The rock volume is split into three major components: pore volume , shale volume and rock martix :| LaTeX Math Block |
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| anchor | Omega_R |
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| alignment | left |
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| \Omega_r = \Omega_e +\Omega_{sh} + \Omega_m |
The usual practice is to use relative volumes: | LaTeX Math Block |
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| anchor | Omega_R |
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| alignment | left |
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| \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 |
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| \phi_e +V_{sh} + V_m = 1 |
The relative pore volume is also called effective porosity (PHIE) and contains free and connate fluids (water, oil , gas).The log name is PHIE.
It corresponds to air porosity of the dried laboratory cores: | LaTeX Math Inline |
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| body | \phi_e = V_{\rm air \, core} |
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|
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The relative shale volume is called shaliness and contains three major components: silt , clay and clay bound water :| LaTeX Math Block |
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| 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 |
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| 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 |
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| \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 |
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| \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 |
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| \phi_{\rm open} = \phi_{\rm free} + \phi_{\rm connate} |
Finally, the pore volume available for flow is represented by the following formula: | LaTeX Math Block |
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| \phi_{\rm flow} = \phi_e \cdot (1 - s_{\rm connate}) |
where
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| s_{\rm connate}=\frac{\phi_{\rm connate}}{\phi_{\rm open}} |
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a fraction of pore volume, occupied by connate fluid (usually water or oil) and estimated in laboratory Special Core Analysis (SCAL) |
As one may expect the value has the best correlation with permeability. | 