A measure of formation porosity change due to reservoir pressure variation:

c_\phi = \frac{1}{\phi} \frac{\partial \phi}{\partial p}

Pore compressibility stays constant for small pressure variations but in a wide range of pressure variations the dependence on ambient pressure can not be neglected and should be tabulated from laboratory core tests or estimated from compressibility-pressure correlations.

The typical values are:

c_ϕ = 0.5 ÷ 1.5 GPa^-1

but may go higher for poorly consolidated rocks.

In many practical cases the pore compressibility can be considered as poorly dependent on reservoir pressure variation: .

In this case porosity dependence on reservoir pressure can be simulated as:

\phi(p) = \phi_i \cdot \left[  1 + c_\phi \, (p-p_i)  + 0.5 \, c^2_\phi \, (p-p_i)^2 \right]

But in case the reservoir pressure is changing substantially one may need to account for the effect it takes on pore compressibility (see Pore compressibility @model) and then reservoir pressure - porosity model is going to take the following form:

\phi(p) = \phi_i \cdot \exp \left[ \int_{p_i}^p c_\phi\, dp  \right]

See Also