Asphaltene precipitation from crude oil blends, conventional oils, and oils with emulsified water

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Asphaltene precipitation from crude oils is one of the flow assurance issues for the oil industry. Precipitation can be significantly affected by the changes in composition and pressure of the crude oil. For example, asphaltenes precipitate upon mixing of incompatible oils or solvents to crude oils, and pressure depletion in conventional oils containing solution gas. The focus of this thesis was to develop a phase behavior model for predicting the onset and the amount of precipitation from solvent-diluted crude oil blends and oils undergoing depressurization. Furthermore, crude oils with no or very little water are always considered for asphaltene precipitation measurements and modeling. In reality, the crude oils are often co-produced or extracted with water. Hence, the thesis also investigates the effect of water on asphaltene precipitation. Previously, a regular solution approach was successfully used to model the precipitation from heavy oils diluted with pure n-alkanes. The model inputs are the mole fraction, molar volume, and solubility parameter of each component in the n-alkane-heavy oil mixture. Heavy oil was characterized into saturates, aromatics, resins, and asphaltenes (SARA) fractions. Asphaltenes were sub-divided into fractions based on the gamma function to account for the distribution of aggregates resulting from self-association. Precipitation was modeled assuming liquid-liquid equilibrium between an asphaltenic and a non-asphaltenic phase. The only unknown parameter in the model was the average molar mass of the asphaltenes. The average molar mass was determined by fitting the model to precipitation yield data for n-heptane diluted heavy oil. In this thesis, the regular solution model was first extended for crude oil blends and then modified to handle conventional oils. Blends were prepared from oils from ten different sources. A methodology was proposed to estimate the composition of the blends and to calculate the molar mass distribution of the asphaltenes in the blends. The mass fraction of each SARA fraction in the blends was experimentally confirmed as a weight average of the respective fraction in the constituent oils. The asphaltene distribution in the blends was calculated assuming either an interaction between the asphaltene distributions from each constituent oils or no interaction. The model methodology was then tested on blends where only one of the constituent oils contained asphaltenes and also on blends where both oils contained asphaltenes. It was found that the model could predict onsets and amounts of precipitation for all the blends when no interaction between the asphaltene distributions was assumed. For conventional oils, a compositional characterization methodology was developed for a recombined oil based on gas chromatography analysis and SARA fractionation. The characterization included the determination of effective molar volumes and solubility parameters for light hydrocarbons. Both the characterization and the model were then tested against measured precipitation data from the dead and live oils. The common oil characterization methodology captured the behavior for both solvent- and pressure­induced asphaltene precipitation. However, the yield predictions were very sensitive to the fitted average molar mass of asphaltenes, limiting the predictive capability of the model. As part of the modeling methodology, the densities of the recombined oil were remarkably well predicted above the bubble point. To investigate the effect of water, precipitation yields were measured and compared for oils with and without emulsified water at different dilution ratios of n-heptane. At dilution ratios above the onset of precipitation for water-free oils, asphaltene yields were observed to be the same for both water-free oils and oils with emulsified water. For dilution ratios below the onset for water-free oils, there was no detectable precipitation but asphaltenes adsorbed on the water-oil interface appeared as yield. Hence, the solubility of asphaltenes was not affected in the presence of water.
Bibliography: p. 159-174
Tharanivasan, A. K. (2012). Asphaltene precipitation from crude oil blends, conventional oils, and oils with emulsified water (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from doi:10.11575/PRISM/4677