Characterization of crude oils and their blends: A study of blending effects on solids precipitation properties

Solids precipitation from crude oils is a major problem faced by the petroleum industry. The precipitated solids are either paraffinic waxes or asphaltic solids or a mixture of both. The blending together of different oils may cause a change in composition leading to a change in the propensity for solids precipitation. The effect of blending on the solids precipitation behavior of the oils was investigated in this study.; A Fourier transform-infrared spectroscopy (FTIR) technique was used to estimate the wax precipitation temperatures (WPT) and weight percent precipitated solid versus temperature. The onset points of asphaltene precipitation were determined using near-infrared spectroscopy (NIR). A detailed characterization was performed on the oils and their blends, including a classification into compound classes: saturates, aromatics, resins, and asphaltenes (SARA). Blends of differing compositions were created from three different crude oils and also from their SARA fractions. The asphaltene content of the blends was not an average of the asphaltene content of the oils in the blend and exhibited different trends based on the constituents in the blends. A comparison of the WPT values obtained from FTIR showed that the WPT of the blends was lower than or equal to the WPT of the oils in the blends. The change in the WPT of the blends was attributed to the solvent effect of the lower-molecular weight paraffins in a paraffinic oil in the blends. Weight percent solids precipitated from the blends were an intermediate value compared to those for the blend constituents. The presence or absence of asphaltenes in the oils affected the WPT and amounts of solids precipitated indicating some interaction between the asphaltene and wax molecules. NIR onset measurements for asphaltene precipitation revealed that all the samples and blends tested were undersaturated with respect to asphaltenes. A general decrease in the onset values was observed on blending. Saturates were observed to decrease the solubility of the asphaltenes and thus the stability of the oil, and the presence of resins seemed to enhance the stability of the asphaltenes in the blend. Thermodynamic modeling of asphaltene precipitation based on the Flory-Huggins model did not yield results comparable with the use of a fixed solubility parameter for the asphaltenes considering molar volumes of asphaltenes in the range of 1000--5000 cm3/mol. A linear function of solubility parameter with a weak dependence on asphaltene concentration was found, however, to adequately model the experimental data.