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 CO<sub>2</sub> (scCO<sub>2</sub>) and N<sub>2</sub> 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 CO<sub>2</sub> huff-n-puff process than in the N<sub>2</sub> process. The CO<sub>2</sub> huff-n-puff operation could produce 29.9% oil in the ultralow permeability core, which is obviously higher than 20.2% for N<sub>2</sub>. With increasing core permeability, the oil recovery ratio for both the scCO<sub>2</sub> and N<sub>2</sub> huff-n-puff processes increases as well. Laboratory results show oil recovery ratios of 33.1% and 47.3% for the scCO<sub>2</sub> process in 0.5 mD and 10 mD cores and 36.3% and 49.1% for the N<sub>2</sub> process in 0.5 mD and 9.5 mD cores, respectively. Although the oil recovery rates for the N<sub>2</sub> and CO<sub>2</sub> huff-n-puff processes are close in the 0.5 mD and 10 mD cores, the effective oil extraction pressure for the scCO<sub>2</sub> puff process is found to be 5~10 MPa, which is much higher than 0~2 MPa for the N<sub>2</sub> 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.