Pore-Scale Oil Mobilization during CO2 Water-Alternating-Gas Flooding in a Low-Permeability Medium-Viscosity Oil Reservoir

Abstract Water-alternating-gas (WAG) flooding has been widely applied for enhanced oil recovery. However, how it mobilizes oil in pores of different sizes remains poorly understood in low-permeability reservoirs with medium-viscosity oil. Core flooding experiments with online nuclear magnetic resonance (NMR) measurements compared water flooding, CO2 flooding, and WAG flooding using cores and fluids representative of the Niuquanhu block. A calibrated conversion between the transverse relaxation time T2 and the pore diameter, with its uncertainty propagated, related the class boundaries of 10 and 100 ms to 0.5 and 5 μm. A segmentation algorithm showed that these boundaries lie within the two pooled transition bands of the mobilization profiles, which shift with the pore-size modes of each core. The mobilization degree of each pore class was quantified by validated trapezoidal integration of the T2 spectra. Shifting the residual-oil spectra by 0.05 decade, which bounds the effect of dissolved CO2, changed the class-wise values by at most 8.45 percentage points. A shift of 0.10 decade changed the values of WAG flooding on Core 1 by at most 4.70 percentage points, and the ranking remained unchanged. On the metered basis, WAG flooding recovered 88.82% of the oil, 1.32 times that of CO2 flooding, and 3.60 times that of water flooding. On the NMR basis, its ratio to water flooding was 2.30. WAG flooding raised the micropore mobilization degree from 25.57% under water flooding to 78.45%. Most oil was produced within six injection cycles, and the second cycle alone recovered 40.30% of the initial oil. Smaller slugs performed better, and the recovery reached 75.54% at a slug size of 0.1 pore volume (PV). The best gas-water ratio tested was 3:1, giving a recovery of 76.76%. The trends are consistent with a capillary-viscous interpretation, and the optima are expressed in the dimensionless form for similar reservoirs.