A measure of rock volume change due to pressure p variation:
(1) | c_r = \frac{1}{V} \frac{\partial V}{\partial p} |
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).
The typical values are in
0.5 \div 1.5 \cdot 10^{-6} \, {\rm kPa}^{-1} but may go higher for poorly consolidated rocks.