPD)dosage and treatment temperature on the rheological properties were investigated.It was indicated that the NPPD in waxy crude oils decreases the gelation temperature,yield stress and apparent viscosity below abnormal point.And the modification effects become greater as the NPPD dosage increases and the treatment temperature increases.For instance,compared with crude oil free of NPPD,the gelation temperature reduction,viscosity reduction rate and yield stress reduction rate at 8?of waxy crude oil dopped with 100ppm NPPD and treated at 70?are 13.0?,91%and 92%,respectively.Then,the wax deposition experiments were carried out with waxy crude oils of different modification effects.The formation rate and compoisiton of deposits were analyzed.It was found that the wax deposition rate decreases but wax content increases as the modification effects become greater,which were caused by the reduction gelation temperature and apparent viscosity of wax curde oils treated with NPPD.What's more,the aging process of wax deposits is accelerated in the presence of NPPD.The heavy n-paraffin components are more enriched while the light n-paraffin components are less enriched.At same time,the CCN of deposits increases from C₂₃ to C₂₆.These phenomena aggregate as the modification effect becomes greater.Furthermore,the content of asphaltenes and resins entrained in the deposits generated with waxy crude oils treated with NPPD were measured.It was presented that the asphaltene content and resin content both increase as deposition time elapse.At the deposition time of 24 h,the asphaltene content of the deposits was found to be nearly 2times that of the original oils,while the resin content nearly 1.5 times.The enrichment of asphaltenes and resins into waxy deposits was limited as the modification effect becomes greater,which was probably because the porosity of the deposits was decreased or the wax content of the deposits was increased caused by the NPPD.Additionly,the yield stresses of the deposits were tested.It was found that the yield stress of the deposits significantly decreases in the presence of NPPD and the depression rate of the deposit increases as the yield stress of waxy crude oil decreases.For examples,compared with the crude oil deposit(12 h)without NPPD,the yield stress of wax deposit with dosage of 100 ppm(treatment temperature 70?)decreased by 91%Eventually,the heterogeneous structure of waxy deposits was analysed by macroscopic observation and DSC.DSC tests show that the wax content and WAT increase from the outside to the inside of the deposit.The surface deposits are bulk condensate,and the aging effect is not obvious as deposition time elapses.NPPD accelerated the aging process of inner wax deposits.According to the above conclusion,the rheological properties of crude oil dopped with PPD are the decisive factors to determine the wax deposition characteristics:the decrease of the gelation temperature and apparent viscosity of the crude oil,decrease the deposition rate of crude oil wax significantly,and accelerate the aging process.With the decrease of yield stress of crude oil,the structural strength of wax deposits decrease significantly.