This work investigates the saturation pressure of crude oil−CO 2 mixtures and how the oil plus-fraction characterization methods affect it. Different distribution functions were tested to split the plus-fraction as well as correlations to calculate the critical parameters and the acentric factor. The Soave−Redlich−Kwong (SRK) equation of state was used to calculate the saturation pressure of the crude oil and its mixtures with pure and impure CO 2 . The results were compared with experimental data and reveal that the effect of the characterization methods on the modeling of the saturation pressure of reservoir fluids is strong mainly for CO 2 molar fraction values higher than 50%. Article pubs.acs.org/jced
The expanded fluid (EF) model is known for its capacity to calculate the viscosities of crude oil and its mixtures with solvents at high pressure and temperature using a cubic equation of state with a minimum amount of experimental data. The main drawback of the EF model is usually the requirement of a proper plus-fraction characterization and accurate density input data. In this study, the sensitivity of the EF model to the characterization method and the density of the oil was evaluated against viscosity data on reservoir fluids. The oil viscosity was calculated above and below the saturation pressure in order to compare the EF model with other viscosity models available in the literature. The results confirmed that the viscosity strongly depends on the quality of the density predictions as well as on a good description of the phase behavior below the saturation pressure. This demonstrates that proper characterization is needed in order to calculate the oil viscosity accurately. Furthermore, a new tuning method with minimum experimental data improved the viscosity prediction as a function of pressure.
Asphaltene precipitation is a complex and serious problem in all sectors of the oil industry because it has a severe and detrimental impact on oil production. Thus, it is crucial to investigate under which conditions asphaltenes precipitate in order to prevent or mitigate the effects. Several approaches have been reported in the literature regarding the modeling of asphaltene precipitation during oil production. The simplest one is the single-component solid model in which the precipitated asphaltene is considered a pure solid and the oil and gas phases are described by a cubic equation of state (EOS). These are the basic assumptions of the Nghiem and Coombe’s model. In this paper, based on this model, we make numerous improvements and simplifications such as reducing the number of parameters to be estimated and considering that the number of asphaltene fractions can vary according to the oil characteristics. The characterization method is performed using the exponential distribution and only the binary interaction parameter is fitted to the experimental oil saturation pressure data. The reference pressure is calculated according to de Boer’s method instead of only extrapolating precipitation depletion experimental data. Furthermore, the solid molar volume is predicted using two correlations from literature, expressed as a function of the molecular weight of the asphaltene. The developed model is capable of calculating the amount of precipitated asphaltene as a function of pressure. Our results indicate that the proposed approach is quite accurate even having a smaller number of parameters to be estimated compared to the original model.
Thermodynamic calculations involving crude oils require a proper characterization of the heavy-fraction in order to describe the phase behavior. Bubble pressure is a key property, and its calculation depends on splitting the heavyfraction of the crude oil through a distribution function to obtain properties such as molecular weight and specific gravity. Small changes in these properties can significantly affect phase behavior calculations. The exponential and gamma distributions are the most used for splitting but they fail in characterizing many oils. Alternatively, the q-Weibull distribution is proposed to the splitting step instead of the exponential and gamma distributions in order to calculate the molecular weight and specific gravity. These three distributions are critically tested and compared for 13 oils with extended composition selected from the literature, stressing the effect of the characterization methods on the calculation of bubble pressure for reservoir fluids.
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