Paraffin deposition under two-phase gas-oil slug flow in horizontal pipes

Paraffin or wax deposition under two-phase gas-oil slug flow in horizontal pipes was studied experimentally and theoretically. The experiments were conducted using 2.067-in TUPDP multiphase flow loop under the operating pressure of 350 psig. Testing fluids were Garden Banks condensate and natural gas. Hydrodynamic experiments were performed prior to the wax deposition experiments to obtain fluid properties, verify the flow patterns, and examine the flow behavior. Liquid holdups calculated using TUFFP unified model were consistent to the values estimated from the hydrostatic pressure drop data. Consequently, hydrodynamic and heat-transfer variables were estimated using the unified model. Wax deposition experiments were conducted under single-phase and slug flows with parametric variation of oil and gas superficial velocities and testing durations of 4, 12, and 24 hr. The bulk fluid and initial pipe wall temperatures were kept below WAT and relatively constant. A new pigging tool was developed for selective wax sampling at each circumferential location of the removable spool piece (top, side, and bottom). Deposit thickness was calculated from the direct measurements of deposit mass and density.;From the experiments, deposit thickness increased with time. The thickness decreased with increasing superficial liquid velocity; whereas, it increased with increasing superficial gas velocity. The trends of the thickness were found to crossover each other at a certain time for different superficial gas velocities. In slug flow cases, the deposit at the top wall was thicker, softer, and contained more oil than the bottom. Wax composition was analyzed by using DSC and HTGC. Local carbon number distributions of the deposit under single-phase and slug flows indicated typical wax deposition mechanisms. Wax fraction increased with time reflecting the aging. Moreover, wax fraction increased with higher superficial liquid and gas velocities at a given time. The crossover of the wax fraction trends with different superficial gas velocities occurred. Average carbon number and melting point of the deposit relatively increased with wax fraction. Wax deposit under slug flow tended to form with longer chain than the single-phase flow with the same wax fraction. Wax mass at the top wall was higher than the bottom. In this study, the impacts of hydrodynamics and heat transfer were coupled. The wax mass flux analysis and hydrodynamic investigation using high speed camera revealed that there was wax deposition at the gas core region in all slug flow cases.;A new mechanistic model of wax deposition was developed for both single-phase and gas-oil slug flows. The model requires two fitting parameters to account for wax precipitation kinetic, deposit growth reduction by shear stress, and the impact of possible undiscovered wax deposition phenomena. Correlations were developed for these parameters. Model prediction of average deposit thickness and overall wax fraction had the uncertainties of plus-minus +/-15 and +/-20 %, respectively, when compared to the experimental results.