@wikipedia
Part of the bulk rock volume 

A pore volume 

\phi_e  = \frac{V_f\phi}{V_r}

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.

The reason to introduce this concept is that a part of the actual inter-grain void is filled with shale thus reducing the actual volume available for fluids.

It splits into two components:

subject to condition:

\phi_e = \phi_{\rm opn} + \phi_{\rm cls}

The log name is PHIE. 
Effective porosity is a function of reservoir pressure at a given location

\phi_e(\mathbf{r}, \ p) = \phi_0{ei}(\mathbf{r}) \exp \left[ - \int_{p_i}^p c_r\phi(p) \, dp \right]

where

This leads to the effect of Porosity Shrinkage.

Since the pore compressibility is very low (~ c_ϕ  = 0.5 ÷ 1.5  GPa^-1) and has a weak dependence on  reservoir pressure  for subsurface rocks in petroleum reservoirs the 

\phi_e(\mathbf{r}, \ p) = \phi_{ei}(\mathbf{r}) \cdot \left[ 1 + c_\phi \, (p-p_i) + 0.5 \, c_\phi^2 \, (p-p_i)^2 \right]

Most Subsurface E&P Disciplines (except Petrophysics) usually omit index "_e" and denote Effective porosity as 

See also

...

Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petrophysics (PP) / Volumetric Rock Model

 [ Basic reservoir properties ] [ Pore volume ] [ Connected pore volume ] [ Closed pore volume ] [ Porosity ] [ Open porosity ] [ Closed porosity ][ Initial Porosity ϕ_i ]

[ Pore compressibility @model ]