The primary objective of this study is to investigate the evolution mechanism of CO 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 CO 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 CO 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 μm to 1.81×10 μm. 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 CO transport increases with increasing the separation pressure of the water films

leading to high CO 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 CO migration.