Pore-scale monitoring of CO_2 and N_2 flooding processes in a tight formation under reservoir conditions using nuclear magnetic resonance (NMR): A case study

摘要

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 N-2 flooding in the Lucaogou tight formation at 35.0 MPa and 80 degrees 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 N-2 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 T-2 signals occurred in the large pores (4.2 ms < T-2 < 252.4 ms), whereas in the case of N-2 flooding, the main change was found both in small pores (T-2 < 4.5 ms) and large pores (4.5 ms < T-2 < 432.3 ms). The results of this study supplement earlier observations from pore scale and can provide insights into oil recovery improvement of tight reservoirs.