An Experimental Study and Genetic Algorithm (GA) Correlation to Explore the Effect of nC5 on Impure CO2 Minimum Miscibility Pressure (MMP)

Abstract

Abstract Limited experimental work has been performed to study the effect of nC5 on Minimum Miscibility Pressure (MMP) when added to impure CO2 as an injection gas for flooding processes. Furthermore, current MMP correlations have been developed based on data where the injection gas contains no nC5 fractions. Consequently, their MMP prediction is not highly accurate when nC5 exists in the injection gas. In order to further investigate the effect of nC5 on impure CO2 MMP, this work presents the experimental investigation and subsequent development of a correlation to model this effect. Using Rising Bubble Apparatus (RBA), MMP data points were measured on three reservoir fluids and four injection gases at three temperatures. The three reservoir fluids were relatively light with stock tank gravities in the range of 44-52 °API and GOR's 100-1300 scf/stb. The base gas composition was 92mol% CO2 and 8mol% C1. To this nC5 was added to create gases with 1mol%, 3mol%, and 5mol% nC5. The nC5 was noted to decrease the MMP. Based on the experimentally measured MMP data set, a strong trend was found. The effect of addition of nC5 on the impure CO2 MMP has been correlated using Genetic Algorithm (GA). This correlation depends on the injected gas composition and critical properties. The GA-based correlation predicted the nC5 effect with greater accuracy than the currently available correlations, giving an absolute error of only 3.4%. It is envisaged that this work can be used to investigate the possible benefits in retaining the nC5 fraction in the injection gas and to provide a method of evaluating whether it is worthwhile to add or retain a small amount of nC5 to the injection gas to improve miscibility with the reservoir fluid. Thus, more oil can be recovered and the added injection gas expense may perhaps be justified.