Page 278 - Petrophysics 2E

RESISTIVITY OF SHALY (CLAYEY) RESERVOIR ROCKS 25 1

This expression, which is referred to as the total shale model or simply
the Simandoux equation, gives good results in clean and uniformly
water-wet systems. The saturation exponent in these systems is
approximately 2. A common method of estimating the percentage of
shale (clay) present in the reservoir rock, vsh, and the shale-corrected
porosity is to solve simultaneously the following pair of equations:

$DC = $D-Vsh$Dsh (4.94)
@NC = @N-Vsh$Nsh (4.95)

The formation porosity, 9, is obtained from the following root-square
formula:

$ = 0.707,/$& 4- $6, (4.96)

where $D and $N are, respectively, the uncorrected density and neutron
porosities of the formation, and @Dsh and @Nsh are the density and neutron
log readings in the shale portion or adjacent shale bed. If Equation 4.94
is
yields a negative @D~, then @D~ assumed to be zero. The subscript c
represents “corrected.
Hilchie defined two types of clay influences on logs: effective
(montmorillonite and illite) and noneffective (kaolinite and chlorite).
The influence of clays, effective or non-effective, on the density log is
solely a function of the clay density. For instance, montmorillonite, which
has a density (2.33 g/cm3) lower than that of sandstone (2.65 g/cm3),
causes the density porosity to be higher than the true porosity. On the
other hand, illite, which has a density (2.76 g/cm3) greater than that of
sandstone, causes the density porosity to be lower than the true porosity.
Kaolinite, with a density (2.69 g/cm3) approximately equal to that of
a sandstone, cannot be detected by a density log when mixed with sand.
The effect of chlorite (2.77g/cm3) on the density log becomes significant
only when the reservoir porosity is lower than 9%. The influence of
shale on the neutron log varies from one service company to another
depending upon the instrumentation. In general, the apparent porosity
derived from modem neutron logs is greater than the actual effective
porosity of the reservoir rock.
Another common method for estimating the fraction of shale (clay),
Vsh, present in reservoir rock is to use one of the following correlations
between the shale volume and the gamma ray index [27]. For tertiary
sediments (less than 4,000 ft deep):