A number of wax models currently in use by the oil industry are based on parameters that were empirically determined to match available data for black oils. These data are often not very precise. The recently developed predictive local composition model is, however, a predictive, theoretically well-defined model based on highaccuracy thermodynamic data. The paper describes how the predictive local composition model can be used in conjunction with conventional cubic equations of state to perform wax equilibrium calculations for black oils. Examples are given that show how the model can predict both wax appearance temperature and the amount of wax precipitated at varying temperatures with or without detailed n-paraffin analyses. The examples presented include the effect of pressure on live oils. The improved thermodynamic modeling of wax formation will allow for better prediction of wax deposition rates for flow assurance.
A method is presented for simulating wax deposition in pipelines
in which the wax phase is represented as a continuous distribution
of n-paraffin components. The thermodynamic properties
of the wax are predicted using a previously developed thermodynamic
model. We have adopted a mass transfer model to predict the likely
rate of wax deposition. Predictions from the model are compared with
observations of flow-loop experiments, and predictions for full-scale
pipelines are presented. The impact of the removal of deposited wax
by shearing is also investigated. We demonstrate the effect of assuming
molecular diffusion to be the dominant mechanism of wax deposition,
and we also show that not representing the wax with its full n-paraffin distribution leads to serious distortions of
the pipeline simulations. Practical software development issues in
the implementation of the model are outlined.
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