The term effective porosity lacks a single or straightforward definition. Even some of the terms used in its mathematical description ("” and “”) have multiple definitions. However, it is most commonly considered to represent the porosity of a rock or sediment available to contribute to fluid flow through the rock or sediment, or often in terms of "flow to a borehole". Porosity that is not considered "effective porosity" includes water bound to clay particles (known as bound water) and isolated "vuggy" porosity (vugs not connected to other pores). The effective porosity is of great importance in considering the suitability of rocks or sediments as oil or gas reservoirs, or as aquifers.
Summary of terms
Utilising the Eslinger & Pevear concept, the following terms are illustrated in Figure 2.
The volume of the reservoir rock which is fluid (oil, water, gas) filled, expressed as a percentage or a fraction of the gross (bulk) rock volume.
The sum of all the interconnected pore space. In the vast majority of cases, this core analysis and Petroleum Engineering definition of effective porosity equates to total porosity.
Effective porosity measured on core samples which are dried in a humidity oven so that clays retain one or two molecular layers of bound water—however, this CBW tends to a minimum and is likely not reservoir representative.
Total porosity minus clay-bound water (CBW).
Log effective porosity. In essence, total porosity minus shale water, where solid minerals and the volume of shale (Vsh) constitute the matrix (non-effective porosity) and the remaining volume constitutes the effective porosity. For practical purposes, Vsh includes solid clays and the clay-sized and silt-sized fraction of non-clay minerals plus CBW and capillary bound water associated with shale micropores.
In a hydrocarbon-bearing reservoir above the transition zone, only that pore space which is filled with hydrocarbons. From the NMR log, this equates to the Free Fluid Index (FFI), in other words, all pore space above the T2 cut-off.
The volume of pore space which contains only producible hydrocarbons.
Clay-bound water (CBW)
The amount of Clay-bound water is determined by the following equation