Abstract

As the world searches for more oil and energy, heavier oils are being found and moved through pipelines. Many of these are Non-Newtonian fluids which have very different modelling needs than Newtonian fluids. Others such as WCS (Western Canadian Select) or Alaskan crudes are truly Newtonian fluids when tested in the lab but can behave like non-Newtonian fluids in low flow and cold temperatures. Also, systems which are normally not modelled for refined products or light oils become issues for these heavy crudes especially in colder times of the year. There are other modeling traps which may result in poorly sized lines or unrealistic predictions of capacity of pipelines. This paper will address some of these issues and provide some back ground information on fluid properties. The modelling processes discussed are software package independent.

Introduction and Background

This section provides information on the physics and basic physical properties which combine to create issues when transporting heavy crudes and trying to effectively model them.

Fluids

A brief refresher on the fluid properties which have significant effects on the behavior of heavy crudes in pipeline systems is provided.

Types of Fluids

There are two basic types of fluids which can flow through pipelines Newtonian and Non-Newtonian. The basic definitions of the two are:

  • Newtonian - Viscosity is independent of shearing force and rate and is a constant at a given temperature. The majority of crude oils are Newtonian.

  • Non-Newtonian - Viscosity is dependent on shearing force and rate and, in some cases, can be time dependent.

See Figure 1 - Fluid Viscosity of Newtonian and Non-Newtonian Fluids. Newtonian fluid viscosity goes through the origin of the curve. Bingham plastic fluids have an offset which is the gel strength of the fluid and then have a straight line relationship.

Fluid Properties
Viscosity

Viscosity is the fluid property which enables it to resist a shearing force. It is a measure of the resistance to flow exerted by a fluid. In other words the more viscous a fluid the harder it is to get it flowing. See Figure 2 - Newtonian fluid Viscosity. There are two different measurements of viscosity:

  • Dynamic Viscosity (μ) – Which is also called Absolute Viscosity, is the measure of the shearing stress necessary to induce a unit of velocity shear gradient in a fluid. Often measured in centipoise (CP).

  • Kinematic Viscosity (ν) – is a measure of the time required for a given volume of liquid to flow through an orifice or restriction. Often measure in Centistokes (CST).

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