Effect of Pressure on the Crystallization of Crude Oil Waxes. I. Selection of Test Conditions by Microcalorimetry

摘要

Evaluations of the potential for formation of wax deposits in laboratories generally are performed on stabilized oil samples, that is, at atmospheric pressure and without the presence of light components. Therefore, the effects to two important factors that influence the solubility of waxes, namely light fractions and pressure, are not considered. As a consequence, the results may not reflect what really happens in production lines and equipment. The high-pressure microcalorimetry technique (HPμDSC) is indicated for determining the wax appearance temperature (WAT) and crystallization enthalpy of waxes in oils, but due to the complexity of petroleum systems, the reliability of the results depends on the experimental conditions chosen. This paper reports experiments to evaluate the factors that influence the accuracy of the experimental results for determining the WAT and crystallization enthalpy of samples of Brazilian crude oil and condensate, saturated with different types of gases, by using high pressure microcalorimetry (HPμDSC). The results show that heat flow values of more than twice the baseline oscillation, that is, 48μW, could be considered as a thermal event. It is fundamental to investigate the gas phase saturation envelope to evaluate theviability of pressurizing the reference cell and to determine the most adequate conditions of temperature for the pressurization of the oil samples. Still better results were obtained without pressurizing the reference cell and with exchange of test aliquots after analysis at 50 bar. The minimum saturation time necessary for the oil and condensate samples evaluated was 180 min, and the ideal mass for analysis was around 100 mg. The most suitable procedure was to pressurize the samples rather than depressurize them, with the deviation between the two procedures being < 1 ℃. The standard deviation of the values found in the conditions specified was < 0.5 ℃.