Xiao Meng, Zhang Yongbo, Shan Xuanlong, Wang Shuyang, He Wentong, Shen Yufeng, Bian Ying, Jiang Nan, Kong Le
World Geology. 2026, 45(1): 104-114.
To further elucidate the microscopic mechanism by which supercritical carbon dioxide (SCCO2
)
alleviates water-lock in tight sandstones, the authors selected tight sandstones from the Shahezi Formation in Lishu
Fault Depression of Songliao Basin as the study object and systematically investigated the changes in pore structure
and water occurrence state of the reservoir during water saturation, water-locking formation, conventional N2
flooding,
and SCCO2
miscible treatment. The experiments comprised vacuum water saturation and high-pressure water-retention
water-lock simulation, N2
flooding under different pressure differentials, water-SCCO2
miscible treatment followed by
N2
gas flooding, and together with nuclear magnetic resonance (NMR) T2
spectrum inversion, pore size distribution
analysis, and cast thin-section observations, quantitatively characterized the stage-by-stage variations in porosity, pore
throat scale, and movable water proportion. Water saturation experiments indicate that the initial water saturation of the
tight sandstone is lower than the bound-water saturation, and the water-phase invasion process can be divided into
three stages: 1 min to 5 min is a rapid saturation stage, during which a large amount of water rapidly occupies
micro-nano pores; 5 min to 5 h is a slow invasion stage with continuously increasing water content; after 5 h, the
system becomes essentially stable, suggesting a short formation time and strong stability of the water-locking.
Conventional N2
flooding exhibits pronounced pore-size selectivity: when the pressure differential reaches 3 MPa,
water in large pores ( >300 nm) is displaced first; when the pressure differential increases to 4-6 MPa, water in
80-300 nm pores is only partially removed, whereas water in finer pores remains difficult to displace, and the
overall flooding efficiency remains below ~30%. SCCO2
miscible treatment markedly alters the pore-throat structure
of the cores: the low viscosity and high diffusivity of SCCO2
enable it to enter dominant flow pathways and react with
carbonate cements in pores via acid dissolution, resulting in a 40%-80% increase in porosity, an enlarged mean
pore-throat radius, and enhanced pore connectivity. The results demonstrate that, after SCCO2
treatment, the sand
stone pore system becomes more open and the degree of water-phase confinement decreases; compared with conven
tional N2
flooding, the miscible-treatment-gas-flooding combination is more favorable for alleviating water-locking in
micro-nano pores, and this coupled workflow shows clear advantages in improving pore structure and enhancing
flooding efficiency in tight sandstones.
