Investigation of CO2 Displacement Characteristics of Highly Water-Sensitive and Low-Permeability Oil Reservoirs: A Case Study of Block A in Eastern China

Highly water-sensitive, low-permeability reservoirs are difficult to develop by water flooding because clay activation and pore-throat plugging impair injectivity. This study combined X-ray diffraction, scanning electron microscopy, water-sensitivity tests, long-core CO2 flooding, and response surface methodology to investigate Block A in eastern China. The sandstone contained 17.0% clay minerals, mainly illite and kaolinite. After water flooding, clay accumulation, fines migration, and pore-throat bridging produced cumulative permeability damage of 63.09%, confirming substantial water-sensitive impairment. CO2 flooding comprised stable displacement, gas breakthrough, and stable gas channeling. Increasing the injection pressure from 20 to 35 MPa increased oil recovery from 40.78% to 57.91%. The measured CO2–oil minimum miscibility pressure (MMP) was 32.70 MPa. Accordingly, the experiments at 20, 25, and 30 MPa were conducted below the MMP, whereas 35 MPa was slightly above the MMP, indicating that the CO2–oil system had the thermodynamic conditions required to develop multiple-contact miscibility. In contrast, increasing the initial water saturation from 36.59% to 62.04% reduced the oil recovery from 52.13% to 44.21%. Permeability had a minor effect, while stronger heterogeneity reduced recovery to 41.67%. RSM identified the permeability ratio as the dominant factor and revealed significant pressure–saturation and saturation–permeability-ratio interactions. The model predicted a maximum oil recovery of 62.33% at an injection pressure of 35.00 MPa, an initial water saturation of 46.71%, a permeability of 89.58 mD, and a permeability ratio of 8.89. A confirmation experiment conducted near the predicted optimum conditions yielded an oil recovery of 60.42%, corresponding to a relative error of 3.05% from the predicted value. These results support CO2-flooding design for highly water-sensitive, low-permeability reservoirs.