The rock volume is split into three major components: effective pore volume , shale volume and rock martix : LaTeX Math Block |
---|
anchor | Omega_R |
---|
alignment | left |
---|
| \Omega_R = \Omega_e +\Omega_{sh} + \Omega_m |
The 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 |
The relative effective pore volume contains free or connate fluids (water, oil , gas) and called effective porosity.The log name is PHIE.
It corresponds to air porosity of the dried laboratory cores: LaTeX Math Inline |
---|
body | \phi_e = V_{\rm air \, core} |
---|
| .
The relative shale volume is called shaliness and contains three major components: silt , clay and clay bound water : LaTeX Math Block |
---|
| V_{sh} = V_{\rm silt} + V_c + V_{\rm cbw} |
The log name is VSH.
The clay bound water is usually measured as the fraction of shale volume:
LaTeX Math Block |
---|
| V_{\rm cbw} = s_{\rm cbw} \cdot V_{sh} |
where is called bulk volume water of shale (BVWSH).
The total porosity is defined as the sum of effective porosity and clay bound water : LaTeX Math Block |
---|
| \phi_t = \phi_e + V_{\rm cbw} = \phi_e + s_{\rm cbw} V_{sh} |
The log name is PHIT.
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.
The effective porosity is not a final measure of the volume available for flow. It includes the unconnected pores which do not contribute to flow: LaTeX Math Block |
---|
| \phi_e = \phi_{\rm open} + \phi_{\rm closed} |
Besides the connected effective pore volume 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_{\rm open} = \phi_{\rm free} + \phi_{\rm connate} |
Finally, the useful porosity which represents a volume available for flow can be LaTeX Math Block |
---|
| \phi_{e \ \rm useflow} = \phi_e \cdot (1 - s_{irr}) |
where
LaTeX Math Block |
---|
| s_{irr}=\frac{\phi_{\rm connate}}{\phi_{\rm open}} |
|
a fraction of pore volume, occupied by irreducible fluid (usually water) |
|