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Summary

Heavy oils represent a large quantity of hydrocarbon resources. Unfortunately, their high viscosities make it difficult to produce and transport them. Different industrial solutions have been developed to transport this specific type of oil in pipelines. The most spread-out way is to blend the crude oil with a light hydrocarbon to decrease the viscosity. In this study, we investigate a technique based on two-phase flow: pipeline lubrication. A thin water film is injected around the internal oil core, which leads to the core-/annular-flow (CAF) regime. Water lubricates the heavy oil, and the longitudinal pressure gradient is then largely reduced.  

We experimentally investigated the flow, the stop and restart of a viscous heavy oil with coinjection of water or brine as the lubricant fluid. The tests were conducted in steady laminar flow at moderate flow rates. The results show a pressure-drop reduction of more than 90% compared with the same product without lubrication. These results confirm the effectiveness of the lubricating process for heavy-oil transport.

We also measured restart pressure with different salts in the water phase, and we show that in some cases, the restart pressure can be limited.

Introduction

Oil flow as a single phase in a pipeline is a spread-out way of transporting hydrocarbon resources for very long distances. The relatively low viscosity of conventional crude oil is a key issue for such a way of transport because of low resistance to flow. For higher viscosity, the pipe diameter can be increased or, eventually, the mean flow velocity can be decreased. But for very high viscosity, large pipe and small velocity lead to a noneconomical transport technique.

Multiphase flow is also widely used now because it allows the mixing of gas, water, and oil together in a single pipe. However, flowing conditions are more difficult to predict because the phase configurations vary. CAF is one particular two-phase-flow regime in which the oil phase is in the center of the pipe and water is flowing near the wall surface. A very interesting characteristic of this flow is that it is stable for an acceptable range of velocities. The pressure drop is also very small, with only a weak dependence on the oil viscosity. Moreover, it is well suited for heavy oils. Indeed, in this case, densities are close to the water, so stratification of the phases is limited. Moreover, high viscosities slow down the core deformation and limit any modification of the flow regime.

Properties of CAF have been observed for a long time, and industrial interest was first noticed approximately 100 years ago. A 1904 patent of Isaacs and Speed (Isaacs and Speed 1904) in the U.S. first mentioned the ability to transport viscous products through “water lubrication.” Despite this early concern, large-scale industrial pipelines for heavy oil are scarce; the first one was built only in the 1970s. The Shell line near Bakersfield, California, was 38 km long with a tube diameter of 15 cm. For more than 10 years, a viscous crude oil has been produced in a water-lubricated flow regime.

Since then, several studies have been dedicated to the CAF regime, and different reviews of the published work have been written in Oliemans and Ooms (1986) and Joseph et al. (1997). It has been shown experimentally and theoretically that this particular flow regime is stable for a specific range of velocity (Joseph et al. 1997) and produces a very small pressure drop. Below a certain velocity limit, the capillary instability breaks the inner core into slugs, and at rest, stratification occurs in the system. So it is necessary to reach a certain flow rate to transport oil in the CAF regime. The different flow configurations of oil and water have been investigated, and flow charts are available (Joseph and Renardy 1993).