(69a) Evaluating Wax Yielding with a New Model

Determination of the yield stress of waxy crude oils is important to understand the restart of pipelines containing oil that has gelled during a shutdown. While it has been observed by several researchers that the pressure required to restart flow is not directly proportional to the yield stress, this rheological value is nevertheless important in estimating an approximate value for the restart pressure.

We have worked internally and with different collaborators to experimentally and theoretically study the rheology of waxy oils. One of the outcomes is a model called the IKH (Isotropic Kinematic Hardening) Model recently developed by MIT (Dimitriou et al, J. Rheology, 57, 2013). This model considers the kinematic hardening process related to the plasticity of the gel and the isotropic hardening associated with the structural changes of the gel.

Our rheological experiments with a number of crude oils as well as model oil systems have shown that, upon application of a constant shear stress, the gel creeps until a critical strain value of about 1% is reached, whereupon a catastrophic failure is observed.

In this presentation, we discuss these experimental results and a way to model the results with the IKH model. Specifically, the ability of the model to predict the experimentally observed breakdown strain limit and the implications on predicting restart pressure in pipelines are discussed.