effective porosity

1. n. [Geology]
The interconnected pore volume or void space in a rock that contributes to fluid flow or permeability in a reservoir. Effective porosity excludes isolated pores and pore volume occupied by water adsorbed on clay minerals or other grains. Total porosity is the total void space in the rock whether or not it contributes to fluid flow. Effective porosity is typically less than total porosity.
2. n. [Formation Evaluation]
In the original definition of core analysts, the volume of connected pores in a unit volume of rock. Effective porosity in this sense is the total porosity less the isolated porosity. It is the porosity measured by most core analysis techniques that do not involve disaggregating the sample. In these techniques, the porosity is usually measured on totally dried core samples. Drying removes most of the clay-bound water. In log interpretation, effective porosity means the total porosity less the clay-bound water. The definition is based on the analysis of shaly formations, in which the clay-bound water is considered immobile and hence ineffective. Isolated porosity is rare in such formations and is ignored, being included in the effective porosity. Effective porosity on dried core samples is therefore greater than effective porosity from log analysis, and close to the total porosity from log analysis. In humidity-dried cores, part of the clay-bound water is not removed, and the difference is reduced. In some usage, the capillary-bound water is not considered part of the effective porosity. In this case effective porosity is synonymous with free fluid. Effective porosity is measured in volume/volume, percent or porosity units, p.u.