It can be assessed from laboratory stress tests on cores.

There is also a number of popular correlations on initial formation porosity $\begin{array}{l}\phi_i\end{array}$ summarized in the table below:

Corrrelation

Name/Author

Scope

(1)	$\begin{array}{l}\displaystyle c_\phi = 2.71 \cdot \phi_i^{-0.415} \cdot 10^{-7} \ \textrm{kpa}^{-1}\end{array}$

Hall

Consolidated sandstone under typical formation pressure

(2)	$\begin{array}{l}\displaystyle c_\phi = 1.45 \cdot \exp \big[ 4.026 - 23.07 \ \phi_i + 44.28 \ \phi_i^2 \big] \cdot 10^{-7} \ \textrm{kpa}^{-1}\end{array}$

Newman

Consolidated limestone, $\begin{array}{l}0.02 < \phi_i < 0.33\end{array}$

(3)	$\begin{array}{l}\displaystyle c_\phi = 1.45 \cdot \exp \big[ 5.118 - 32.26 \ \phi_i + 63.98 \ \phi_i^2 \big] \cdot 10^{-7} \ \textrm{kpa}^{-1}\end{array}$

Newman

Consolidated sandstone, $\begin{array}{l}0.02 < \phi_i < 0.33\end{array}$

(4)	$\begin{array}{l}\displaystyle c_\phi = 1.45 \cdot \exp \big[ 34.012 \cdot ( \phi_i - 0.2) \big] \cdot 10^{-7} \ \textrm{kpa}^{-1}\end{array}$

Newman

Unconsolidated sandstone, $\begin{array}{l}0.2 < \phi_i < 0.5\end{array}$

Reference

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