Numerical study of gelation and flow restart in a waxy crude oil riser

Abstract

Precipitation of paraffin wax during crude oil transportation in low ambient temperature presents many technological challenges in petroleum industries. During a complete shutdown, the precipitation intensifies to form a gel-like structure which usually demands high restart pressure during flow restart operation. Numerical simulations are performed to investigate the gelation process in the shutdown pipeline and subsequent non-isothermal flow start-up operation in a heterogeneously gelled pipeline. A Finite volume method is used to solve the mass, momentum and energy conservation-based governing equations over an orthogonal static grid setup. The gelation model utilizes a modified Eyring-based viscosity model to account for temperature-dependent changes in gel strength. On the other hand, the flow restart process follows the deformation and temperature-dependent shear-thinning based rheological model. The pipeline under consideration in the present work is vertical, replicating flow in risers. Our gelation model considers natural convection-driven flow inside the pipeline, which incites the hot oil in the core to mix with cooler fluid at the wall. The gelation is achieved through cooling the static crude oil in the pipeline for different timespans varying from 3 h to 15 h. The gelation stage ends up producing spatial heterogeneity in gel strength. Such heterogeneity leads to weaker gel structure in the core causing early flow restart due to cohesive failure of the gel medium at some interior radial location. Hence, it can be concluded that depending on the cooling stage, the heterogeneous gel with a weaker gel in the pipe core can restart faster than when the pipeline is filled with the gel having an equivalent average temperature. The findings provide realistic insights into the mechanism of gel failure in the pipeline, and the knowledge can be utilized to develop a better flow assurance strategy.