A measure of formation porosity \phi change due to pressure p variation:
(1) | c_r = \frac{1}{\phi} \frac{\partial \phi}{\partial p} |
There is a statistical correlation between initial formation compressibility c_r(\phi) and formation porosity which can be picked by various compressibility-porosity models.
It stays constant for small pressure variations but for a wide range pressure variations the dependence on ambient pressure c_r(p) can not be neglected and can be tabulated experimentally or estimated from compressibility-pressure correlations.
The typical values are in
0.5 \div 1.5 \cdot 10^{-6} \, {\rm kPa}^{-1} but may go higher for poorly consolidated rocks.