Page 330 - Petrophysics 2E

FORMATION EVALUATION 299

Interpretation of Well Log Data
The analysis of logs obtained from the 38 new wells required several
phases. First, the logs were digitized and corrections for hole size and
invasion were determined. Then, cross-plots were made of the corrected
data from the three porosity logs (acoustic, density, and neutron),
and formation minerals were identified. The log data from the three
porosity devices were corrected for shale (clay) volume by the following
expressions:

(4.165)

(4.166)

(4.167)

where: t, Pb, Qn = acoustic, density, and neutron log readings,
respectively.
tc, pk, @nc = three-device data values corrected for shale
(clays) content.
Tsh, Psh, @sh = three-device readings for 100% shale.

These corrected data become the input to a matrix solution for primary
(matrix) and secondary (fractures and vugs) porosity indices, and three
minerals, as shown in Table 4.22. The term “trilith” describes the
porosity-lithology matrix obtained from the combination of the three
porosity devices. In less complex formations (no secondary porosity),
a simpler matrix called “bilith” is constructed from the combination of
two porosity devices only: acoustic and density. Finally, these matrices
were solved by a computer program called “Bitri.” Figure 4.55 shows
a good agreement between the profile of core-derived porosity and the
Bitri-computed porosity. In addition to porosity, the program computed
permeability from Equation 4.159, irreducible water saturation from
Equation 4.163, and water saturation from Equation 4.54 for m = 2.1
and n = 2.20:

(4.168)

where Rt is the true resistivity from a deep-reading resistivity log,
corrected for borehole and invasion effects, and @ is the porosity
obtained from Bitri. Figure 4.56 shows a comparison of the core-derived