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The threshold pressure difference required for a nonwetting fluid to invade a connected pore throat. In a simple cylindrical throat of radius $r$, the magnitude is approximately $2\gamma |\cos \theta |/r$, using surface tension $\gamma$ and contact angle $\theta$. A sealing caprock can fail to retain a buoyant phase if its pressure exceeds this threshold or if connected fractures bypass the fine pore network.

Potential leakage paths are permeable or reactivated faults and connected fractures cutting the sealing sequence; lateral migration through connected reservoir layers into an unsealed outcrop or spill point; and natural or artificial penetrations such as poorly sealed abandoned wells or a compromised injection well. Injection raises pore pressure, potentially overcoming the capillary entry pressure of a seal, hydraulically opening fractures, or causing fault reactivation through reduced effective stress.

Seal continuity and porous permeability matter more than the rock name alone: a fractured or porous limestone need not be an effective caprock. Dissolution, chemical alteration or well-cement degradation can change leakage pathways over time. The underlying salt may be an effective barrier beneath the reservoir, but it does not seal the roof. Conversely, dissolved or residually trapped carbon dioxide is less mobile than a connected buoyant free-fluid phase, so connected porosity and fluid state also affect leakage.

An aquifer containing saline rather than fresh water. A deep saline aquifer can provide pore space for carbon capture and storage, but safe retention depends on reservoir connectivity, continuous caprock and pressure management.