Reynolds Number Is Important in Understanding Wax Deposition
Authors: Carpenter Chris 
Published: October 2018

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 28053, “Reynolds Number Influence on Wax Deposition,” by R.C. Albagli, Petrobras, and L.B. Souza and A.O. Nieckele, Pontifical Catholic University of Rio de Janeiro, prepared for the 2017 Offshore Technology Conference Brasil, Rio de Janeiro, 24–26 October. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission.

Comprehension of the mechanisms that influence wax deposition in oil-production systems has not yet been achieved fully. Given the absence of a theory able to explain the evolution and characteristics of these deposits, the resulting production limitation is one of the main issues in flow assurance. This paper investigates the influence of the Reynolds number on wax deposition.

Introduction

Several numerical models and experiments have been conducted to understand wax deposition in flowlines. A typical experiment consists of injecting a heated solution (wax dissolved in a solvent) through a test section with one of its surfaces kept at a colder temperature than the wax appearance temperature (WAT). As the fluid temperature is reduced below the WAT, the first wax crystals appear, initiating deposit growth.

Typically, oil flow in production lines is turbulent. As the Reynolds number increases, deposit thickness decreases. Experiments have shown that composition changes with time, with the increase of shear stress (higher Reynolds numbers) resulting in deposits with higher percentages of solids.

In this work, an enthalpy/porosity model was coupled with a kinetic energy/specific dissipation (kappa/Ω) turbulence model to allow an investigation of this flow regime. The outer pipe is maintained at a constant high temperature equal to the inlet temperature, and the inner pipe is cooled.

Mathematical Model

The enthalpy/porosity model was based on applying conservation equations in the liquid region of the domain, identified by its volume liquid fraction, and using a Darcy term at the linear momentum equation to account for the solid interactions in the porous region. To take turbulence into account, the Reynolds average/Navier-Stokes model was applied to determine the time average value of all relevant quantities. The fluctuation in the average flow was modeled on the basis of the Boussinesq approximation, and the kappa/Ω model was used because it usually presents good results for low-Reynolds-number flow. In addition to these characteristics, the kappa/Ω model does not need a wall function, which is convenient in this case because the liquid/deposit interface moved as time progressed. The equations necessary for building the mathematical model, as well as the methodology of determining initial states and boundaries, are provided in the complete paper.

To complete the model, a thermodynamic equilibrium was included to determine which components were and were not precipitating and the new composition was provided by the equilibrium. This was achieved by comparing the component’s fugacity in a mixture with the pure component’s fugacity in an interactive method.