Modeling wax thermodynamics

Written by: Alessandro Speranza and Xiaohong Zhang

Transportation of waxy crudes in pipelines in a cold environment is one of the most important flow assurance challenges for the oil industry, particularly with the increasing trend of exploration and development offshore. When the temperature of a crude oil or gas condensate drops below the wax solubility limit, known as Wax Appearance Temperature (WAT), the waxes start to crystallize and may precipitate. Waxes form deposits on the pipe walls if a radial heat flux to the surroundings exists. This phenomenon, known as wax precipitation and deposition, can result in significant operational and remedial costs, reduced or delayed production, well shut-ins, and pipeline replacement or abandonment. Therefore, prevention or mitigation for controlling wax precipitation and deposition is one of the key aspects of flow assurance, which depends on reliable modelling to predict the likelihood and the location of wax deposition.

The ability to predict accurately the rate of the wax deposition depends on a number of key factors:

• Correct fluid characterization with n-paraffins
• An accurate thermodynamic wax model that describes the phase behavior of solid waxes
• Flow hydrodynamics for establishing the temperature and pressure conditions along pipelines
• The mechanisms for modelling the process of the wax deposition

This whitepaper describes the thermodynamic theories of waxes, the fundamental and essential part of wax deposition. Validation and model performance are also provided, based on Multiflash.