N2 and CO2 Huff-n-Puff for Enhanced Tight Oil Recovery: An Experimental Study Using Nuclear Magnetic Resonance

Gas huff-n-puff (HnP) is considered a promising method for enhanced oil recovery (EOR) in tight reservoirs. An accurate comparison of the EOR performance between N2 and CO2 HnP at the pore level is still lacking. In this work, N2 and CO2 HnP experiments were performed in tight cores with a nuclear magnetic resonance (NMR) instrument. The pore-level oil distribution at reservoir temperature and pressure was monitored online by NMR signals, and the ability of NMR technology to measure the oil recovery in different pores was explored. The result shows that both N2 and CO2 HnP can significantly improve tight oil recovery, but CO2 HnP presents better performance. There is less difference in oil recovery between N2 and CO2 HnP after the first cycle; however, the difference in oil recovery increases with increasing HnP cycles. The variation of the NMR transverse relaxation time (T2) spectrum indicates that CO2 HnP is significantly more capable of enhancing oil recovery in large pores than N2 HnP, but N2 and CO2 HnP is comparable in their ability to enhance oil recovery in small pores. However, the above conclusion is not rigorous as a result of the error of NMR technology in measuring the oil recovery of specific pores. The composition of the remaining oil changes during gas HnP, resulting in no longer a fixed correspondence between the T2 value and the pore size. This may cause the oil recovery calculated using the T2 spectrum to be high for large pores and low for small pores, especially for CO2 HnP.