Flow Properties of Difficult-To-Handle Waxy Crude Oils
Authors: Sifferman Thomas R. 
Published: December 2007

This paper presents viscosity, gel strength (yield value), and pour-point data for Dickinson (ND), Udang (Indonesia), and Amna (Libya) crude oils, with and without flow improvers (wax crystal modifiers). All three flow properties are necessary to characterize the oil. Low ambient temperatures properties are necessary to characterize the oil. Low ambient temperatures relative to the pour point can cause start-up problems for these crude oils.

Introduction

The pour point often is used as a total indicator of a crude oil's flow properties. It is not proper to compare only the pour points of crude oils themselves or only pour points relative to ambient conditions in which these pour points relative to ambient conditions in which these oils mill be handled. The Dickinson (ND) crude oil with a pour point of 95 degrees F supposedly would be difficult to work with at temperatures below 95 degrees F; however, this oil actually flows (pours) at room temperature (about 70 degrees F). A 75 degrees F pour point for the Amna crude oil from Libya might not indicate problems. However, this is misleading since a 45 degrees F North Sea temperature could cause unloading problems at the tanker. The Udang (Indonesia) crude oil has a 100 degree F pour point and is actually solid at room temperature. Therefore, although ambient conditions are very important for understanding the flow properties of crude oils, viscosity, gel strength (yield properties of crude oils, viscosity, gel strength (yield value), and pour point are all considered together. Unfortunately, many pipelines still specify pour point as the sole criterion for determining acceptability of a crude oil. This often is an unrealistic and unnecessary constraint because viscosity and gel strength indeed may be more important.This paper combines for the first time data on crude oils treated with flow improvers (wax crystal modifiers) and emulsifiers (surfactants). It also compares various additives, rather than just one type. A variety of crude oils from different parts of the world are considered.Waxy crude oils are becoming more important since they often have low sulfur contents, making them desirable from an environmental viewpoint. Also, waxy crude oils are available, and the need for new supplies has encouraged production of these difficult-to-handle crudes. However, the economic situation now requires better design of new installations because of the high cost involved.Flow improvers (wax crystal modifiers) often decrease the pour point slightly, while both viscosity and gel strength can be reduced appreciably.

Previous Investigations Previous Investigations Much of the earlier research was conducted on methods to move the crude oil below its pour point. Above the pour point, the crude oil typically behaves as a pour point, the crude oil typically behaves as a Newtonian fluid. However, as it cools, it becomes non-Newtonian; therefore, its viscosity varies as a function of shear rate (flow rate in a pipe). A crude oil can become time dependent so that there is hysteresis (thixotropic behavior). Much of the waxy crude-oil pipeline problem information was presented in the early pipeline problem information was presented in the early 1970's, particularly by Ford et al. Both a 75 degrees F pour-point crude oil from Libya and a 55 degrees F pour-point pour-point crude oil from Libya and a 55 degrees F pour-point Nigerian crude oil were discussed for pipelining operations. Start-up was the chief concern.Uhde and Kopp discussed various methods of transporting waxy crude oils.

JPT

P. 1042