Part of the bulk rock volume containing the hydrodynamically connected fluids (also called free fluids) within each pore element.
Although the fluid inside the pore is movable but some pores maybe isolated from each other and do not contribute to rock permeability.
This splits the effective pore volume into two components: open pore volume and closed pore volume:
| V_\phi = V_{\rm open} + V_{\rm closed} |
The log name is PHIE.