An Experimental Study of Paraffin Deposition under Two-Phase Gas-Oil Slug Flow in Horizontal Pipes

Abstract

Paraffin or wax deposition under two-phase gas-oil slug flow in horizontal pipes was studied experimentally. The experiments were conducted using a 2.067-in large-scale multiphase flow loop under an 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 verify the flow patterns and examine the flow behavior. The hydrodynamic and heat-transfer variables were estimated using the unified mechanistic model (Zhang et al., 2003). 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 hrs. The bulk fluid and initial pipe wall temperatures at a removable spool piece were kept below WAT and relatively constant to control the initial concentration driving force. In this study, unprecedented detailed measurement and analysis of the circumferential local samples were conducted. A new pigging tool was designed and constructed for selective wax sampling at top, side, and bottom quarters of the circumference of the removable spool piece. Local deposit thicknesses were calculated from the direct measurements of deposit mass and density. Wax samples were analyzed by using DSC and HTGC for wax content and composition. The impact of the physics of the slug flow on wax deposition was investigated. The results indicated that deposit thickness, wax content, and wax mass were affected by the change in superficial velocities or the flow rates of the phases. It was found that the 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, circumferential variations of the deposit characteristics were analyzed. The deposit at the top wall was thicker, softer, and contained more oil than the bottom. 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. In slug flow, deposit at the top wall always had lower wax fraction than the bottom. Average carbon number and WAT of the deposit relatively increased with wax fraction. Wax deposits under slug flow had longer chain paraffins compared to the single-phase flow with the same wax fraction. Wax mass at the top wall was higher than the bottom. This new set of experimental data can be used as a verification tool for further development of wax deposition mechanistic model under such conditions.

Wax Formation Mechanisms, Wax Chemical Inhibitors and Factors Affecting Chemical Inhibition

“…In contrast, the deposition-hindering factor is attributed to the surface area of the crude oil pipeline which is in-contact with the oil when the liquid holdup decreases as the gas velocity increases. Few studies have found that the deposition-hindering factor is amplified for bubbly, wavy, and intermittent flow patterns [49,54].…”

Wax Deposition in Crude Oil Transport Lines and Wax Estimation Methods

“…Wax deposition models are developed based on consideration of assumptions and selection of parameters. Four main models are discussed in this book chapter: film mass transfer model (FMTM), equilibrium model (EM), Matzain's model (MM), and Venkatesan's model (VM) [24][25][26][27][28].…”

A model of wax deposition under oil-gas two-phase stratified flow in horizontal pipe

“…Matzain et al [8], Apte et al [9] and Rittirong [10] observed that the wax deposit only buildup on the wall wetted by the oil, and there was no deposit on the wall wetted by gas, and the deposit emerged a crescent shape at the pipe cross section, through the wax deposition in oil-gas stratified flow experiment. A semi-empirical model for wax deposit in oil-gas stratified flow was developed, based on the molecule diffusion mechanism and the assumption that all precipitated wax crystals on pipeline wall can deposit.…”

“…A semi-empirical model for wax deposit in oil-gas stratified flow was developed, based on the molecule diffusion mechanism and the assumption that all precipitated wax crystals on pipeline wall can deposit. The model takes the mutual effect of molecule diffusion and shear dispersion into consideration [9][10][11]. The wax deposit experiment for oil-gas stratified flow was also carried out by Gong et al [12] with Daqing crude oil and air.…”