= Capillary residual trapping
{title2=$s\in[0,1]$}
When a nonwetting phase recedes from pores, <surface tension> and pore geometry can retain a residual saturation $s$. If $h$ is the mobile invaded thickness and $M$ its maximum past value, stored phase volume per plan area is $\phi[(1-s)h+sM]$. Newly invading pores have $M=h$; during recession $M$ remains fixed. With mobile <Darcy velocity> $u$, conservation gives advance speed $u/\phi$ and recession speed $u/[\phi(1-s)]$. The latter is faster for $0<s<1$.
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