A comparative study of CO2 and N2 huff-n-puff EOR performance in shale oil production

In this study, Huff-n-Puff (HnP) experiments were conducted to investigate the potential for producing hydrocarbons from the shale core samples using CO2 HnP and N-2 HnP, respectively. The compositions of crude oil, produced oil, and gas in the system were analyzed by Gas Chromatography/Mass Spectrometer (GC/MS). The results showed that at the end of six injection cycles, the cumulative oil recovery factors were 61.49% for CO2 injection and 34.85% for N2 injection. CO2 had better injectivity in the ultra-low permeability shale cores than N-2. The injected gas extracted and vaporized the light hydrocarbon components C3-8 into the gas phase. After six HnP cycles, the recovery factors of C3-4 were 91.6% for CO2 HnP and 75.1% for N-2 HnP. For the other components, the recovery factors were about 60% for CO2 HnP and 30% for N-2 HnP. After that, a core scale model was established to mimic the process of HnP and the mechanisms leading to the different performances of CO2 HnP and N-2 HnP were discussed. Different properties relating to the EOR mechanisms during HnP process were investigated. The results indicated that a significant amount of CO2 could dissolve into the crude oil and deposit as free gas phase existing in the core. During the CO2 HnP process, the oil and gas interfacial tension reduced to zero, and the oil viscosity was reduced by 40%-60%. The gas cap drive, dissolved gas drive, and miscible drive during CO2 HnP are more efficient than that during N-2 HnP. The change of the molecular diffusion rate resulted in an obvious change in oil recovery factor. Although simultaneous CO2 and N-2 is still improving the recovery, pure CO2 shows the best result. This study provides operators with valuable laboratory data about the produced hydrocarbon analysis on CO2 HnP and N-2 HnP in shale oil production and a better understanding of gas HnP mechanisms.