Modeling wax deposition in pipes: Developing a closure relationship for improving the prediction accuracy

Abstract

Wax deposition remains one of the most challenging flow assurance issues faced by the oil and gas industry. The first cases of paraffin deposition problems in pipelines started to draw attention more than 50 years ago. However, no unequivocal model for its prediction has yet been proposed and accepted. The most used models for paraffin phenomenon prediction usually return over or under-predictions up to 100% in relative errors. Accurate prediction of this phenomenon will lead to proper flowline sizing, insulation requirements, pigging strategies, chemical treatments, etc. With the recent improvements in mechanistic understanding of wax deposition mechanisms based on in-situ microscopic visualization, it became clear that precipitated wax crystals occur within the boundary layer. Therefore, more than one mechanism governs the deposition process. These observations are the core for proposing a new estimation model that accounts for wax crystal precipitation. The model determines the concentration of precipitated crystals of wax through a material balance involving the soluble amount of wax in the system, the degree of supersaturation, and the precipitation rate constant (kp). Deposition data obtained from a laboratory-scale flow loop was used to validate the proposed model, resulting in great improvement in the relative errors between estimated and measured values for deposit thickness and wax fraction. Comparatively, the SD (Santos-Daraboina) Model resulted in predictions with relative errors as low as 12.8% against experimental data, while the FMT model, in some cases, resulted in overpredictions up to 299%.