NMR-Based Characterization of Pore Structure Alterations in Tight Sandstones Due to scCO ₂ -Water-Rock Interactions during CO ₂ Displacement

CO 2 enhanced oil recovery (CO 2 -EOR) serves as an effective method for developing tight reservoirs while sequestering CO 2 . However, CO 2 -water-rock interactions alter rock properties, impacting recovery and storage. This study examines these interactions in tight sandstone through a combination of static high-pressure reactor tests and dynamic displacement experiments, with pore-scale changes monitored using Nuclear Magnetic Resonance (NMR) and Scanning Electron Microscopy (SEM). Results show that, under precipitation-dominated conditions, scaling ions in formation water decrease significantly, especially Ca 2+ (from 5746.32 to 4997.64 mg/L as pressure and temperature rise). Pore blockage evolves as the injected volume increases, reaching 18.74% at 8 pore volumes (PV), progressing from large to small pores. Increased temperature enhances carbonic acid dissociation and precipitation, reducing permeability and porosity by 20.37% and 19.68% at 60 °C. Pressure exerts a stronger influence: at 15 MPa, blockage peaks at 23.67%, while at 20 MPa, dissolution dominates, creating secondary micropores, improving connectivity, and reducing the blockage rate to 15.31%. Lower permeability cores experience higher blockage (up to 24.68% at 0.003 × 10 –3 μm 2 ), as fine particles and secondary minerals preferentially clog small pores. This work explains how CO 2 -water-rock reactions influence pore structure, clarifies blockage behavior under various factors, and guides efficient tight oil development and field optimization.