Wax Deposition Of Waxy Crude Oil And Water-in-Oil Emulsion

In recent years,multiphase transportation technology has been widely used in the production of offshore oil reservoirs.The wax deposition problem in multiphase pipeline is believed to have posed a serious threat to production safety.In this paper,the wax deposition of crude oil and water-in-oil emulsion is studied experimentally to reveal the variations of the thickness and composition of wax deposit layer under laminar flow condition.Based on these aging characteristics of wax deposit,a dynamic wax deposition model is developed,which can be used to predict the thickness of deposit layer and its wax content profile along the radial direction.First of all,the wax deposition experiments of crude oil are conducted through a cold finger apparatus,and the change of temperature field in these experiments is simulated by numerical software.The influences of oil temperature,time and stirring speed on wax deposition are investigated.Besides,the wax diffusion coefficients under experimental conditions are calculated according to Fick's diffusion law,which are much higher than the values obtained from Hayduk-Minhas correlation and Wilke-Chang correlation.Considering that these correlations were derived from binary solution system,their applicability in crude oil system is poor.Thereby,Hayduk-Minhas correlation is improved by correcting its correlation coefficient,temperature term and viscosity term with the calculation results of Fick's diffusion law.Sequentially,a more applicable correlation of wax diffusion coefficient for crude oil is proposed.Furthermore,the effects of different factors,including time,flow rate,oil temperature and pipe wall temperature,on the wax deposition of crude oil are studied on a flow loop apparatus.The characteristics of wax deposits at different radial positions are analyzed via DSC,GC and microscopy observation.It is found that CCN+ dominates the bottom layer while the content of CCN-is much higher in the surface layer.The variation rates of CCN+content in the bottom layer and CCN-content in the surface layer drop with time.The concept of deposit solidification is put forward after the representation of the wax content variation in deposit.The length-diameter ratios of wax crystals at different radial positions fluctuate slightly around the average value of 1.64,but the irregularity of wax crystal distribution increases from the surface layer to the bottom layer.Also,the yield stress of wax deposit,from 5310 Pa to 24500 Pa,is closely related with its wax content.Meanwhile,the yield stress will grow by increasing time,flow rate and oil temperature.In addition,the droplet size and distribution of the emulsion after different running time in flow loop are represented by number average diameter,specific surface area and polydispersity.The gel point and viscosity of emulsions are measured and it is found that the variation trend of these rheological properties is the same with the droplet size and distribution.Besides,with the growth of either emulsion temperature or flow rate,the deposit mass will first increase and then decrease.The critical shear stress of wax deposit shows an increasing trend with the growth of water content.The wax content of deposit containing water is able to be determined with the correspondence between wax content and wax appearance temperature in DSC result.The water phase in wax deposit is mainly distributed in its surface gel layer,and the water contents in deposit are below 2%,7% and 14% for emulsions with water content of 10%,20% and 30%,respectively.Finally,a dynamic wax deposition model is developed based on the mechanisms of molecular diffusion,aging and shear stripping.It is deemed that whether shear stripping is able to effect wax deposition depends on the structure strength of wax deposit.This model is capable of predicting the thickness of the wax deposit layer in water-in-oil emulsion and the variation of wax content inside the deposit with time and radial position.