Rheological Degradation of Model Wax-Oil Gels

Abstract

Paraffin gel formation in sub-sea petroleum transportation pipelines is a common problem encountered during operational and emergency production shut-down periods. Restart of a gelled oil pipeline usually requires the application of a large pressure drop across the pipeline length. In severe cases, permanent wax plugs have formed, resulting in the loss of production capacity. In this investigation, the structural breakdown of a quiescently-formed model wax-oil gel is measured at shear rates ranging from approximately 10⁻⁵ sec⁻¹ to 1 sec⁻¹. It is demonstrated that gel breakdown can be mathematically modeled using a time-dependent Bingham equation in which the yield stress follows third order degradation kinetics. The Bingham plastic viscosity term becomes significant at shear rates above ∼10⁻¹ sec⁻¹ and follows a similar third order decay profile with time. When the imposed shear rate is altered in a step-wise manner during the course of the gel breakage, transient non-linear viscoelastic effects are also observed in the mechanical stress response. Finally, explicit evidence is presented which indicates that after fracture, the gel strength behaves as a point function of the absolute strain, signifying a path-independent gel structure in terms of the single-dimensional flow history.

Keywords:

• Pipeline restart
• time-dependent breakage
• viscosity decay
• wax-oil gels

The National Research Council of Norway is acknowledged for financial support. StatoilHydro is also acknowledged for financial support.