@wikipedia
...
A pore volume
...
...
fraction of bulk rock volume ...
containing the hydrodynamically connected fluids (also called free fluids) within each pore element:| LaTeX Math Block |
|---|
| Omega_R |
\ Omega_R = \Omega_e +\Omega_{sh} + \Omega_mThe usual practice is to use relative volumes: | LaTeX Math Block |
|---|
|
| anchor | Omega_R |
|---|
| alignment | left |
|---|
|
\phi_e
=
\frac{\Omega_e}{\Omega_R}, \quad V_{sh} = \frac{\Omega_{sh}}{\Omega_R}, \quad V_
m = \frac{\Omega_m}{\Omega_R}which are measured in V/V units (or fracs) and honor the following constraint:
| LaTeX Math Block |
|---|
|
\phi_e +V_{sh} + V_m = 1 |
| Anchor |
|---|
phie | phie | The relative effective pore volume
contains free or connate fluids (water, oil , gas) and called effective porosity.It corresponds to air porosity of the dried laboratory cores:\phi}{V_r}
The log name is PHIE.
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:
...
...
...
...
...
...
subject to condition:
| LaTeX Math Block |
|---|
| V_sh |
V_{sh}\phi_e = V\phi_{\rm siltopn} + V_c + V\phi_{\rm cbwcls} The clay bound water |
Effective porosity is a function of reservoir pressure at a given location
...
...
...
:| LaTeX Math Block |
|---|
| V_sh |
V_{\rm cbw}\phi_e(\mathbf{r}, \ p) = s\phi_{\rm cbw} \cdot V_{sh} where ei}(\mathbf{r}) \exp \left[ \int_{p_i}^p c_\phi(p) \, dp \right] |
where
...
...
...
|
| LaTeX Math Inline |
|---|
| body | --uriencoded--\phi_%7Bei%7D = \phi_e |
|---|
|
|
...
...
This leads to the effect of Porosity Shrinkage.
Since the pore compressibility is very low (~ cϕ = 0.5 ÷ 1.5 GPa-1) and has a weak dependence on reservoir pressure for subsurface rocks in petroleum reservoirs the | LaTeX Math Block Reference |
|---|
|
can be written as
| LaTeX Math Block |
|---|
|
\phi_t = \phi_e + V_{\rm cbw} = \phi_e + s_{\rm cbw} V_{sh} |
The term total porosity is more of a misnomer as it actually does not represent a pore volume for free flow as the clay bound water is essential part of the rock solids.
Nevertheles, the total porosity property has been adopted by petrophysics as a part of interpretation workflow where the intermediate value of total porosity from various sensors leads not only to effective porosity but also to lithofacies analysis.
On the other hand, the effective porosity itself is also not the final measure of the volume available for flow.
...
:
| LaTeX Math Block |
|---|
| phi_t |
\phi_e = \phi_{e(\mathbf{r}, \ \rm connected} + p) = \phi_{e \ \rm unconnected}Besides the connected effective porosity includes the connate fluids which may be not flowing in the practical range of subsurface temperatures, pressure gradients and sweeping agents: | LaTeX Math Block |
|---|
| \phi_{e \ \rm connected} = \phi_{e \ \rm free flow} + \phi_{e \ \rm irreducible \, fluids} |
Finally, the useful porosity which represents a volume available for flow can be | LaTeX Math Block |
|---|
| \phi_{e \ \rm use} = \phi_e \cdot (1 - s_{irr}) |
where represents a fraction of pore volume, occupied by irreducible fluid (usually water).ei}(\mathbf{r}) \cdot \left[ 1 + c_\phi \, (p-p_i) + 0.5 \, c_\phi^2 \, (p-p_i)^2 \right] |
Most Subsurface E&P Disciplines (except Petrophysics) usually omit index "e" and denote Effective porosity as .
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 ]
