= Capillary entry pressure
{title2=$P_e\simeq2\gamma|\cos\theta|/r$}
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.
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