Experimental study on dispersion characteristics and CH4 recovery efficiency of CO2, N2 and their mixtures for enhancing gas recovery

Abstract

Dispersion is considered as an important indicator to reflect the contamination of produced gas during CO2 enhanced gas recovery (EGR). Reducing dispersion while obtaining a high CH4 recovery is the primary goal of EGR research. In this work, the effects of core permeability and different types of displacing fluids on dispersion and CH4 recovery efficiency were investigated. The CO2 dispersion and the recovery efficiency were compared in the pressure range from gaseous to supercritical state, it was found that the CO2 dispersion coefficient is anomalously large near the critical point. The results reveal that the implementation of EGR at supercritical conditions is most favorable in terms of less CO2 dispersion and higher CH4 recovery. Higher permeability of porous media has a weakening effect on dispersion and increases the CH4 recovery efficiency. In vertical upward displacement experiments, CO2 has a smaller dispersion coefficient and higher CH4 recovery efficiency compared to N2. However, in the horizontal displacement experiments, the gravity effect on CO2 was more pronounced in the high permeability cores, resulting in greater dispersion and lower recovery efficiency. Moreover, it was found that the CH4 recovery efficiency provided by the displacing fluids with high CO2 concentration becomes larger as the injection flow rate increases, while the displacing fluids with high N2 concentration gradually decreases after reaching the maximum recovery efficiency as the injection flow rate increases.