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).
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. NeverthelesNevertheless, 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 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 openconnected} + \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 openconnected} = \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 most linear correlation with permeability. | 