A fundamental model of wax deposition in subsea oil pipelines

Abstract

Wax deposition in subsea pipelines is a significant economic issue in the petroleum industry. A mathematical model has been developed to predict the increase in both the deposit thickness and the wax fraction of the deposit using a fundamental analysis of the heat and mass transfer for laminar and turbulent flow conditions. It was found that the precipitation of wax in the oil is a competing phenomenon with deposition. Two existing approaches consider either no precipitation (the independent heat and mass transfer model) or instantaneous precipitation (the solubility model) and result in either an overprediction or an underprediction of deposit thickness. By accounting for the kinetics of wax precipitation of wax in the oil (the kinetic model), accurate predictions for wax deposition for both lab-scale and pilot-scale flow-loop experiments with three different oils were achieved. Furthermore, this kinetic model for wax precipitation in the oil was used to compare field-scale deposition predictions for different oils.

Online estimation of wax deposition thickness in single-phase sub-sea pipelines based on acoustic chemometrics: A feasibility study

“…Transportation of crude oil from the oil reservoir may result in the deposition of wax from the petroleum fluids. Wax precipitation is a result of cooling of the sub-sea pipeline by the ocean which is typically at a temperature of 4 o C whilst the reservoir temperature and pressure is between 70 -150 o C and > 2000 psi respectively [2]. The heat transfer from the crude oil to the ocean is mainly a result of convection and conduction heat transfer.…”

“…In cases where production must be stopped in order to remove the plugged portions of the pipeline due to large wax deposits, the cost involved in replacement and downtime was estimated by Huang et al as ~$30,000,000 whilst in severe cases the platform could be abandoned at a cost of $100,000,000. Thus wax deposition can cause devastating economic implications to the oil producing company [2].…”

Fundamental Study of Wax Deposition Under Real Flow Conditions

“…This implementation was called solubility model by Huang et al (2011). As one might assume, Huang et al (2011) realized that the lab experimental results fell constantly in between the two scenarios. To accommodate these results, a model that would fall in between was proposed.…”

“…By ignoring the fact that solids might already be precipitated in the liquid, this strategy generates the largest concentration gradient possible and thus is an upper limit of the diffusion mechanism. This implementation was called independent heat and mass transfer (IHMT) by Huang et al (2011).…”

“…The threshold that differentiates between the molecules diffusing out of the deposit and those diffusing into it was named Critical Carbon Number (CCN) (Singh et al, 2001a). Huang et al (2011) discussed the different hypothesis involved in implementing the molecular diffusion model. One way to implement the method is to make the mass transfer model completely independent of the heat transfer model (Singh et al, 2001b).…”