Total porosity may be defined as all the pore space containing fluids (water, oil or gas), whether or not they are mobile. This pore space includes any hydrocarbon fluids, mobile water, capillary bound water and clay-bound water.

There are various definitions of 'effective' porosity e.g.Juhasz (1986), Hill-Shirley-Klein (1979), Clavier-Coates-Dumanoir (1977). However, the most common definition is:

    phiE = phiT - Vd

where phiT is the total porosity of clean (clay free) sand and Vd is the volume of dispersed clay in the sand pore space expressed as a fraction of the bulk volume. In other words, effective porosity is total porosity less the volume of clay-bound water.

Of course, when you decrease porosity by converting from a total porosity to an effective porosity, the hydrocarbon saturations must increase, since the same amount of hydrocarbon is present. In other words, the hydrocarbon volumes estimated using either a total or effective porosity system are the same (i.e. ShT.phiT.h = ShE.phiE.h), it should make no difference which system is utilised.

To have the most confidence in your log evaluations, the core derived measurements should agree with those from your wireline logs. Since it is not possible to measure effective porosities in a reliable and repeatable manner, calibration with core analyses is best achieved by measuring total porosities on core plugs and comparing these with total porosities estimated from logs.

The best way to calculate total porosity is using the density log, correcting for lithology (using grain density) and fluid density (using invaded zone resistivity or neutron logs). For completeness the formula recommended is:

   phiT = (RHOma - RHOB)/(RHOma - (RHOMF.SWXO+RHOHC.(1-SWXO)))

where RHOma is the grain density (normally determined from laboratory measurements on core material), RHOB is the density log measurement, RHOHC is the in-situ hydrocarbon density (from pressure data or sampling), RHOMF is the mud filtrate density (from correlation charts normally) and SWXO is the invaded zone water saturation.

Of course, solving this equation properly requires iteration since SWXO is dependent on phiT, no matter which saturation model you use (Archie, Dual-Water, Indonesia, Waxman-Smits etc.). Unfortunately, this requirement is also why this type of solution is not more commonly used. In actuality, the maths is very straightforward to program nowadays.

In the absence of density log (or NMR) data, total porosities are best calculated by empirical calibration to core data.

From the first equation, to estimate the effective porosity, some estimate of Vd must be made. Typically the density-neutron separation or the gamma ray logs are used to make this estimate. Actually, in the industry today, over 20 different models for the shale fraction Vd are in use. A lot of time is spent in discussions between personnel as to which model is most appropriate in any particular circumstance. This debate is another of the drivers for using total porosities.

Of course, once phiE is known, along with phiT and ShT, ShE can be readily calculated from the second equation.

Once a total or effective porosity has been determined, it must be used in a water saturation equation to determine hydrocarbon saturations. If you've used a total porosity model, then there's no concern about which shale fraction model you've used. If you've gone down the effective porosity route too early, you must adjust the saturation model to account for the conductivity of the clay-bound water that is measured by the resistivity logs, but not accounted for in the porosity.

So why bother with effective porosity at all? - There are a number of reasons:

Since a total porosity system is more reliably calibrated to core and simpler to work in, this is the approach recommended for petrophysical evaluation. If there are conductive shales present, correct for these using total porosity and either the Waxman-Smits or Dual Water models to get water saturations. Use effective porosity only to calibrate a permeability transform. If you need effective porosities and water saturations for some other reason (e.g. your volumetric model is "effective"), calculate them from your calibrated total porosity system.