THE MEASUREMENT AND DESCRIPTION OF THE YIELDING BEHAVIOR OF WAXY CRUDE-OIL

Statically cooled samples of waxy crude oil possess a complex yielding behavior that cannot be described by existing yield stress fluid models nor by the description and modeling of thixotropic materials. Three distinct characteristics of the yielding process, namely, a solid (Hookean) behavior, a slow deformation (creep), and a sudden failure of the sample that closely resembles the brittle or ductile fracture of solids, have been identified by four different techniques-the vane technique, the cone and plate viscometer (constant rotation), constant stress rheometry, and oscillatory (dynamic) testing. A capillary viscometer or pilot scale pipeline presents technical difficulties which make it unsuitable for investigating the yielding behavior of waxy crude oils. The shear stress at fracture is the value of most interest to pipeline designers and the one most often taken as the yield point. Means of obtaining reproducible yield stress and fracture stress data are discussed.