Multiphase Flow Wax Deposition Modeling

Abstract

Results are presented from two-phase flow wax deposition tests using a state-of-the-art, high pressure, multiphase flow test facility. Wax deposition was found to be flow pattern dependent and occurs only along the pipe wall in contact with the waxy crude oil. The deposition buildup trend at low mixture velocities is similar to that observed in laminar single-phase flow tests. The buildup trend at high mixture velocities is similar to that observed in turbulent single-phase flow tests. Thinner and harder deposits at the bottom than at the top of the pipe were observed in horizontal intermittent flow tests. Thicker and harder deposits were observed at low liquid superficial velocity than at high liquid superficial velocity annular flow tests. No wax deposition was observed along the upper portion of the pipe in stratified flow tests. A semi-empirical kinetic model tailored for the wax deposition tests predicted wax thickness with an acceptable accuracy, especially at high oil superficial velocity. Deposition rate reduction due to shear stripping and rate enhancement due to entrapment of oil and other mechanisms not accounted for by the classical Fick’s mass diffusion theory were incorporated through the use of dimensionless variables and empirical constants derived from the wax deposition data. The kinetic model, although semi-empirical, provides an insight for future model development.

Tulsa University Paraffin Deposition Projects

“…The paraffin deposition phenomenon in single-phase flow is not well understood. Lund (1998), Matzain (1996Matzain ( , 1999 and Apte (1999) presented thorough literature reviews on paraffin deposition under single and multiphase conditions during the TU Paraffin Deposition JIP, completed in 1999. The precipitation of paraffin or wax occurs when the wall temperature of a pipe through which paraffinic oil is transported falls below the Wax Appearance Temperature, WAT (also called cloud point).…”

“…Few attempts have been found in the literature to properly model this shear stripping effect. Matzain (1999) modeled the change in the deposit thickness as:…”

“…In single-phase liquid flow, molecular diffusion is generally considered to be the dominant mechanism, while shear dispersion effects are considered negligible. Matzain (1999) addressed the nature of wax deposition in each individual flow pattern. The flow pattern dependent deposit behavior has been observed for South Pelto oil in multiphase deposition experiments.…”

“…After 12 experiments, the start-up procedure was changed in order to reduce possible early deposition. In the new start-up procedure (also used by Matzain, 1999), the gas was cooled and then mixed with the oil. After the gas-oil mixture temperature reached the test temperature of 85°F, the mixture was allowed to flow through the test section.…”

“…Previous numerical and experimental studies (Ghajar et al (1997a), Ghajar et al (1997b), Dougherty et al (1997), Apte (1999) and Matzain (1999)) in JIP project of "Paraffin Deposition Prediction in Multiphase Flowlines and Wellbores" show that wax deposition is sensitive to convective heat transfer in the flow so that it is required to develop a robust prediction method for all possible operating conditions in petroleum multiphase systems.…”