A COMPARATIVE STUDY ON THE OIL RECOVERY PERFORMANCE OF THE CO2 AND N2 HUFF-N-PUFF PROCESSES IN LOW PERMEABILITY CORES

Abstract

The gas huff-n-puff process has been widely employed in low permeability reservoir development practices. At present, the understanding of synergistic effects of different injection fluids and rock permeability is still limited and is worthy of in-depth investigation. Therefore, laboratory studies on oil recovery performance of supercritical CO₂ (scCO₂) and N₂ huff-n-puff processes in ultra-low (< 0.5 mD), extra-low (0.5-1 mD), and average-low permeability (1-10 mD) core samples were systematically investigated. More effective cycles could be achieved in the CO₂ huff-n-puff process than in the N₂ process. The CO₂ huff-n-puff operation could produce 29.9% oil in the ultralow permeability core, which is obviously higher than 20.2% for N₂. With increasing core permeability, the oil recovery ratio for both the scCO₂ and N₂ huff-n-puff processes increases as well. Laboratory results show oil recovery ratios of 33.1% and 47.3% for the scCO₂ process in 0.5 mD and 10 mD cores and 36.3% and 49.1% for the N₂ process in 0.5 mD and 9.5 mD cores, respectively. Although the oil recovery rates for the N₂ and CO₂ huff-n-puff processes are close in the 0.5 mD and 10 mD cores, the effective oil extraction pressure for the scCO₂ puff process is found to be 5~10 MPa, which is much higher than 0~2 MPa for the N₂ puff process. The study's findings can help provide a better understanding of the oil extraction behaviors in the huff-n-puff process with different injection fluids as well as in different low permeability cores.