Genetic Algorithm (GA)-Based Correlations Offer More Reliable Prediction of Minimum Miscibility Pressures (MMP) Between Reservoir Oil and CO₂ or Flue Gas

Abstract

Two new genetic algorithm (GA)-based correlations were proposed for more reliable prediction of minimum miscibility pressure (MMP) between 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 a 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 a 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, SO_x, O₂ and C₁-C₄.

Introduction

The MMP is a vital design parameter for CO₂ miscible flooding projects. 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 the project's 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^(1-3). On the other hand, the oil composition 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 (e.g. C₂-C₄) fraction reduces the MMP⁽²⁾. Furthermore, the presence of non-CO₂ (e.g. H₂S, N₂, SO_x and O₂) in the injected gas 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 SO_x could have a positive impact, as they contribute towards lowering the MMP. The 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 a 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.