Paraffin deposition is a severe challenge facing the oil and gas industry.Wax deposition risks in oil production are usually characterized by the wax deposition rate, wax deposit thickness and hardness.Research efforts have been dedicated to establishing modeling methodologies to predict the severity of wax deposition during oil production.In this thesis, wax deposition modeling investigations were carried out in single-phase oil flow as well as water-oil two-phase flow.Firstly, the existing ;;single-component” wax deposition model was enhanced to simulate the precipitation and deposition of paraffin molecules covering a wide range of carbon numbers.The enhanced model predicts the carbon number distribution of the wax deposit in addition to the deposit thickness.For a waxy crude oil with a paraffin carbon number distribution covering C15 to C45, the enhanced model can predict the carbon number of the most abundant component in the deposit within an error of 2 carbon numbers.The composition of the deposit provides insights to the hardness of the deposit as well as its responsiveness to polymeric inhibitors. Modeling methodologies were also established for wax deposition process in water-oil dispersed flow by analyzing the roles of the dispersed water droplets on the heat and mass transfer aspects of wax deposition.NMR techniques were implemented to uncover and quantify the hindrance to wax molecular diffusion caused by dispersed water droplets.It was discovered that the confusion about the wax diffusivity in water-oil dispersion in the industry can lead to underestimation of the wax diffusivity by as much as 300%.Simulations also revealed the different impacts of the dispersed water phase on heat transfer in a laboratory set-up and in field operations.Dispersed water droplets can also be entrapped in the deposit.It was discovered experimentally that the water content of the deposit depends not only on the water content of the bulk but also the size of the droplets.Incorporation of the droplets in the wax deposit lowers the yield strength of the deposit and causes the deposit to slough-off frequently during wax deposition.In a laboratory flow loop test, the pressure drop across the test section increased by more than 10 psi during a single-phase wax deposition experiment.However, the pressure drop did not increase by more than 5 psi due to frequent sloughing of the deposit when only 10 vol.% of water was dispersed in the oil phase.In contrary to the complex effects of the dispersed water phase on the heat and mass transfer characteristics of wax deposition from water-oil dispersed flow, the water phase has minimal impact on heat and mass transfer in water-oil stratified flow regime, as was discovered by wax deposition experiments.Eventually, the long-existing simplified wax deposition model assuming Newtonian fluid mechanics was enhanced to consider the non-Newtonian fluid characteristics.A comprehensive workflow was established to perform hydrodynamics, heat and mass transfer as well as deposit formation simulations based on fundamental analysis with advanced Computational Fluid Dynamics.It was discovered that for a highly waxy (>15 wt.% wax) and thus highly non-Newtonian crude oil, the wax deposit thickness predicted by the Newtonian model can be 1/3 that of the thickness predicted by the non-Newtonian model.Therefore, misuse of the Newtonian wax deposition model for highly waxy oils can lead to over-optimistic assessments of the wax risk.
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Wax Deposition from Single-Phase Oil Flows and Water-Oil Two-Phase Flows in Oil Transportation Pipelines
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