Quantitative study of CO2 huff-n-puff enhanced oil recovery in tight formation using online NMR technology

Recent evidence suggests that CO 2 huff-n-puff can significantly enhance the oil recovery in tight/shale oil reservoirs after primary depletion. However, the key driving forces of CO 2 huff-n-puff for oil displacement, especially their effect on the enhanced oil recovery are still unclear. In addition, the CO 2 effective distance in matrix, which is a critical parameter impacting the performance of huff-n-puff, has not been accurately studied. In this work, to identify and quantify the contribution of the related enhanced-oil-recovery (EOR) mechanisms, an online nuclear magnetic resonance (NMR) instrument was used to monitor the real-time migration of the matrix oil at all stages of the CO 2 huff-n-puff process. The experiment results show that the dissolved CO 2 drive controlled by molecular diffusion is the dominant mechanism during the huff-n-puff process. This EOR mechanism mainly works on the oil stored in the macropores, while the oil stored in the micropores is hardly mobilized. Base on the dynamic oil saturation distributions converted from the 1-D NMR profiles during huff-n-puff, we designed a novel 2-D evaluation system, which focuses on microscopic displacement efficiency and CO 2 effective distance, to investigate the effect of the operation parameters on oil production. We found that simply increasing the injection pressure mainly improves the displacement efficiency but has less effect on the expansion of the sweep area, which limits the CO 2 huff-n-puff EOR on the reservoir scale. Lengthening the soaking period extends the effective distance significantly but delays oil production. A cyclic huff-n-puff scheme with variable soaking time was proposed to maintain a high oil production rate and a large sweep area. Periodic optimization of the soaking time in the cyclic CO 2 huff-n-puff process is needed for an optimal development plan. • The related EOR mechanisms in the CO 2 huff-n-puff were distinguished and quantified. • The dissolved gas drive is demonstrated as the dominant driving force in the CO2 huff-n-puff process. • A novel 2-D evaluation system was proposed to investigate the CO 2 huff-n-puff EOR. • Periodical optimization of the soaking time in the cyclic CO2 huff-n-puff process is needed for EOR.