| Wax deposition when two-phase natural gas and crude oil flowed in 52.5-mm (2.067 in) diameter horizontal, near-horizontal, and vertical pipes has been studied experimentally and theoretically. A new high-pressure state-of-the-art test facility was designed, constructed and operated to generate wax deposition data. Waxy crude oil from the South Pelto field, Well 10E, in the Gulf of Mexico and natural gas supplied by Oklahoma Natural Gas Co. were chosen as the test fluids. A new device for measuring wax thickness, called liquid Displacement and Level Detection, was installed and was found to he reliable and accurate.; Twenty-three tests at different oil and gas superficial velocities were conducted. The test conditions involved flowing the fluids at horizontal (0°), inclined upward (2°), and vertical (90°) positions for up to 24 hrs, and utilized one inlet mixture temperature and one inlet glycol temperature. Wax deposition was found to be flow pattern dependent and occurs only along the pipe wall in contact with the waxy crude oil. An increase in mixture velocity results in harder deposits, but with a lower deposit thickness. 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. Thicker 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 conducted during the present study 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. |
