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Concept
Effective porosity
Summary
Effective porosity 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.
The term lacks a single or straightforward definition. Even some of the terms used in its mathematical description ("” and “”) have multiple definitions.
"Quartz" (more aptly termed “non-clay minerals”) forms part of the matrix, or in core analysis terms, part of the grain volume.
"Clay layers" are dry clay (Vcl) which also form part of the grain volume. If a core sample is dried in a normal dry oven (non-humidified atmosphere) the clay layers and quartz together form the grain volume, with all other components constituting core analysis “total porosity” (notwithstanding comments in ). This core total porosity will generally be equivalent to the total porosity derived from the density log when representative values for matrix and fluid density are used.
The clay layers contain groups (often termed “structural water”). This structural water is never part of the pore volume. However, since neutron logs sense H (hydrogen) and all hydrogen so-sensed is allocated as pore space, then neutron logs will overestimate porosity in argillaceous rocks by sensing as part of the pore space.
“Clay surfaces and interlayers” comprise electrochemically bound water (clay-bound water or CBW) which varies in volume according to the clay-type, and the salinity of the formation water (see the Attachments section). The most common definition of effective porosity for sandstones excludes CBW as part of the porosity, whereas CBW is included as part of the total porosity. That is:
To assess the effective porosity, samples are dried at 40-45% relative humidity and 60 °C.
Official source
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