Modelling the wax deposition process requires considerations of several disciplines ranging from the thermodynamics of fluids and waxes, the thermo-hydraulics of fluid flow to the kinetics of the deposition process. In this work, we investigate how different fluid flow and thermodynamic wax models may affect the predictions of wax deposition rate and pigging frequency. The fluid flow models considered include the steady-state multiphase OLGAS® and Beggs-Brill model. The thermodynamic wax models of Coutinho and an ideal mixing approach will be compared. Real field cases are considered to assess the impact of different fluid flow and thermodynamic wax models on wax deposition and pigging operation.
The phenomenon of wax precipitation and deposition occurs when transporting waxy crude oils via subsea pipelines or in a cold environment onshore. The presence of wax deposits as a result of wax deposition in a pipeline can lead to significant reduction of production and increasing operational cost. The major driving force of such issues is the fluid temperature and the temperature gradient between the bulk fluid and the surrounding of the transporting pipelines. When the temperature of a waxy crude drops below its wax solubility limit, the Wax Appearance Temperature (WAT), the waxes start to precipitate and form a deposit on the pipe walls if a radial heat flux towards the surroundings exists. To remediate and prevent such flow assurance problems, chemicals such as wax inhibitors, dispersants or wax crystal modifiers are commonly used by the industry, together with mechanical techniques, dewaxing pigging, to remove the wax deposits.