Oil production performance and reservoir damage distribution of miscible CO2 soaking-alternating-gas (CO2-SAG) flooding in low permeability heterogeneous sandstone reservoirs

Miscible CO2-SAG flooding is an enhanced form of CO2 flooding, which mitigates the inadequate CO2-crude oil interaction by adding a CO2 soaking period just after CO2 breakthrough (BT). The addition of the soaking process results in improvements in the recovery of residual oil during the secondary CO2 flooding process, which may vary across the reservoir, and lead to differential changes in the distribution of permeability decline and wettability variation due to asphaltene precipitation. In this work, CO2-SAG and simple miscible CO2 flooding experiments were carried out at reservoir conditions (90°C, 23 MPa) on low permeability long composite cores with progressively decreasing permeability in the direction of injection. The results show that the overall oil recovery factor (RF) was 72.8% after CO2-SAG flooding, 11% higher than simple miscible CO2 flooding (61.8%). The oil RFs of individual core plugs decreased progressively along the core (i.e., with decreases in initial permeabilities). The inadequate interaction occurs more for those cores with low permeability and high residual oil saturation close to the outlet. Consequently, recovery is improved more in lower permeability cores, particularly in medium sized pores of these cores. By contrast, the permeability decline of core plugs after CO2-SAG flooding was 8-20%, 1.0-4.5% higher than that after simple miscible CO2 flooding. The permeability decline of the cores close to the injection end showed a slight increase, and continued to decrease along the injection direction after CO2 flooding. However, the overall decline in core permeability at the injection end was greater than that at the outlet. The soaking process led to a more homogeneous distribution of permeability decline and a greater increase in permeability decline. The distribution of wettability variation was consistent with the distribution of residual oil saturation after CO2 flooding. The wettability variation was larger and the cores in middle had the biggest values after SAG flooding.