Monitoring oil displacement and CO2 trapping in low-permeability media using NMR: A comparison of miscible and immiscible flooding

Abstract Dynamic monitoring of the internal fluid distribution during CO2 immiscible/miscible flooding provides a crucial window for illuminating key processes controlling displacement and transport. We used online low-field nuclear magnetic resonance (NMR) imaging to monitor CO2 flooding under different reservoir pressures in low-permeability siltstone core plugs pre-saturated with stock tank oil from the Chang72 Formation in China. Immiscible and miscible styles of fluid displacement were triggered by flooding at pressures of 8 and 16 MPa, respectively. To visualize the oil distribution, NMR T2 and T1–T2 spectra and one-dimensional frequency (1D-FQ), and magnetic resonance imaging (MRI) were conducted to obtain the overall and spatially distributed saturations. The results show that recovery by miscible flooding (69.4%) was twice more than that by immiscible flooding (32.6%). The effect of the extraction of lighter components on oil viscosity and density after CO2 injection was negligible compared to the changes in miscibility. CO2 concentrations at the leading edge of the miscible zone (0.526) were higher than in the immediately adjacent transition zone (0.1); the dimensionless widths were 0.76 and 0.44, respectively. The CO2 retention rate following miscible flooding was 10.59% higher than that due to immiscible flooding, primarily due to remnant dissolved CO2 in the oil. In contrast, immiscible flooding limited the solubility of CO2 and led to solubility and residual trapping. These observations have a significant impact on defining oil and CO2 budgets, modes of transport, and storage for enhanced oil recovery and CO2 sequestration.