(65a) Investigation on the Effect of Water Droplets on the Diffusion of Wax Molecules in Water-in-Oil Dispersed Flow

Investigation on the Effect of Water Droplets on the Diffusion of Wax Molecules in Water-in-Oil Dispersed Flow

Sheng Zheng and H. Scott Fogler

Water-in-oil dispersed flow in subsea pipelines is increasingly common in transportation of deepwater petroleum fluids.  When the oil phase is the continuous phase in water-in-oil dispersion, dissolved wax molecules can diffuse towards wall through the oil phase to form wax deposit.[1] It was previously found that wax deposition from water-in-oil dispersed flow occurs at a lower rate of wax deposition when compared to wax deposition from single-phase flow.[2]  The reduction in the wax deposition rate could be due to the fact that water droplets can impede the diffusion path of wax molecules because wax molecules are not soluble in water and therefore cannot penetrate water droplets and hence must diffuse around them.  Although considerable research on wax deposition in water-in-oil dispersed flow has been carried out, the spatial effect of water droplets on diffusion of wax molecules has not been fully investigated.  In fact, most of the previous research focuses mainly on experimentation and correlation instead of the fundamentals of transport phenomena.[3]-[4][5]  Therefore, existing models for wax deposition from water-in-oil flow are highly empirical and lack detailed description of fundamental physics:  Bruno et al. modeled wax deposition in dispersed flows using the diffusivity of wax in single-phase flow while neglecting the effect of water droplets.³  Mandar et al. tuned the diffusivity of wax in the presence of water droplets in order to match experimentally measured deposition rate.⁴  Couto et al. treated water-in-oil dispersed flow as a pseudo-single phase flow and estimated the diffusivity of wax using averaged fluid properties.⁵ Panacharoensawad used the oil volume fraction as a tuning multiplier on the wax diffusivity to account for the reduction of available diffusion space due to the existence of the water droplets.[6] Edwards and Davis developed an equation to relate the diffusivity of molecules in dispersed flow to the volume fraction of the dispersed phase.[7]   However, in the equation by Edwards and Davis, the effect of droplet size is not accounted for, which can significantly affect the mass transfer resistance caused by droplets.

In this project, we will measure the diffusivity of wax in water-in-oil emulsion by PFG-NMR techniques and compare the measured diffusivity with values predicted by Monte Carlo simulation.