Predicting the Wax Deposition and Gelation Behavior of Waxy Oils in Subsea Pipelines through Modelling and Numerical Simulation Approach
Authors: Sharma Rohit , Vuthaluru Hari , Mahto Vikas 
Published: November 2020

Abstract
Wax deposition during pipeline transportation of crude oil is one of the serious flow assurance problems faced in the petroleum industry (Deka et al., 2020b) and wax gelation phenomenon is an inherent characteristic of the wax deposition process and it must be investigated to understand the deposition process. Wax-gelation formation was simulated during flow of waxy oils by numerically simulating its gelling behavior during transportation under different operating conditions by developing a computational fluid dynamics (CFD) model. The modelling technique and the obtained results from this work would help to predict the wax deposition and gel formation tendency in different waxy oils. Wax deposition phenomenon was modelled for a model wax-oil mixture (representing a waxy oil). A numerical model was developed to predict the wax gelation with time and length of pipeline. It involves heat and mass transfer calculations and molecular diffusion mechanism is considered as the primary mechanism for wax deposition, with the calculations being performed for 5 days. The CFD model uses enthalpy porosity technique where wax-oil gel is treated as the solid-liquid region with porosity equal to the liquid fraction. Fluid flow was considered laminar with flow rates being 1-5 GPM and ambient temperature was maintained at 281.3 K. The developed numerical model predicts that the wax gelation increases with time initially and then slowed down subsequently and less increment is observed upon 5 days of fluid flow. These results indicate the wax gelation is fast initially and slows down subsequently, showing strong dependence on time. Wall temperature of wax-oil mixture and the gel deposit decreases from bulk fluid temperature (295.2 K) to the ambient temperature (281.3 K). Increase in the temperature difference between wall and ambient temperature leads to higher wax deposition and gelling formation. Wax gelation was observed to be function of time. The CFD model numerically-simulates the wax-oil gelling formation in terms of liquid fraction (gel formation), where the liquid fraction gets reduced up to 44.7 % upon crude oil flow in the pipeline, being highest at the centre and lowest at pipe-walls, indicating the possible gel formation regions on pipewall. The gel formation was strongly affected by the wall temperatures, duration and length. This work would be one of its kind to report CFD-simulations of wax-oil gel formation in pipelines, where interdependency of fluid flow parameters would be assessed. Therefore, in this work, an attempt was made to simulate the gelling behaviour of the wax crystals in pipeline using an enthalpy porosity technique which has been not been tested much before. Computational fluid dynamics is used in this work to simulate the wax-gelling phenomenon and the wax gelation is observed through observing the changes in the liquid fractions of the wax-oil mixture used in this work. The current work would provide a comprehensive understanding about the wax deposition and gelling phenomenon in waxy oils and would help to design waxy-oil pipelines with appropriate measures, especially in subsea conditions.