 Capillary entry pressure

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.