Control Mechanism of Water Shield on Pressure Dynamics of CO2 Huff-n-Puff and Crude Oil Mobilization Effect in Tight Reservoirs

Tight oil reservoirs are characterized by poor petrophysical properties, ultra-low permeability, and limited pressure-transmission capacity. After hydraulic fracturing, retained fracturing fluid can accumulate near the fracture–matrix interface and form a water shield, which restricts carbon dioxide (CO2) seepage, CO2–oil contact, and crude-oil mobilization during CO2 huff-n-puff. To clarify the control mechanism of a fracture-adjacent water shield on pressure dynamics and pore-scale oil mobilization in tight reservoirs, three stepwise core experiments were designed: a non-fractured core without a water shield (#E1), a single-fracture core without a water shield (#E2), and a single-fracture core with a MnCl2-induced water shield (#E3). Pressure monitoring, cumulative nuclear magnetic resonance (NMR) T2 spectra, and NMR imaging were integrated to compare the matrix, fracture, and water-shield effects. The results show that the water shield does not act as a constant resistance during cyclic CO2 huff-n-puff. In Cycle 1, the water shield restricts CO2–oil contact, and the relative pressure-decline amplitude of #E3 is approximately 10% lower than that of #E2. In Cycle 2, the relative pressure-decline amplitude of #E3 increases to 18.4%, approximately 1.8 times that of #E2, indicating a local water-shield transition rather than uniform matrix sweeping. The T2-derived recovery degrees of #E3 are 7.24%, 15.34%, 7.96%, and 4.07% from Cycles 1 to 4, respectively, and its cumulative recovery after four cycles is 34.61%, which is 7.32 percentage points lower than that of #E2. NMR imaging further shows that the swept region in #E3 is mainly concentrated near the fracture after Cycle 2, while matrix regions away from the fracture retain strong oil signals. These results indicate that the water shield first acts as a fracture-adjacent water-phase barrier and then undergoes a local transition into a preferential pressure-dissipation pathway. The findings provide a basis for interpreting pressure decline together with NMR evidence and for optimizing CO2 huff-n-puff operations in tight reservoirs affected by retained fracturing fluid.