Laboratory analyses of the formation of wax deposits in crude oil are generally performed at atmospheric pressure on stabilized samples without the presence of light components. Therefore, the effects of 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. In this work, we evaluated five Brazilian crude oil samples and one condensate, recombined with different gases and under varying pressures, using the high-pressure microcalorimetry technique (HPμDSC). The samples were characterized with regard to the distribution of waxes by the number or carbon atoms to correlate with the results for wax appearance temperature (WAT) and crystallization enthalpy. The results indicate that rising pressure causes the WAT to rise as well and the recombination with gases composed mainly of light hydrocarbons reduces the WAT. Methane has the greatest influence on the oils containing higher amounts of macrocrystalline waxes, an effect that is stronger at higher pressures. Nitrogen does not act as a solvent of the paraffins present, whether of the macro- or microcrystalline type. Fractions lighter than C3, contained in the saturation gas mixtures, are more efficient in the solubilization of waxes, mainly those with lower molar masses. Variations in temperature and enthalpy of crystallization depend upon the pressure, composition of the gas (or mixture of gases), and composition of each crude oil.

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, 48 μW, could be considered as a thermal event. It is fundamental to investigate the gas phase saturation envelope to evaluate the viability 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°C. The standard deviation of the values found in the conditions specified was <0.5°C.

Chemical inhibition is one of the traditional methods used by the petroleum industry to prevent or correct the problems caused by the formation of organic deposits. In this work, three samples of poly(ethylene-co-vinyl acetate)s (EVAs) were added at different concentrations to three different Brazilian crude oils, and their effect on the wax appearance temperature (WAT) and crystallization enthalpy of the oils was evaluated as a function of increasing pressure and the type of saturation gas, using high-pressure microcalorimetry (HPlDSC). The results show that different copolymer compositions present dissimilar performance in reducing WAT, and, in some cases, an optimum concentration was observed. When rising pressure, a positive influ-ence on the WAT is observed, but the performance of organic deposition inhibitors that are ineffective at atmospheric pressure is not enhanced at higher pressures. On the other hand, additives able to modify the crystallization kinetics of waxes at atmospheric pressure show a synergy when evaluated in the presence of light fractions and under high pressures. HPlDSC analysis is an important tool to optimize the additive concentration to be used in oil fields.

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