Compressibility of multiphase fluid in thermodynamic equilibrium at a given pressure $\begin{array}{l}p\end{array}$ and temperature $\begin{array}{l}T\end{array}$ is a simple sum of its single-phase components:

(1)	$\begin{array}{l}\displaystyle c_f(p, T) = \sum_{\alpha} s_\alpha \cdot c_\alpha(p,T)\end{array}$

where

$\begin{array}{l}s_\alpha\end{array}$	$\begin{array}{l}\alpha\end{array}$ -phase saturation, subjected to $\begin{array}{l}\sum_{\alpha} s_\alpha = 1\end{array}$
$\begin{array}{l}c_\alpha(p, T)\end{array}$	$\begin{array}{l}\alpha\end{array}$ -phase compressibility as function of pressure $\begin{array}{l}p\end{array}$ and temperature $\begin{array}{l}T\end{array}$

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

(2)	$\begin{array}{l}\displaystyle c_f = s_w \, c_w + s_o \, c_o + s_g \, c_g\end{array}$

where $\begin{array}{l}\{ w, \, o, \, g \}\end{array}$ mean water phase, oil phase and gas phase.

Some applications (like multi-phase pressure diffusion) account for the impact of phase exchange on the total compressibility which require some corrections to equation (2):

(3)	$\begin{array}{l}\displaystyle c_t(s,P) = c_r + c_w s_w + c_o s_o + c_g s_g + s_o [ R_{sp} + (c_r + c_o) R_{sn} ] + s_g [ R_{vp} + R_{vn}(c_r + c_g) ]\end{array}$

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