PETROPHYSICAL MODELS - DEFINITIONS
The rock-fluid model used for the analysis methods described in this Handbook is shown in the illustration below. From this model, we can generate a series of equations that can be used to calculate the petrophysical properties of a rock. These equations have been derived by many researchers across a long period of years. Some equations are tuned to local areas, and may not be universally applicable. The mathematical algorithms selected for inclusion in this Handbook were chosen for their universal applicability, although many regionalized variations probably exist for most of them. Because you might need to modify existing models, or develop a new one of your own, the basic reservoir model should always be in the back of your mind.

The Rock-Fluid Model for Petrophysical Analysis                                                          

DEFINITIONS OF PETROPHYSICAL PROPERTIES
Here are the definitions that derive from the rock/fluid model shown above.

By definition, Vrock + PHIe + Vsh = 1.00

The mineral mixture can be quite complex and log analysis may not resolve all constituents.

The sum of the three water volumes in a particular rock is called clay bound water (CBW). CBW varies with shale volume and is zero when Vsh = 0.

By definition, Vsh = Vcl + Vsilt + CBW. Sometimes Vsilt is considered to be part of Vrock, especially in fine grained unconventional reservoirs.

Some of the “free water” is not free to move - it is, however, not “bound” to the shale.

BVI is sometimes called “bound water”, but this is confusing (see definition of clay bound water above), so “irreducible water” is a better term. Note that BVWm = BVW – BVI.

This definition is needed for the nuclear magnetic log (NMR, CMR, etc), since it cannot see BVWir. Non-useful porosity also occurs as tiny pores that do not connect to any other pores. They are almost invariably filled with immoveable water and do not contribute to useful reservoir volume or energy. Such pores occur in silt, volcanic rock fragments in sandstones, and in micritic, vuggy, or skeletal carbonates. The NMR may see some of this non-useful porosity – the jury is still out.

This is the standard definition of “water saturation”. Older books use this term to define total water saturation. Since all interpretation methods described here correct for the effects of shale, we are not normally interested in the total water saturation, except as a mathematical by-product. As effective porosity approaches zero, the water saturation approaches one (by edict, if not by calculus).

The amount of free water in the invaded zone is usually higher than in the un-invaded zone, when oil or gas is present. Thus Sxo >= Swe. The water saturation in the invaded zone between the flushed and un-invaded zone is seldom used.