The thermodynamic equilibrium of the reservoir fluids may end up with the pressure difference between fluid phases (called capillary pressure) due to difference in fluids interaction with rocks:

(1)	$\begin{array}{l}\displaystyle P_{cow} = p_o - p_w\end{array}$

(2)	$\begin{array}{l}\displaystyle P_{cog} = p_o - p_g\end{array}$

In most popular practical case of a 3-phase fluid model this will be:

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See also