Detailed Modelling of Displacement and Hot Oiling of a Waxy Non-Newtonian Fluid in a Mature West Africa Deep Water Production Facility

Abstract

Abstract This study investigates the feasibility of a changeover of displacement and hot oiling fluid from diesel to dead crude in a mature West Africa deep-water production facility from a flow assurance perspective. It addresses the practical challenges associated with the dead crude displacement and hot oiling procedures as well as the technical challenges of transient modelling of a waxy non-Newtonian fluid through both insulated and uninsulated piping. Modelling was performed using commercial software packages to simulate the existing diesel displacement and hot oiling procedures and to tune the model of the system to data from the field. The non-Newtonian behavior and wax precipitation and deposition properties of the dead crude were then characterized and tuned based on laboratory rheology and flowloop testing. Cases of dead crude treated with different levels of wax inhibitor were also considered. Multiple cycles of displacement and hot oiling were then modelled to evaluate the wax buildup over time and the ability of topside pumps to provide sufficient flowrate to displace production fluid. Modelling in commercial software packages was performed in parallel with modelling of the system using correlations found in literature. The modelling of uninhibited dead crude showed strong non-Newtonian behavior when under flow through pipe without insulation as the fluid rapidly cooled below the wax appearance temperature. The uninhibited fluid also showed gelling behavior when cooling down from hot oiling, and was therefore not considered a feasible option. Crude dosed with a wax inhibitor showed a reduced pour point and viscosity, which also reduced gelling effects. The fluid was able to be circulated by the pumps at a sufficient flowrate to remove production fluid from the pipe but also exhibited significant wax deposition due to rapid temperature changes when entering uninsulated pipe. The resulting wax deposit poses the risk of interfering with pipe integrity assessment. Further work is now ongoing to identify more effective wax inhibitors and alternative methods of assessing pipeline integrity. This study offers further insights into wax deposition and non-Newtonian fluid modelling in both insulated and uninsulated flowlines as well as the effectiveness of modelling non-Newtonian frictional pressure drop and wax deposition in parallel with commercial software packages to develop more effective models.