Pore-scale monitoring of CO2 and N2 flooding processes in a tight formation under reservoir conditions using nuclear magnetic resonance (NMR): A case study

• Published in: Fuel (Guildford) 2019-06, Vol.246, p.34-41
• Main Authors: Wei, Bing, Zhang, Xiang, Wu, Runnan, Zou, Peng, Gao, Ke, Xu, Xingguang, Pu, Wanfen, Wood, Colin
• Format: Article
• Language: English
• Subjects: Carbon dioxide; Case studies; CO2 and N2 flooding; Depletion; Displacement; Flooding; Gas injection; High temperature and pressure; Low-field nuclear magnetic resonance (NMR); NMR; Nuclear magnetic resonance; Oil recovery; Pore-scale displacement behaviors; Pores; Porosity; Reduction; Reservoirs; Resonance; Saturation; Spatial distribution; Tight sandstone reservoirs
• ISSN: 0016-2361; 1873-7153
• DOI: 10.1016/j.fuel.2019.02.103

•CO2 and N2 flooding experiments were performed in tight cores under high pressure and temperature.•Low-field NMR technique was used to monitor pore-scale displacement and quantify improvement.•CO2 flooding created a uniform displacement front due to the local oil-CO2 interactions.•The small and large pores nearly contributed equally to the overall oil recovery for N2 flooding.•The incremental oil recovery was mainly contributed by large pores for CO2 flooding. Recent reports have demonstrated that gas injection can improve oil recovery of tight reservoirs after natural depletion, with major projects in progress worldwide. There is however a lack of understanding of the gas displacement behaviors at pore-scale especially under reservoir conditions. Herein we conducted an experimental investigation of CO2 and N2 flooding in the Lucaogou tight formation at 35.0 MPa and 80 °C. The flooding dynamics were continuously monitored by use of low-field nuclear magnetic resonance (NMR) to map the displacement processes at pore-scale and quantify the reduction of oil saturation in-situ. We observed that the overall oil recoveries of CO2 and N2 flooding were 38.4% and 35.1%, respectively. Due to the interactions between the oil phase and CO2, a uniform displacement along the core plug was yielded by CO2 flushing, indicated by the 1D spatial distribution of local oil saturation and 2D transverse images. Interestingly, during CO2 flooding, the major reduction of NMR T2 signals occurred in the large pores (4.2 ms