(see Compressibility @ Wikipedia)
A measure of formation porosity \phi change due to pressure p variation:
(1) | c_r = \frac{1}{\phi} \frac{\partial \phi}{\partial p} |
The above correlation assumes constant which holds true in many practical applications.
It stays constant for small pressure variations ( c_r(p) = \rm const) but for a wide range pressure variations the dependence on ambient pressure c_r(p) can not be neglected and can be estimated experimentally or via popular compressibility-porosity correlations on c_r(\phi).