N2and CO2Huff-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 N2and CO2HnP at the pore level is still lacking. In this work, N2and CO2HnP 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 N2and CO2HnP can significantly improve tight oil recovery, but CO2HnP presents better performance. There is less difference in oil recovery between N2and CO2HnP 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 CO2HnP is significantly more capable of enhancing oil recovery in large pores than N2HnP, but N2and CO2HnP 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 T2value and the pore size. This may cause the oil recovery calculated using the T2spectrum to be high for large pores and low for small pores, especially for CO2HnP.