Abstract: The primary objective of this study is to investigate the evolution mechanism of CO₂ breakthrough pressure in mudstone caprock under different effective stresses. This study specifically investigates the silty mudstone caprock in the Bohai Bay Basin, China, and conducted a series of experiments on breakthrough pressure and permeability under different effective stresses. Whereas, the evolution process of CO₂ breakthrough pressure in mudstone caprock was investigated, and the mechanism of pore water film affecting the breakthrough pressure was discussed. The results of this study illustrate that during the CO₂ injection process (reduction in effective stress on the caprock), the effective stress decreases from 27 MPa to 7 MPa, while the caprock permeability increases from 1.46×10⁻⁶ μm² to 1.81×10⁻⁶ μm². When the effective stress is 5.2 MPa, the minimum breakthrough pressure of caprock is 3 MPa, which exceeds the minimum sealing threshold of 2 MPa. The results of breakthrough pressure tests indicate that the caprock possesses effective sealing capability. The disjoining pressure and distribution characteristics of the pore water films are the main factors influencing the breakthrough pressure. The resistance of CO₂ transport increases with increasing the separation pressure of the water films, leading to high CO₂ breakthrough pressure. The pore throat radius of mudstone samples range from 0.1 nm to 2.5 nm in the Bohai Bay Basin, and the corresponding disjoining pressures of water films range from 5.2 MPa to 50 MPa. The water films has strong constraint ability on CO₂ migration.

Key words: CO₂ geological storage, silty mudstone, effective stress, breakthrough pressure, water film separation pressure, caprock sealing capacity