Two new genetic algorithm (GA)-based correlations were proposed for more reliable prediction of minimum miscibility pressure (MMP) between the reservoir oil and CO2 or flue gas. Both correlations are particularly useful when experimental data are lacking and also in developing an optimal laboratory program to estimate MMP.
The key input parameters in GA-based CO2-oil MMP correlation, in order of their impact were: reservoir temperature, MW of C5+, and volatiles (C1 and N2) to intermediates (C2-C4, H2S, and CO2) ratio. This correlation, which has been successfully validated with published experimental data and compared to common correlations in the literature, offered the best match with the lowest error (5.5%) and standard deviation (7.4%).
For the GA-based flue gas-oil MMP correlation, the MMP was regarded as a function of the injected gas solvency into the oil, which in turn is related to the injected gas critical properties. It has also been successfully validated against published experimental data and compared to several correlations in the literature. It yielded the best match with the lowest average error (4.6%) and standard deviation (6.2%). Moreover, unlike other correlations, it can be used more reliably for gases with high N2 (up to 20%) and non-CO2 components up to 78% (e.g., H2S, N2, SOx, O2, and C1-C4).
The MMP is a vital design parameter for CO2 miscible flooding project. It impacts both operational and reservoir engineering aspects during the flood. Therefore, an operator must investigate the effects of various factors on the MMP, and consequently, their eventual impact on project operational strategy and surface facilities. The main factors affecting MMP are: reservoir temperature, oil composition, and injected gas purity. The reservoir temperature has a big impact on MMP, as the MMP increases with the increase in the temperature and decreases with its decrease M(1)(2)(3). On the other hand, the oil composition, too, has a significant impact on MMP in that the MMP increases with the increase in oil molecular weight. A high oil volatiles fraction (e.g., C1) in the reservoir oil causes the MMP to rise; whereas, a high intermediates (C2-C4) fraction reduces the MMP(2). Furthermore, the presence of non-CO2 (e.g., H2S, N2, SOx, O2, and C1-C4) in the injected gas, too, affects MMP, either raising or lowering it depending on the type of the component. From an operational perspective, the existence of these non-CO2 components in the injected gas should not be treated as a rigid impediment. In fact, the existence of certain components such as H2S and SOx could have a positive impact as they contribute towards lowering the MMP. Presence of C1 and N2, on the other hand, could be detrimental as they cause the MMP to rise. As the separation of such components from the gas is difficult and costly, the current trend is to use the flue gas stream as it is, provided such impurities are below certain optimum level. A gas stream containing such non-CO2 components are referred to as either low-purity CO2 or flue gas in the context of this paper.