Rheometry and direct flow observations of a model wax-oil system

Abstract

Waxy crude oils are a commonly encountered type of crude oil which must be dealt with by oil production companies. They are characterized by a gel-like behavior at temperatures below their wax appearance temperature (Twa) and often exhibit a yield stress. The restart of pipelines containing gelled waxy crude is a commonly encountered problem in production scenarios where low ambient temperatures are encountered, especially due to the increasingly important role that deep-water oil exploration has in meeting production needs. In this thesis, a model wax-oil system is formulated which closely mimics the thermorheological complexity of an actual waxy crude oil. The composition of the model system is characterized using chemical characterization techniques, and the basic rheological properties of these components are probed using temperature sweeps and stress sweeps. Large amplitude oscillatory shear (LAOS) is also used to gain additional insight into the rheology of the model system. The results from the LAOS measurements show that the wax-oil system exhibits a strain stiffening and shear thinning response under oscillatory shear. The system is then studied using a combined approach of rheology and flow measurement techniques. We describe the design, fabrication and calibration of a quantitative flow imaging apparatus that enables measurements of the local kinematics within the model fluid using Particle Image Velocimetry (PIV). This Rheo-PIV approach forms the basis for the study of the model fluid using Flow Assurance Rheometry. The data from this combined approach shows that the model fluid exhibits a complex behavior due to heterogeneities within the domain of the fluid. Specifically, the wax-oil system is shown to consist of two distinct phases: rigid clumps or fragments that are formed by aggregates of wax crystallites, and fluid-like regions which contain fewer wax precipitates. It is shown that under imposition of a steady shear stress, these rigid fragments break down over time into smaller pieces, and this contributes to the fluidization of the model wax-oil. The effect of surface roughness on the dynamics of this shear-induced fragmentation process is also investigated. It is shown that when the gelled system is in contact with a roughened surface, the fragmentation process happens faster.