Influence of Thermal History on the Waxy Structure of Statically Cooled Waxy Crude Oil

Summary The effect of the thermal history on the waxy structure of two statically cooled waxy crude oils is examined using a controlled stress rheometer for rheological measurements and a microscope to observe microstructure of wax crystals. A three-yield-stress model is used to describe the yielding of the samples at low temperatures. The elastic-limit, static, and dynamic yield stresses as a result of different static cooling processes were determined from controlled stress and oscillatory rheological measurements. The types of histories examined include the temperature, cooling rate (0.5 to 4°C/min), and isothermal holding time before testing (up to 15 hours). In addition, the reversibility over time of the waxy structure after yielding was investigated. The results from both yield stress measurements and microscopic observations show that the waxy structure strongly depends on both the temperature and the cooling rate. The isothermal holding time both before and after yielding was found to have no observable influence on the structure over the time period studied in these experiments.