Experimental and Numerical Study on CO2 Sweep Volume during CO2 Huff-n-Puff Enhanced Oil Recovery Process in Shale Oil Reservoirs

CO2 huff-n-puff has been proven to be the most effective enhanced oil recovery (EOR) method in shale oil reservoirs. The injected CO2 will replenish reservoir energy and penetrate the reservoir matrix to extract oil. However, the CO2 sweep volume during the huff-n-puff process has not been accurately evaluated by existing studies. In this paper, the CO2 sweep volume was investigated through experimental and numerical simulation methods. In the experimental study, the CO2 sweep areas were depicted by X-ray computed tomography scan technology. The results indicated that the ratio of the CO2 sweep area was 78.63% in the seventh huff-n-puff cycle, leading to a total oil recovery of 56.80%. The numerical simulation considered the mechanisms of molecular diffusion and nanopore confinement. The results showed that in the first huff-n-puff cycle, the gas sweep volume percentage was 9.47% after 100 days of huff period. In the gas swept volume, oil viscosity was reduced by 25.9% to 68.2%. After three cycles of CO2 injection, the oil recovery manifested a 1.5% increase compared to the case without huff-n puff. The contributions of different parameters on gas sweep volume and cumulative oil recovery were investigated. The results illustrated that the nanopore confinement effect and molecular diffusion had significant impacts on the gas sweep volume and cumulative oil recovery. Higher injection pressure, longer huff time, and more huff-n-puff cycles lead to larger gas sweep volume, as well as cumulative oil recovery. A suitable primary depletion period and a huff-n-puff schedule should be determined based on the requirements of field production. The investigations in this study provide insights into better understanding of the EOR mechanisms and optimizing the design of CO2 huff-n-puff operations in shale oil reservoirs.