Page 279 - Petrophysics

252 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

For older rocks (4,000 to 8,000 ft deep):

For very hard compacted formation (at a depth of 8,000 ft or more),
Vsh = IRA, where the gamma ray, or radioactive index IRA, is defined as
follows:

(4.99)

The gamma ray deflection, GR, is obtained from the log at a zone
of interest; Ghs is the gamma ray, or radioactive log, reading in a
clean (shale-free) sand zone and GRsh is the radioactive log reading in
a shale zone.

EXAMPLE

A well is drilled in an oil zone, where the pressure is above the bubble
point. This shaly sandstone formation has the following characteristics
1553:

Rw = 0.05 ohm-m Rt = 5 ohm-m
Rsh = 1 ohm-m @e = 0.18

Estimate the oil saturation, assuming (a) vsh is negligible, Vsh = 0.20, and
(c) Vsh = 0.40. Use Equations 4.86 and 4.93, and compare.

SOLUTION

(a) For Vsh = 0, Equation 4.86 reduces to Archie equation, i.e. Eq. 4.87.
The formation factor is calculated from Humble equation 4.39b:
0.81
0.81 --
F=-- - - 25
$2 0.182

So = 1 - S, = 1 - 0.5 = 0.5

(b) For vsh = 0.20, Eq. 4.88 gives a shale correction factor of:

VshRw - 0.20 x 0.05
-
Fsh = = 0.14
0.40 X QeRsh 0.40 X 0.18 X 1