Page 151 - Petrophysics 2E

124 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

pores and is held by surface tension and high capillary pressure. In such
a case bulk volume water is termed bulk water volume irreducible @%VI)
and is estimated as:

One may consider the bulk volume of irreducible water saturation @VI)
to be represented by the consistent minimum value of the BVW curve.
The BVW concept generally provides a good estimate of the irreducible
water saturation if the porosity is intergranular, not secondary, and if
the rock contains little clay in pore throats. Swir cannot be determined
confidently from resistivity logs when the reservoir is not at irreducible
conditions and when the pay zone produces water. In this case, local
experience is considered the best guide to the percentage of water
saturation likely to be irreducible.
The free fluid index is defined as the product of hydrocarbon
saturation and porosity. It is a measure of movable liquids, oil and/or
water, and therefore it is connected to the flow unit. It is obtained from
the MNL tool. Mathematically, it is expressed as:

Coates and Denoo [30] related permeability to FFI as follows:

(3.58)

The correlation constant 10 limits this equation to reservoirs in which
(a) the irreducible water saturation is well defined, (b) the porosity
is intergranular, and (c) the rock contains little clay in pore throats.
Combining this equation and the definition of RQI yields a useful
relationship between RQI and FFI:

RQI = 3.14 (-) FFI fl (3.59)
0-rn
where FFI and porosity are expressed as a fraction, permeability in mD,
and RQI in pm.
Taking the logarithm of both sides of Equation 3.59 yields:

LogRQI = Log (e) +Log (3.14&) (3.59a)

Thus a log-log plot of the reservoir quality index versus @ should yield a
straight line of slope unity, assuming the reservoir is a clean homogeneous