Study of thickened CO2 flooding for enhancing oil recovery in Low-Permeability sandstone reservoirs

This study aims to explore the application of thickened CO2 flooding technology in low-permeability sandstone reservoirs. Through minimum miscibility pressure tests, core flooding experiments, CT scanning, and nuclear magnetic resonance (NMR) imaging, the study focuses on analyzing the enhanced oil recovery (EOR) effect and mechanisms. The results of the minimum miscibility pressure tests show that at 40 °C, the minimum miscibility pressure of thickened CO2 and crude oil is between 16–17 MPa. The competition for dissolution between the thickener and crude oil in CO2 leads to a slight increase in the minimum miscibility pressure. Core flooding experimental results indicate that after waterflooding, the oil recovery increments for 0.5 %, 1.0 %, 1.5 %, and 2.0 % thickened CO2 continuous gas flooding and 1.0 % thickened CO2 water-alternating-gas (WAG) flooding with a 1:1 ratio are 22.61 %, 26.80 %, 26.46 %, 27.78 %, and 27.67 %, respectively, which are significantly higher than the 8.07 % obtained by pure CO2 flooding. The use of thickened CO2 for WAG flooding reduces the amount of thickener by about 2/3 while maintaining the same oil recovery. NMR T2 spectra and oil signal imaging results show that thickened CO2 effectively mobilizes small and intermediate pores that were not affected by waterflooding and pure CO2 flooding, significantly improving oil recovery. CT scanning further reveals that the thickener can be retained in pores with a radius of 20–68 μm, causing pore shapes to shift from triangular and circular to square. The retained thickener increases the flow resistance of CO2, forcing the flow to divert, which enhances sweep efficiency. However, the retention of the thickener also reduces the ability of CO2 to mobilize residual oil in the low-permeability zones at the core's end. In conclusion, thickened CO2 flooding technology demonstrates significant technical advantages in low-permeability sandstone reservoirs, providing effective theoretical and experimental support for improving oil recovery in low-permeability sandstone reservoirs. © 2024 Elsevier Ltd