Yansong Zhao

Polymeric Wax Inhibitors and Pour Point Depressants for Waxy Crude Oils: A Critical Review

Paraffin precipitation during pipeline transport of waxy crude oils gives rise to several challenges, including wax deposition, flow reduction, and gel formation, which adversely impacts pipeline performance. Small dosages of polymeric wax inhibitors or pour point depressants comprise an effective preventative measure. In this article, the structural character, functionality, and mechanisms of the polymeric additives targeting paraffin wax are reviewed, and factors influencing product efficacy are summarized. Most polymeric additives contain a nonpolar moiety as well as a polar moiety, with the exception of crystalline-amorphous copolymers. Via nucleation, adsorption, co-crystallization and solubilization interactions, polymeric additives alter the morphology and interface of precipitated wax crystals, inhibiting wax deposition, improving flow, and impeding gel formation. The presence of asphaltenes significantly impacts wax crystal morphology and interfaces, thus influencing the mechanism of polymeric additives. Most polymeric additives fall into the categories of crystalline-amorphous copolymers, ethylene-vinyl acetate copolymers, comb polymers and nanohybrids. Factors influencing polymeric efficacy include molecular structure, fluid composition, and pipeline transport conditions.

Utilization of DSC, NIR, and NMR for wax appearance temperature and chemical additive performance characterization

Wax crystallization processes are investigated using differential scanning calorimetry, near-infrared spectroscopy, and nuclear magnetic resonance spectroscopy. The performance of a chemical additive is assessed using calorimetry and NMR. Heat flows of model waxy oils are obtained using differential scanning calorimetry, providing the wax appearance temperature and crystallization profiles. The effect of cooling rate, wax content, asphaltene, and chemical additive on the wax appearance temperature is investigated. The wax appearance temperature increases with increasing wax contents. The wax appearance temperature decreases in the presence of chemical additive, effectively increasing the instantaneous supersaturation. Furthermore, near-infrared spectroscopy and nuclear magnetic resonance spectroscopy are utilized to determine wax appearance temperature. The NMR experiments quantify liquid and solid fractions at thermal equilibrium conditions, effectively circumventing the need for dynamic thermal techniques.

Strain-Dependent Rheological Model and Pressure Wave Prediction for Shut in and Restart of Waxy Oil Pipelines

Waxy oil gelation and rheology is investigated and modeled using strain-dependent viscosity correlations. Rotational rheometry shows a sharp viscosity increase upon gel formation. High creeping flow viscosities are observed at small deformation conditions prior to yielding. A new strain-dependent rheological model, following analogous formulation to the Carreau–Yasuda shear rate-dependent model, captures viscosity reduction associated with yielding. In addition, shear viscosity and extensional viscosity are investigated using a capillary rheometry method. Distinct shear-thinning behavior is observed in the shear mode of deformation, while distinct tension-thinning behavior is observed in the extensional mode of deformation for the model fluid systems. High Trouton ratios are obtained for the gelled model fluid systems, confirming strongly non-Newtonian fluid rheology. Finally, axial pressure wave profiles are computed at real pipeline dimensions for idealized moderate yield stress fluids using a computationally efficient 1D pipeline simulator. The Rønningsen time-dependent gel degradation model is used to emulate the fluid rheology in the simulator. Axial stress localization phenomena are shown to depend on the overall magnitude of gel degradation as established by the reduction in yield value. A high degree of gel degradation serves to afford flow commencement in a timely manner.