Phase pressure = pw, po, pg

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

(3)	$\begin{array}{l}\displaystyle P_{cgw} = p_g - p_w\end{array}$

where

Some engineering applications make fair use of the concept of Average phase pressure $\begin{array}{l}p\end{array}$ :

(4)	$\begin{array}{l}\displaystyle p = \frac{1}{3} \, ( p_w + p_o + p_g )\end{array}$

(5)	$\begin{array}{l}\displaystyle p = \frac{1}{2} \, ( p_w + p_o )\end{array}$

(6)	$\begin{array}{l}\displaystyle p = \frac{1}{2} \, ( p_o + p_g )\end{array}$

(7)	$\begin{array}{l}\displaystyle p = \frac{1}{2} \, ( p_w + p_g )\end{array}$

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