The rock volume is split into three major components: pore volume , shale volume and rock martix :
The usual practice is to use relative volumes:
which are measured in V/V units (or fracs) and honor the following constraint:
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: . The relative shale volume is called shaliness and contains three major components: silt , clay and clay bound water :
The log name is VSH. The clay bound water is usually measured as the fraction of shale volume:
where is called bulk volume water of shale (BVWSH). The total porosity is defined as the sum of effective porosity and clay bound water :
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:
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:
Finally, the pore volume available for flow is represented by the following formula:
where
As one may expect the value has the best correlation with permeability. |