Just Paraphrase :

Theory:
Porosity in definition is the pore volume over the bulk volume of a rock (eq. 1). The pore volume of a rock is responsible for storing any fluid present in the reservoir. However, there are some insulated pores within the rock that fluids can’t access. Therefore, effective porosity is the pore volume of the connected pores of the rock that can store the fluids. The effective porosity is the one needed when evaluating the reservoir properties and making necessary calculation for the reservoir.

Porosity can be measured in different ways including well logging and core analysis. In well logging, specific logging tools are driven down the hole and it measures the neutron and density porosity immediately. In core analysis, a sample of the reservoir or the target zone it taken to the lab to be tested. Although core analysis consumes more time and it could be costly, it is a very useful technique because core analysis gives much more than porosity. In core analysis, petroleum engineers and geologists are benefitted from the overall results. For petroleum engineers, the core analysis can show porosity, permeability, fluid saturation, grain size and distribution, capillary forces, etc. Those parameters allow the engineer to design the best drilling tools and casings for the drilling process. On the other hand, geologists can determine lithology, minerology, geochemistry, etc.

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