The long-term storage of CO₂in deep geological formations would lose effectiveness if CO₂leaks through the cap rock, joints, and improperly grouted wells. Plug tests conducted on shale and cement specimens highlight the importance of hairline cracks in otherwise intact small-pore materials. Crack sealing is attempted under pressure by injecting suspensions of bentonite and kaolin; the sub-micron particles successfully fill cracks, and the CO₂-breakthrough pressure increases significantly above initial values. The sealing strategy can be optimally engineered for field applications during or after CO₂ injection in order to prevent CO₂ leakage.