Eﬀect of Emulsiﬁed Water on Gelled Pipeline Restart of Model Waxy Crude Oil Cold Flows Yichen Wang, † Jules Magda, † Ramachandran Venkatesan, ‡ and Milind Deo* ,† † Department of Chemical Engineering, University of Utah, Salt Lake City, Utah 84112, United States ‡ Chevron Energy Technology Company, Houston, Texas 77002, United States ABSTRACT: Recent work has shown the promise of cold ﬂow technology for transporting waxy crude oils below their wax appearance temperatures. The thermal ﬂux that results due to the diﬀerence between the temperature of the transported oil and the ambient is minimized in the implementation of cold ﬂow, thus signiﬁcantly lowering wax deposition on pipeline walls. Restarting pipelines carrying waxy oils under cold ﬂow conditions is an important consideration. In previous work from our laboratory, we have shown that gelled pipeline restart pressures for water-free systems are signiﬁcantly lower for pipelines shut down starting from pre-established cold ﬂow conditions in comparison to pipeline shutdown starting from hot ﬂow conditions. In this paper, we examine the applicability of this conclusion when varying amounts of emulsiﬁed water are present within the oil. We investigate model oils containing 7.0 wt % wax, with water cuts ranging from 0 to 60 wt %. Restart studies were conducted in a bench-scale, 0.4 in. diameter, 4 ft long ﬂow loop. The pressure required to restart the gelled pipeline decreases with a decrease in the cold ﬂow shutdown temperature and with an increase in the water content. Complementary rheological measurements show that the presence of water reduces the yield stress of model waxy crude oil gels. 1. INTRODUCTION In crude oil production, the use of in-place long-distance pipelines, both subsea and underground, is an inexpensive and high-capacity method for transporting oils. 1-3 Petroleum companies need to handle oils containing solids, such as asphaltenes, salts, hydrates, and waxes, and to assure heavy oil ﬂow through a long pipeline continuously and eﬃciently, especially for high-pressure and low-temperature deepwater environments. 4,5 In this paper, we investigate the inﬂuence of waxes and emulsiﬁed water. Suspended long-chain paraﬃn waxes in crude oils tend to deposit on interior pipeline walls during regular transportation and reduce the cross-sectional area for ﬂow. 4,6-8 Therefore, waxy oils require much higher pressure drops to maintain the same ﬂow rate as nonwaxy oils with the same API value, thus adding more burden on pumping stations. 1,9,10 More importantly, when the temperature of a solution of waxy oils is below its wax appearance temperature (WAT), wax crystals precipitate out of the solution and entangle to form a crystal-crystal interaction matrix. 8 If ﬂow shutdown then occurs, the entangled wax crystal network will transform into a solid-like particle gel that must be disrupted to restart ﬂow in the pipeline. This cross-linked particle gel may sometimes stretch over several miles and block pipelines during a shutdown in the cold environment. For strong gels, the amount of pressure required to break gels and restart ﬂow may exceed the limitations of the pipeline walls or the pump, and hence cause severe issues like fractures and oil leakages. 5,11,12 The minimum pressure required to restart a gelled pipeline can be estimated by measuring the gel yield stress or perhaps better yet the yield stress proﬁle vs deformation in conjunction with modeling. 13,14 Previous studies show that the yield stress of waxy oil gels strongly depends on the condition of the gel formation. 8 A gel cannot form while it is being sheared at a high stress level, hence gels form only after ﬂow ceases. The maximum yield stress value occurs for gels formed when ﬂow shutdown occurs at temperatures above the WAT. For a water- free gel, the lower the temperature at which ﬂow shutdown occurs, the lower the yield stress of the gel so formed. 15 In fact, the yield stress value approaches zero when the ﬂow shutdown temperature approaches the temperature at which the yield stress is measured. 16,17 However, in many cases, waxy oils produced at an undersea well head are emulsiﬁed with a signiﬁcant amount of water (0- 60 wt %). 18-20 The eﬀects that this water content has upon gel yield stress and upon gelled pipeline restart behavior are largely unknown. Hence, these are the topics that we investigate in this paper. We investigate emulsions stabilized by the addition of chemical surfactants, as opposed to asphaltene-stabilized emulsions in indigenous crude oils. In the previous work, Paso et al. 21 and Sun et al. 22 investigated the eﬀects of the presence of water on the yield stress of waxy crude oils. Water cuts were investigated between 0 and 70 vol %. The yield stress values of waxy crude oils were reported to systematically increase with an increase in water cut, as determined by rheology measurements. However, no gelled pipeline restart results were reported. Here, we report both rheology and gelled pipeline restart results for a model waxy crude oil emulsiﬁed with water. The trends we report for the yield stress are opposite to those reported by Paso et al. 21 and Sun et al. 22 The eﬀect of solid wall roughness upon gel slip is also investigated. Received: August 8, 2019 Revised: October 17, 2019 Published: October 18, 2019 Article pubs.acs.org/EF Cite This: Energy Fuels XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.energyfuels.9b02625 Energy Fuels XXXX, XXX, XXX-XXX Downloaded via UNIV OF WINNIPEG on November 8, 2019 at 21:07:49 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.