2-phase fluid model based on three pseudo-components $\begin{array}{l}C = \{ G, W \}\end{array}$ :

$\begin{array}{l}G\end{array}$	dry gas pseudo-component
$\begin{array}{l}W\end{array}$	water pseudo-component, which may include minerals (assuming formation water and injection water composition is the same)

existing in two possible phases $\begin{array}{l}\alpha = \{ g, w \}\end{array}$ :

$\begin{array}{l}g\end{array}$	gas phase, consisting of natural gas pseudo-component and vaporized dead oil pseudo-component (called volatile oil)
$\begin{array}{l}w\end{array}$	water phase, consisting of water component only

The volumetric phase-balance equations is:

(1)	$\begin{array}{l}\displaystyle s_g+s_w =1\end{array}$

where

$\begin{array}{l}s_g = \frac{V_g}{V}\end{array}$	share of total fluid volume $\begin{array}{l}V\end{array}$ occupied by gas phase $\begin{array}{l}V_g\end{array}$
$\begin{array}{l}s_w = \frac{V_w}{V}\end{array}$	share of total fluid volume $\begin{array}{l}V\end{array}$ occupied by water phase $\begin{array}{l}V_w\end{array}$

The accountable cross-phase exchanges are illustrated in the table below:

	$\begin{array}{l}g\end{array}$	$\begin{array}{l}w\end{array}$
$\begin{array}{l}G\end{array}$	x
$\begin{array}{l}W\end{array}$		x

It's a typical case for reservoirs with poor or no aquifer.

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