CO₂‐oil saturation pressure and onset asphaltene precipitation

Abstract

Partial miscible flooding using CO2 injection has been shown to be a promising method for enhanced oil recovery, but this injection could result in asphaltene precipitation, which clogs the reservoir and production equipment. Therefore, an important parameter to be monitored is the onset of asphaltene precipitation. The efficient displacement of oil by CO2 depends on a variety of factors, including phase behaviour of CO2/crude‐oil mixtures that requires an accurate description of the CO2‐oil mixture bubble pressure. Observing bubble pressure behaviour, it is possible to exactly determine the CO2 molar fraction for onset asphaltene precipitation, which is expected to occur when the selected injection pressure is equal to the bubble pressure at the same CO2 molar fraction. However, assessment of oil characterization relating CO2‐oil mixture bubble pressure and onset asphaltene precipitation has not been found in the literature and should be properly addressed. In this work, we employed the Soave‐Redlich‐Kwong equation of state and used CO2‐oil mixture bubble pressure experimental data from literature to show that the accuracy of onset asphaltene precipitation is quite affected by the binary interaction parameter between methane and C7+ fractions, and the density of this heavy fraction.

Modeling the Saturation Pressure of Systems Containing Crude Oils and CO2 Using the SRK Equation of State

“…If the phase boundaries are not properly identified, then a second phase could appear, impairing the transport processes. A proper description of the saturation pressure curve is useful even in the identification of asphaltene precipitation conditions, using the minimum miscibility pressure (MMP) of CO 2 …”

“…A proper description of the saturation pressure curve is useful even in the identification of asphaltene precipitation conditions, using the minimum miscibility pressure (MMP) of CO 2 . 9 For these reasons, the same evaluation was done with gas injection to predict the mixture saturation pressure for different gas concentrations. This kind of analysis of characterization methods applied to the mixture saturation curve has never before been reported in the literature and is extremely important because an accurate description of the oil−gas phase diagram enables the easy identification of the phase boundaries and increases the efficiency of EOR processes.…”

“…Thus, asphaltene onset precipitation does not occur at pressures or CO 2 mole fractions on the left side of the saturation pressure diagram. On the basis of this fact, Oliveira et al 9 proposed a method to predict the amount of injected CO 2 that promotes asphaltene precipitation based on the saturation pressure curve of CO 2 −crude oil mixtures and CO 2 MMP. According to this approach, the CO 2 mole fraction that causes the onset of asphaltene precipitation corresponds to the point where the MMP line crosses the saturation pressure curve, indicated by letter B in Figure 1.…”

“…As a result, it is possible to know if the system remains miscible with the gas, which is required for a more efficient residual oil displacement and recovery process. Moreover, CO 2 injection has a significant effect on asphaltene precipitation, and the proper thermodynamic modeling can aid in accurate predictions of the asphaltene onset precipitation curve (AOP). , For models that describe the asphaltene precipitation through regular solution theory, as cubic and noncubic equations of state, parameter estimation requires experimental data of the saturation pressure and AOP to calculate the AOP for a wide range of gas–oil mixture compositions. Therefore, if the model does not provide a good description of the saturation pressure, then the AOP calculation is also impaired. , …”

“…Oliveira et al’s method based on the saturation pressure curve and the CO 2 MMP versus CO 2 molar fraction.…”