Influence of water saturation on CO 2 flooding performance and screening of WAG injection timing in tight reservoirs

To clarify water saturation effects on CO2 flooding and optimize post-waterflood WAG, this study integrated PVT phase behavior tests, MMP measurements, microscopic visualization, and WAG displacement experiments at varying water saturations in block X of tight reservoirs. Results show that at 70% water saturation vs. anhydrous conditions, viscosity reduction ratio decreases from 87% to 35%, volume expansion factor from 1.9 to 1.3, density reduction narrows to 15.6%, and solution GOR increase is limited to ∼100 m3/m3. As water saturation increases from 20% to 70%, MMP rises from 20.26 MPa to 23.18 MPa, indicating hindered miscibility. microscopic visualization reveals that high water saturation does not prevent CO2 penetration through water films for miscibility, but markedly inhibits oil-gas mass transfer, causing delayed gas breakthrough, early water production, and reduced light-component extraction (