A pore volume V_\phi fraction of bulk rock volume V_r containing the hydrodynamically connected fluids (also called free fluids) within each pore element:
| (1) | \phi_e = \frac{V_\phi}{V_r} |
The reason to introduce this concept is that a part of the actual inter-grain void is filled with shale thus reducing the actual volume available for fluids.
It splits into two components:
Open porosity \phi_{\rm opn} | Closed porosity \phi_{\rm cls} |
subject to condition:
| (2) | \phi_e = \phi_{\rm opn} + \phi_{\rm cls} |
Effective porosity is a function of reservoir pressure at a given location
p({\bf r}):
| (3) | \phi_e(\mathbf{r}, \ p) = \phi_{e0}(\mathbf{r}) \exp \left[ \int_{p_i}^p c_r(p) \, dp \right] |
where
c_r(p) | pore compressibility (see also Pore compressibility @model ) |
p_i | initial formation pressure |
This leads to the effect of Porosity Shrinkage.
Since the pore compressibility is very low (~ cr = 0.5 ÷ 1.5 GPa-1) for all subsurface rocks in petroleum reservoirs the
(3) can be written as:
| (4) | \phi_e(\mathbf{r}, \ p) = \phi_{e0}(\mathbf{r}) \cdot \big[ 1 + c_r(p) \, (p-p_i) \big] |
Most Subsurface E&P Disciplines (except Petrophysics) usually omit index "e" and denote Effective porosity as
\phi = \phi_e.
Effective porosity at the initial formation pressure p_i is usually denoted as \phi_i.
See also
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petrophysics (PP) / Volumetric Rock Model
[ Basic reservoir properties ] [ Pore volume ] [ Connected pore volume ] [ Closed pore volume ] [ Porosity ] [ Open porosity ] [ Closed porosity ][ Initial Porosity ϕi ]
[ Pore compressibility @model ]