Relationship between Waxy Crude Oil Composition and Change in the Morphology and Structure of Wax Crystals Induced by Pour-Point-Depressant Beneficiation

The morphology and structure of wax crystals is one of the most important factors influencing flow properties of waxy crude oils. The pour-point depressant (PPD) improves flow properties of waxy crude oil by modifying the morphology and structure of wax crystals, and this modification, i.e., the effectiveness of PPD, is greatly affected by the oil composition. There have been some investigations into how the oil composition affects the effectiveness of PPD. However, these studies were basically qualitative. The objective of this study is to quantitatively reveal the relationship between the change induced by PPD beneficiation in the morphology and structure of wax crystals and the oil composition. PPD treatment was made on eight waxy crudes, and microscopic observations were performed on the morphology and structure of wax crystals before and after PPD beneficiation. The fractal dimension of the wax crystal microstructure, which was used to characterize the morphology and structure of wax crystals, was determined on the basis of the microscopic images of wax crystals. A total of 11 parameters of oil composition, which are thought to have influence on the effectiveness of PPD and the morphology and structure of wax crystals, were measured. Among them, 5 were determined to be the representative parameters of oil composition using the cluster analysis, i.e., the concentration of precipitated wax at the studied temperature, the average carbon number of wax, the viscosity of the liquid continuous phase at 60 °C, the wax content, and the content of resins and asphaltenes. Further, a correlation was developed using the multiple regression analysis between the fractal dimension change induced by PPD treatment and three representative parameters of oil composition, i.e., the concentration of precipitated wax at the studied temperature, the wax content, and the content of resins and asphaltenes. This helps to quantitatively understand the relationship between the oil composition and microstructure of wax crystals

Effect of Carbon Number Distribution of Wax on the Yield Stress of Waxy Oil Gels

Wax deposition is one of the most important problems in flow assurance of petroleum pipelines. Pigs are commonly employed for the removal of wax deposit (actually a wax–oil gel consisting of liquid oil and solid wax particles) on the pipe wall. Understanding of the wax deposit strength, which may be a function of the carbon number distribution of wax, assists in preventing the pig from getting stuck in the pipeline. This study focuses on the effect of the carbon number distribution of wax on the yield stress of waxy oil gels. Waxes with different carbon number distribution were dissolved into a crude oil to prepare the model oils. The vane method was used to determine the yield stress of waxy oil gels formed under quiescent or shear conditions, in which an applied shear stress was maintained during the process of cooling and isothermal holding. The results showed that the yield stresses dramatically decrease with increase of average carbon number of wax regardless of the quiescent or shear conditions. However, the applied shear stress has little effect on the yield stress of the wax–oil gel 12.5% W1 + oil-A and no effect on the yield stresses of 12.5% W2 + oil-A and 12.5% W3 + oil-A. Under quiescent conditions, the changing rate of the yield stress with respect to the solid wax content reduces as the average carbon number of wax increases. The morphology and structure of the wax crystals were also observed using optical microscopy. Microscopic observation indicated that the average size and boundary fractal dimension of the wax crystals decrease but the aspect ratio increases with the increase of the average carbon number of wax

Thermal, Macroscopic, and Microscopic Characteristics of Wax Deposits in Field Pipelines

Wax deposition is an important issue in crude oil transportation. Understanding the deposit properties assists in making a suitable schedule for removing wax deposits. However, there was little work published on the nature of wax deposits in field pipelines. This study focuses on the thermal, macroscopic, and microscopic characteristics of wax deposits obtained from the field pipelines. First, differential scanning calorimetry (DSC) was used to investigate the thermal properties of wax deposits and crude oils. The results showed that the wax appearance temperature (WAT) and wax content of wax deposits is much higher than that of crude oils, and both of two parameters increase with the increasing radial distances. For all deposits, the precipitated wax concentration increases significantly with the decreasing temperature near the WAT, while it almost increases linearly when the temperature decreases about 25 °C below the WAT. Based on DSC results, a correlation between the solid wax content and wax content and temperature was developed. Verification experiments showed that this correlation calculates the solid wax content with the absolute deviation within ±4% and the average relative deviation of 3.8%. Second, the macroscopic structure of wax deposits was visually observed. The yield stresses were also determined using the vane method. The results showed that the loose structure of the original deposits could result in small values of yield stresses, and the yield stresses increase as the radial distances increase. Third, the optical microscopy was used to observe the microscopic characteristics of wax crystals. The average size, aspect ratio, and boundary fractal dimension of wax crystals in deposits vary in the ranges 9.1–14.3 μm², 1.55–1.77, and 1.13–1.20, respectively. The average size and boundary fractal dimension of wax crystals in deposits are higher than that of the crystals in crude oils

Modeling the Thixotropic Behavior of Waxy Crude

Waxy crude shows viscoelastic, yielding, and thixotropic behaviors below the gelation point. In this paper, to depict those complex rheological behaviors a new thixotropic model is proposed. In the model, the total shear stress is considered to be a combination of an elastic stress and a viscous stress. The elastic stress, assumed to be structure- and strain-dependent, is described by a nonlinear damping function together with a structural parameter. The evolution of structural parameter is described by a new kinetic equation, whose breakdown term is assumed to be dependent on energy dissipation rate rather than on shear rate. The proposed model is validated respectively by the test of stepwise increases in shear rate and the hysteresis loop test for waxy crude and further validated by predicting the transient flow curve of hysteresis loop test with the model parameters obtained from the test of stepwise increases in shear rate. The results showed that the model’s capability of fitting experimental data and prediction is satisfactory, and the model nicely predicts the transition from an “elastically” dominated region to a “viscous” dominated region without a discontinuity in the stress–strain curve

Experimental Study on the Strength of Original Samples of Wax Deposits from Pipelines in the Field

The yield stress of the wax deposit, characterizing its mechanical strength, provides critical design basis for pigging. The deposits naturally formed in a pipeline (hereafter, “natural wax deposits”) and those artificially generated from model wax–oil mixtures (hereafter, “model wax deposits”) usually present different yield stress due to structural variations. Investigations on the distinctive yielding characteristics between natural and model deposits are limited in the literature. In this research, we present a comprehensive comparative mechanical and structural analysis of natural and model wax deposits, based on which representative laboratory tests can be designed to guide pigging operations. A rheometer with the vane geometry was enhanced to preserve the microstructure of the deposit sample collected from the field prior to the yielding test. Field wax deposits from different radial positions of the pipe were analyzed. It was discovered that the yield stress of the natural wax deposits increases exponentially with solid paraffin content. Moreover, the deposit layer closer to the center has lower solid paraffin content and lower resulting yield stress than the layer in the vicinity of the inner pipe wall. The original sample of natural wax deposits (called “original sample” for short following) was heated until completely melted and cooled again for the reformed solid sample similar to the model wax deposits in common use. The tested yield stress for the newly formed deposits can be 5–13 times that of the original sample at the same temperature due to the compact microstructure. Consequently, the required pressure to remove the wax deposits in the pipeline could be relatively high estimated based on the yield stress of model wax deposits. On the other hand, the natural wax deposits and model wax deposits formed on the coldfinger or in the flow loop are more alike in structure. So model deposits obtained in these ways should be used in the studies relative to pig motion, rather than the wax–oil gel which is currently very popular

Study on the Effect of Dispersed and Aggregated Asphaltene on Wax Crystallization, Gelation, and Flow Behavior of Crude Oil

Asphaltene can exist in both the dispersed state and the aggregated state in crude oil. Because of the changes in crude oil composition, pressure, or temperature, the asphaltene transition from dispersed asphaltene to aggregated asphaltene will occur and then influence the wax crystallization, gelation, and flow behavior of crude oil. In this paper, the asphaltene transition was realized by mixing two different crude oils for different times. The aggregated asphaltene was characterized by the optical microscopy and centrifugation-based separation method. The effects of asphaltene transition on wax crystallization, gelation, and flow behavior of crude oil were investigated by differential scanning calorimetry and rheological measurements. The results show that the aggregated asphaltene can serve as a crystal nucleus for wax molecules, promoting the wax precipitation, weakening the strength of the network of wax crystals, and delaying the gelation process of crude oil. On the other hand, the dispersed asphaltene can serve as the connecting point between wax crystals, accelerating the gelation of crude oil, and increasing the gel strength. The viscosity measurements below the wax appearance temperature show that the viscosity of crude oil increases because of the interaction between aggregated asphaltene and wax