A major problem faced by the petroleum industry is the deposition of wax during the pumping of waxy crude oils. This precipitation occurs at “normal” temperature, typically 20-30°C in Libya. It could occur during the journey from well to terminal through hundreds of miles of pipelines. This kind of transportation is expensive in terms of pumping costs. The pumping has to be continuous; otherwise wax can build up in the pipeline, reducing the pumping or even stopping it. The property that defines this characteristic is the yield stress which depends on wax concentration and cooling rate. The build-up of paraffin and asphaltenes can lead to serious problems in formation, tanks, and pipelines. Blockages can be expensive and time-consuming to deal with; this is precisely the topic of this research. For this research, model and real waxy crude oils are formulated and their rheology systematically measured under various cooling rates to determine the yield stress. A pipeline loop has been designed to measure the start-up pressure of stagnant oil which has been allowed to precipitate wax. The start-up pressure and the thickness of deposited wax are used in a simple mathematical model to calculate the yield stress. This research thus provides two independent means of predicting the yield stress. This research studied three different waxy crude oils. An MCR-301 Anton Paar rheometer was used to measure the rheology of the oils, and a pipeline rig was used to obtain the start-up pressure to calculate the yield stress of each type of oil after different stoppage times. Also, the thickness of the precipitated wax is measured to calculate the yield stress precisely. The data show that the layer thickness has significant effect on the yield stress and start-up pressures and corresponding yield flow stresses have been found to underpin the crystallisation process of the wax and slow cooling rate produce stronger structures requiring higher stresses to fracture and induce flow. Also, longer shutdown times make these structures even stronger and therefore require even larger stresses for flow to commence.

[pt] Esta tese é sobre a reologia de óleos parafínicos crus. Um protocolo baseado na literatura relevante é desenvolvido para garantir que históricos térmico e de cisalhamento bem definidos sejam impostos em amostras que serão submetidas à medições reológicas. Com este protocolo, uma caracterização reológica quase completa de um óleo parafínico cru brasileiro é executada . Quatro experimentos reométricos são realizados, a saber, escoamento em estado estacionário, taxa de cisalhamento constante, tensão constante e rampa oscilatória de amplitude de tensão. Propriedades reológicas importantes, tais como, viscosidade, tensão limite de escoamento e módulos elástico e viscoso são medidos com o auxílio de técnicas diferentes em uma ampla faixa de condições. Além disso, uma comparação entre essas técnicas de medição é realizada. O processo de quebra da microestrutura do óleo parafínico brasileiro, assim como os efeitos na reologia do histórico do material e do tempo de repouso isotérmico são estudados. Fenômenos interessantes são observados, incluindo bifurcação da taxa de cisalhamento, dependência da tensão limite com os históricos térmico e de cisalhamento e independência dos resultados de regime permanente com relação à condição inicial após o resfriamento. Por fim, oportunidades para futuros desenvolvimentos e investigações são enfatizadas. / [en] This thesis deals with the rheology of waxy crude oils. A protocol based on the relevant literature is developed to guarantee well defined shear and thermal histories to waxy crude oil samples to be submitted to rheological measurements. With this protocol, a fair complete rheological characterization of a Brazilian waxy crude oil is performed. Four rheometric experiments, namely steady-state flow, constant shear rate, constant shear stress, and dynamic-stress-amplitude sweep are carried out. Important rheological properties, such as, viscosity, yield stress, and storage and loss moduli are measured with di↵erent techniques in a wide range of conditions and a comparison between the techniques is presented. The yielding as well as the history effects and the e↵ects of isothermal holding time on the microstructure and rheology of a Brazilian waxy crude oil are investigated. Interesting phenomena are observed, including bifurcation of shear rate, dependence of the yield stress with thermal and shear histories, and independence of the steady-state results with the post-cooling initial condition of the experiment. At last, opportunities for further developments are highlighted.