A measure of formation porosity $\begin{array}{l}\phi\end{array}$ change due to pressure $\begin{array}{l}p\end{array}$ variation:

(1)	$\begin{array}{l}\displaystyle c_r = \frac{1}{\phi} \frac{\partial \phi}{\partial p}\end{array}$

There is a statistical correlation between initial formation compressibility $\begin{array}{l}c_r(\phi)\end{array}$ and formation porosity which can be picked by various compressibility-porosity models.

Pore compressibility stays constant for small pressure variations but in a wide range of pressure variations the dependence on ambient pressure $\begin{array}{l}c_r(p)\end{array}$ can not be neglected and should be tabulated from laboratory core tests or estimated from compressibility-pressure correlations.

The typical values are:

(2)	$\begin{array}{l}\displaystyle c_r = 0.5 \div 1.5 \cdot 10^{-6} \, {\rm kPa}^{-1}\end{array}$

but may go higher for poorly consolidated rocks.

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