Controlling Wax Deposition in Slug Flow: Coupled Effects of Multiphase Hydrodynamics and Chemical Inhibition

Abstract

Wax deposition under slug flow remains one of the most severe flow assurance risks in offshore and subsea pipelines because slug flow combines large hydrodynamic fluctuations, rapid thermal transients, and intermittent oil-gas contact with the pipe wall. These features make wax deposition behavior fundamentally different from that in a steady single-phase flow. Although chemical inhibitors are widely used to mitigate wax deposition, their performance in slug flow remains poorly understood and inadequately predicted. Most existing qualification methodologies are based on steady-state, single-phase experiments that fail to capture the complex interactions among multiphase hydrodynamics, inhibitor transport, and wax crystal evolution during sludging. This study critically examines the role of chemical inhibition of wax deposition in slug flow by using an in-house flow loop apparatus. The principal findings of the study are that slug-flow hydrodynamics alone resulted in only a marginal reduction in wax deposition at low gas superficial velocities (Vsg = 0.05–0.1 m/s), where prolonged bottom-wall liquid holdup sustained deposit masses comparable to those observed under single-phase flow. A significant reduction in deposition was observed only at Vsg = 0.2 m/s, where increased turbulence and wall shear reduced the deposit mass, whereas in the presence of INHI-1 at 500 ppm, substantially reduced deposit accumulation under all tested conditions, with the lowest deposit mass observed at the highest gas velocity due to the combined effects of chemical inhibition and enhanced hydrodynamic removal. Although INHI-1 reduced total deposit mass, it increased the wax fraction and thermal stability of the deposits, resulting in WDT values more than 10 °C higher than those of uninhibited deposits. The findings obtained in this study show that effective wax control requires integrating both the chemical and hydrodynamic understanding. INHI-1 provides robust suppression of deposition across flow regimes, while slug flow at a sufficiently high gas velocity enhances its performance. These mechanistic insights guide the design of integrated wax management strategies for multiphase pipelines.