To assess the relationship of wax chemical structure and viscoelastic properties of waxy gels, model oils composed of single and blended waxes were prepared at a fixed concentration of 7.5 wt % and then gelled. The investigation encompassed four different well-characterized commercial paraffin waxes, solubilized in a mineral oil matrix. Previous rheological measurements pointed out that these systems reproduce essential features of crude oil gels (e.g., gel-like mechanical response) when gelled. Among the employed waxes, two are predominantly linear, whereas the others are nonlinear branched molecules. The waxes were molecularly characterized to aid in the investigation by means of GC-FID, 13 C NMR, DSC, FT-IR, and XRD. Rheological properties were measured via a controlled-stress rheometer by oscillatory shear experiments. Polarized light microscopy was adopted for morphological characterization of precipitated crystals. It was found that yield stress and elastic modulus at linear viscoelastic region are highly correlated (R 2 = 0.94). For single wax systems, the increase in chain length resulted in a yield stress increase, although there is a competitive effect among chain length (positive effect) and branching content (negative effect). The results indicated that for blended systems the small-chain linear wax was able to interact favorably with the long-chain nonlinear wax, possibly due to its ability to accommodate within the later molecule, ensuring the highest yield stress value (630.2 Pa). The wax structural arrangement of 37 carbon atoms on average, including approximately three tertiary carbons, was effective for lowering the yield stress of particular blended systems. The lowest viscoelastic properties were measured for a blended system composed of nonlinear waxes, which was also characterized by the smallest and rounded crystals visible by optical microscopy.

The influence of six experimental variables (cooling rate, weight fraction of linear wax, weight fraction of branched wax, shear rate during cooling, final cooling temperature, and aging time) on the yield stress of gelled waxy oils was simultaneously investigated using an experimental design. Rheological tests were performed with systems comprising spindle oil and a blend of paraffin waxes. The waxes were characterized by differential scanning calorimetry, gas chromatography–flame ionization detection, Fourier transform infrared and 13C NMR spectroscopy, and X-ray diffraction, pointing to predominantly linear- and branched-type wax. Also, wax crystal morphology was assessed through polarized light microscopy. Cooling rate, wax content, shear rate during cooling, final cooling temperature, and aging time were the variables analyzed. The obtained results disclosed the relative importance of these variables on the viscoelastic properties of the gelled oils. Multiple linear regression was used to propose a mathematical expression for yield stress estimation. It was also found that the storage modulus in the linear region was highly correlated with the yield stress; thus, an alternative method to estimate the yield stress was proposed. The wax network restructuring level for different aging times are also discussed, and evidence indicates that after fracture the gel is unable to rebuild its original structure. The presented information may be useful for start-up prediction and/or the feasibility of a remediation procedure aimed at helping the removal of wax plugs.

The observation of lamellar liquid crystals (LCs) is reported in fractions of Brazilian crude oil emulsions. Through polarized light microscopy, several structures exhibiting Maltese cross-optical pattern were observed, mostly in aqueous-rich fractions (bottom fractions). Microscopy observations were carried out immediately after emulsion preparation and after a destabilization protocol by centrifugation. Four oils were used: two waxy crude oils and two non-paraffinic oils. The experimental variables investigated were oil composition, aqueous content, and saline solution concentration. The liquid crystalline structures had a similar pattern for all samples observed with a thickness of 1.77 ± 0.14 μm surrounding droplets of 4.57 ± 0.47 μm diameter. Neither the presence of wax nor NaCl (up to 35 g/L) exhibited any correlation with liquid crystal (LC) appearance. However, the increase of the aqueous phase added to the systems favoring LC formation.

The effects of apparent wall slip on rheometric measurements of waxy gels are quantified for gels consisting of a macrocrystalline wax added in mineral oil in concentrations of 3.0 and 7.5 wt. %. The waxy gels are formed in situ in a stress-controlled rheometer, and rheological properties are then obtained. Seven different geometry configurations, including parallel plates, concentric cylinders, and vane, are employed. The surface roughness of the different geometries and wax crystals size are assessed to provide insight into the apparent wall slip phenomenon. In the presence of smooth surfaces, a decrease in approximately 80% in the measured yield stress value is observed in oscillatory tests, regardless of system composition. In addition, the storage modulus measurements are substantially different when obtained from smooth or grooved geometries. In creep experiments, the yield stress measurements decrease by 68%. As expected, apparent wall slip markedly affects the shape of the flow curves obtained, causing the appearance of kinks for smooth Couette geometry. Apparent slip velocities are calculated in the regime just above the yield stress, and it is found that for low concentrations apparent slip increases with concentration and decreases with the ratio between the shear stress and dynamic yield stress. The gathered results also demonstrate that the reproducibility of rheological measurements is improved by using geometries with grooved parts. By quantifying the effects of apparent wall slip on rheological measurements of waxy gels for different geometries, this work can provide useful information for the design of pipelines and oil transportation processes.

The prediction of yield stress for waxy crude oil emulsions and the understanding of oil gelification process play a major role in flow assurance. Oil characterization and its wax content can also provide useful data, specially for engineers engaged in production procedures at offshore drilling plataforms. Since petroleum is a complex mixture of many chemical compounds, a systematic study involving model oil is suitable to account for specific influences on yield stress of systems under gelling conditions. The present work aims to study the behavior of systems at gelation conditions. Therefore, waxy crude oil emulsions and model oil systems were investigated. Rheological and thermal analysis were employed for yield stress measurments and estimations of wax amount precipitated, respectively. Characterizations by DSC, C 13 RMN, SARA and GC-FID were also carried out. It was verified that cooling was the most import step on yield stress appearance whilst shear rate had the biggest impact on gelification for both systems. The chemical nature of parrafins employed (branched or linear) also played an important role on yield stress values. From the results, it could be shown that aging times larger than 15 minutes did not affect yield stress appreciably.