Effects of Kerogen on the Flow and EOR Performance of Oil in Shale Cores during CO2 Flooding Process Investigated by NMR Technology

Summary Carbon dioxide (CO2) flooding is a common method for enhancing oil recovery in conventional and tight sandstone reservoirs. Nevertheless, CO2 flooding suitability for shale oil reservoirs with low permeability, low porosity, and rich organic matter (kerogen) remains controversial. In this study, the kerogen effects on the flow and enhanced oil recovery (EOR) behavior of oil in shale cores during CO2 flooding were investigated using nuclear magnetic resonance (NMR). The effects of injection pressure, temperature, and injected pore volume (PV) on the EOR properties of shale cores were considered. Moreover, the flow mechanism of oil displacement by CO2 flooding in shale was discussed by measuring the oil content variations in different core sections. The recovery was lower for the shale cores than for the tight sandstone cores; moreover, the shale oil recovery increased with increased injection pressure, temperature, and injected PV. The recovery was approximately 30% even after 7 PV of CO2 flooding at 18 MPa and 45°C. During the CO2 flooding process, the oil near the core inlet flowed more easily than that near the outlet, indicating CO2oil miscible fluid formation near the inlet, hampering the displacement of further saturated oil in the core. The kerogen presence in shale reduced the oil mobility and substantially decreased the oil recovery owing to the strong interactions between kerogen and oil. High injection pressure and injected PV increased the oil-in-shale flow performance and enhanced the shale oil recovery in CO2 flooding to a certain extent.