Effective porosity

The effective porosity is of great importance in considering the suitability of rocks or sediments as oil or gas reservoirs, or as aquifers.

This means that one to two molecular layers of CBW can be retained, and a form of “effective porosity” can be measured on the samples.

[7] Log derivation of effective porosity includes CBW as part of the volume of shale (Vsh).

Capillary water generally forms part of the effective pore space for both log and core analysis.

[8] If we humidity dried core samples, (some of) the electrochemically bound CBW would be retained, but none of the capillary-bound microporous water (notwithstanding comments in [7]).

Effective porosity (with reference to the figure below) can be classified as only the hydrocarbon-filled large pore spaces above the transition zone.

Greensands are green because of iron-bearing glauconite which is usually recognized as illite/mica or mixed layer illite-smectite clay by x-ray diffraction.

More importantly for the consideration of effective porosity, though, glauconite grains (part of the Vsh) have intra-particular microporous pore space which retains capillary-bound water.

Glauconite can constitute a large percentage of the reservoir rock, and therefore the associated intra-particular pore space can be significant.

[citation needed] The difference is the glauconitic microporosity which contains water at reservoir conditions and is included as part of the Vsh (non-effective porosity) by log analysis.

However, glauconitic microporosity is measured as part of the effective porosity in core plugs, even if they are humidity dried.

Therefore, it is essential to have a core - or at least a good understanding of the geology - before invoking total vs effective porosity relationships.