Influence of thermal history on the waxy structure of statically cooled waxy crude oil

The effect of the thermal history on the waxy structure of two statically c ooled waxy crude oils is examined using a controlled stress rheometer for r heological measurements and a microscope to observe microstructure of wax c rystals. 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 degre esC/min), and isothermal holding time before testing (up to 15 hours). In a ddition, the reversibility over time bf the waxy structure after yielding w as investigated. The results from both yield stress measurements and micros copic observations show that the waxy structure strongly depends on both th e temperature and the cooling rate. The isothermal holding time both before and after yielding was found to have no observable influence on the struct ure over the time period studied in these experiments.