(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:
| (2) | c_r(p) = \rm const |
For wide range pressure variations formation compressibility the dependence of ambient pressure can not be neglected:
| (3) | c_r = c_r(p) |
The pressure dependence can be estimated experimentally or via popular compressibility-porosity correlations on
c_r(\phi).