A measure of formation porosity change due to pressure
variation:
c_r = \frac{1}{\phi} \frac{\partial \phi}{\partial p} |
It stays constant for small pressure variations:
)
but for a wide range pressure variations the dependence on ambient pressure can not be neglected and can be tabulated experimentally or estimated from popular compressibility-porosity correlations on
.
The typical values are in but may go higher for poorly consolidated rocks.