Prediction of Wax Deposition in Pipelines and Field Experience on the Influence of Wax on Drag-Reducer Performance
Authors: Hamouda A.A. , Ravneøy J.M. 
Published: October 2008

A simulation program for pressure and temperature profiles in a pipeline was developed taking into account the wax deposition tendency and prediction of the amount and location of the wax deposited in the pipeline. The data from the simulation program when compared to the: field data, enabled us to explain the drag reducer behaviour in a major oil pipeline in the North Sea.

Several mechanisms contribute to the wax deposition. These are molecular diffusion. shear dispersion, brownian diffusion and gravity settling. Earlier works by others, showed that the molecular diffusion is the main mechanism.

The model developed here was based on numerical heat transfer calculations, molecular diffusion .rate and rheological properties of the oil fitted to Bingham plastic model. In order to make this model practical to use, a collective factor "Wax Deposition Tendency Factor" was chosen to eliminate the necessity to calculate the concentration gradient and the weight fraction of the wax in the total deposit. This model requires rheological properties measurements which can be obtained from laboratory tests.

The developed test setup is discussed in this paper. The simulated pressure profile was compared with actual field data. The simulated data and the field data were within 10% agreement. However, after careful evaluation of the two pressure profiles obtained from the field and simulation, we believe that the difference may be narrowed further.

Furthermore, the model enabled us to explain a phenomenon of the change in drag reducer performance in the pipeline. This will, also, be discussed in the paper.

Introduction

Crude oils are generally complex chemical systems containing from hundreds to thousands of individual components in the range from low molecular weight (nalkanes) to high molecular weight waxes and asphaltenes. The high molecular weight waxes and asphaltenes are dissolved under reservoir conditions. These components may precipitate in production wells, processing equipment and/or pipelines as the thermodynamic equilibrium is disturbed(1) which is caused by changing of the pressure and temperature. The formed wax crystals, during the crystal growth process, may develop an interlocking 3D- structure that can entrap the carrier oil. This will lead to increase of the viscosity of the transported oil. It was reported that as low as 2% precipitated wax, gelling process may occur(2). Various papers(3–6) have addressed problems in transportation facilities and production equipments associated with wax deposition. Wax deposition mechanisms such as molecular diffusion, shear dispersion, brownian diffusion and gravity settling are encountered to contribute to the deposition of wax in pipelines.

Several workers(7,8) have found that molecular diffusion is the main wax deposition mechanism. deposition in pipelines based on the molecular diffusion mechanism was developed.

Theoretical Approach of the Model

Oils in pipeline are cooled from the inlet temperature asymptotically towards the ambient (seabed) temperature. While the temperature has a relatively uniform lateral distribution in the turbulent core of the flow, there is a layer of laminar flow adjacent to the pipeline wall, where a temperature gradient exist.