Chemical Modification of Paraffin Wax-Oil Gels
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Paraffin wax crystallized in petroleum production line is well known in causing flow assurance challenges. Wax crystallization behavior is influenced by its interaction with other components co-exist in petroleum fluids. Synthetic wax inhibitors or pour point depressants (PPD) are commonly added to prevent problems related to wax formation. Microcrystalline wax, one of natural components in petroleum, demonstrates overall beneficial effects on rheological properties of waxy model oil containing macrocrystalline wax, the main component causing wax related problems. The strength of formed waxy oil gel is substantially reduced by the microcrystalline wax addition. Modification in wax crystal morphology from long platelet to be more spherical has provided an explanation on the significant yield stress reduction. Although beneficial effects of PPD in on rheological properties of waxy oils are proven, the underlying phenomena and mechanisms remain a subject of debates. An effective PPD changes n-alkane crystal morphology from thin plate to dendrite-like or branched needle. The crystal growth mechanism leading to altered wax crystal morphology involving wax-PPD interactions is described sequentially. Investigation on thermodynamic phase partitioning behavior provides qualitative and quantitative evidences of wax-PPD interactions. It is indicated that PPD may enhance wax solubilization effect at temperature closely below the wax crystallization temperature. At below polymer cloud points, polydisperse polymers split into solid precipitate fraction comprising of high MW polymer components and soluble fraction comprising of lower MW components. In presence of precipitated solid wax, the compositional split shifts as such the concentration of PPD polymer in the liquid phase is reduced compared to that of in the solutions without wax. Some amount of polymer interact and co-precipitate with the solid wax. Selectivity of PPD molecular weight is studied against paraffin waxes that crystallize at different temperature conditions. High MW polymer components that precipitate out prior to wax crystallization do not contribute to wax inhibition. On the other hand, low MW polymer components partition preferentially to the liquid phase and thereby exhibit low treatment efficacy. Hence, an optimal polymer MW exists for maximum fluid treatment at isothermal crystallization conditions without polymer loss to solid phase polymer precipitates and without polymer loss to the liquid phase. These results imply that PPD tailoring practices should utilize polymers that are more monodisperse in nature and optimized for the targeted wax compositions as well as the range of wax crystallization temperatures.