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:

c_r = 0.5 ÷ 1.5 GPa^-1

but may go higher for poorly consolidated rocks.

In many practical cases the pore compressibility can be considered as poorly dependent on reservoir pressure variation.

But in case the reservoir pressure is changing substantially one may need to account for the effect it takes on pore compressibility (see Pore compressibility-pressure correlations).

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