Shut in and Restart of Waxy Crude Oil Pipelines: Gelation, Rheology Model Development, and Application of Polymer/Ionic Liquid Based Additive
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The processes of shut in and restart of waxy crude oil pipelines are important for crude oil flow assurance strategies. During the flow shut in process at low temperature, strong gel may form in the waxy crude oil pipelines. However, the formed gel needs to be broken during the restart of waxy crude oil pipelines process. Therefore, gelation and gel breakage process during the shut in and restart of waxy crude oil pipelines is necessary to investigate. Gelation and gel breakage models need to be established. In addition, oil additives are important for the processes of shut in and restart of waxy crude oil pipelines. In this thesis, wax appearance temperature of waxy oil samples is measured by differential scanning calorimetry, near-infrared spectroscopy, and nuclear magnetic resonance. It shows the method using differential scanning calorimetry, near-infrared spectroscopy, and nuclear magnetic resonance is accurate to determine the wax appearance temperature of waxy oils. In addition, a non-oscillatory rheological gelation point determination method is developed using rheometer. It shows the repeatability and accuracy of non-oscillatory rheological method is excellent to determine gelation point of waxy oils. It is a promising method for gelation point determination. In addition, effect of thermal history, shear history, wax type, wax content, asphaltene and additive on gelation process of waxy oil samples is investigated. Void formation, wax deposition, heat transfer during the gelation process, and gel buildup are discussed in the thesis. Effect of temperature, cooling rate, shear stress, shear rate and wax content on gel strength is also investigated. Subsequently, thermal history dependent, shear history dependent, and solid fraction dependent gel strength models are established or discussed according to the experimental results from rheometer and differential scanning calorimetry, as well as the previous theory. Moreover, constant shear rate, constant shear stress, dynamic shear rate, dynamic shear stress, oscillatory, and extensional rheological experiments are performed to investigate the rheological properties of waxy oil samples. Two types of rheological models are established as follows: strain dependent model for stress prediction, strain dependent model for viscosity prediction. Shear history dependent rheological model and gel breakdown kinetics are also discussed in this thesis. Furthermore, wax inhibitor is investigated for industrial applications. Effect of wax inhibitor on wax appearance temperature, gelation point, viscosity, and yield stress are investigated. New polymer/ionic liquid based additive is investigated. The ionic liquid based additive has potential scientific, economic and environment friendly benefits for oil industry. Experimental results show that the conventional ionic liquid can change the oil property significantly, the property of which can be utilized in some branches of oil industry. However, for waxy oil industry, novel polymer ionic liquid based additive is desirable to synthesize in order to prevent or reduce wax precipitation at low temperature. In summary, the ionic liquid based additive is promising to utilize in oil industry for oil property modulation.