The difference between phase pressures due to difference in fluids interaction with rock matrix:

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

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

One of the strong assumptions in reservoir fluid dynamic modelling is that 3-phase capillary pressure is a function of one saturation only:

(3)	$\begin{array}{l}\displaystyle p_{cog} = p_{cog}(s_o)\end{array}$

a function of oil saturation only $\begin{array}{l}s_o\end{array}$

(4)	$\begin{array}{l}\displaystyle p_{cow} = p_{cow}(s_w)\end{array}$

a function of water saturation only $\begin{array}{l}s_w\end{array}$

Mathematical model of capillary pressure is given by interpolation of laboratory data and also by a number of physical and empirical capillary pressure correlations

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