The rock volume is split into three major components: effective 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 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 |
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body | \phi_e = V_{\rm air \, core} |
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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.
On the other hand, the The effective porosity itself is also not the 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_{e \ \rm connectedopen} + \phi_{e \ \rm unconnectedclosed} |
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 |
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| \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 |
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| \phi_{e \ \rm use} = \phi_e \cdot (1 - s_{irr}) |
where represents a fraction of pore volume, occupied by irreducible fluid (usually water). |