Pore scale performance evaluation and impact factors in nitrogen huff-n-puff EOR for tight oil

Abstract

Nitrogen huff-n-puff (N2 HnP) appears to be an economical and high-efficiency enhanced oil recovery (EOR) technique for tight oil reservoirs. There is however a lack of understanding of the pore-level EOR performance of N2 HnP under tight reservoir conditions. In this work, a non-magnetic reactor was created and combined with a nuclear magnetic resonance (NMR) device for real-time monitoring of oil distribution in the HnP experiment. N2 HnP experiments were then performed in a tight sandstone core sample at a temperature of 353 K and an injection pressure ≥ 24 MPa. The pore-level oil distribution under reservoir conditions was monitored and the EOR performance of N2 HnP in specific pores was analyzed. The pore throat structures of the core sample and the phase behavior of the N2-oil system were analyzed to elucidate the EOR mechanism of N2 HnP. An oil recovery factor of 37.52% can be achieved after four cycles, which proves the EOR potential of N2 HnP for tight reservoirs. The highest recoveries after N2 HnP are obtained in the large pores, followed by the medium pores, the small pores, and finally the micro pores. Increases in soaking time and injection pressure resulted in slight and pronounced increases in oil recovery, respectively, both of which are mainly reflected in the first cycle. Specifically, increasing the soaking time only slightly improves the cumulative oil recovery in the small pores while increasing the injection pressure significantly improves the cumulative oil recovery in the small, medium, and large pores simultaneously. However, variations in both injection pressure and soaking time have a negligible effect on the cumulative oil recovery of the micro pores.