Asphaltenes precipitation within reservoir pores or production flowlines can severely hamper the petroleum-extraction process. Although the effect of temperature and pressure on asphaltene deposition is well-known, the manner in which the variations in oil composition affect the asphaltenes-precipitation mechanism requires more clarity. This work investigates the effect of crude-oil compositional change on asphaltene stability. The impact of oil composition is analyzed by preparing pseudocomponents through blending the crude oil with their own saturates fractions.
A systematic characterization of 11 different bitumen and crude-oil samples was carried out by density and viscosity measurements and the determination of the elemental composition and saturates, aromatics, resins, and asphaltenes (SARA) contents. Further analyses were conducted on the asphaltenes separated by use of n-pentane. The cluster size was determined by a particle-size analyzer, and the stability of asphaltenes was evaluated by zeta-potential. The molecular structure of SARA fractions and bulk crude-oil samples was analyzed by Fourier-transform infrared (FTIR) spectroscopy. Onset-of-asphaltenes-precipitation (OAP) tests on crude-oil samples were achieved by the addition of different solvents (n-pentane, n-heptane, and crude-oil saturates fraction).
While the physical characterization studies could only provide weak relations between the density/viscosity and the asphaltene content of the bulk samples, it has been found that mainly the ratio of the heavy (resins + asphaltenes) to light (saturates +aromatics) fractions controls the viscosity and the °API value of the crude oils. As this ratio increases, the crude oil becomes more viscous and dense. Also, the asphaltene/resin ratio was found to be critical because of its impact on asphaltene stability, which was determined through zeta-potential measurements. The high asphaltene/resin ratios result in low asphaltene stability; however, this effect is surpassed by the higher aromatics fraction in the bulk oil. Asphaltene stability was further studied with OAP tests. The OAP-test results provide the behavior of asphaltenes after the interaction of bulk oil samples with normal saturated hydrocarbons; however, our study improves the OAP-test procedure by conducting OAP tests with crude oil's own saturates fractions. The interaction of saturates fraction with crude oil resulted in more asphaltenes precipitation compared with interaction of n-pentane and n-heptane. The FTIR analyses indicate the presence of impurities in saturates fractions, and these impurities are believed to cause higher asphaltenes precipitation as a result of the polar nature of the impurities.