Page 322 - Petrophysics 2E

FORMATION EVALUATION 293

exceptionally high for a carbonate formation. The core analysis involved
five phases.
During the first phase, 15 lb of whole-core sections were analyzed.
Because of the complex lithology of the San Andres Formation, the whole-
core technique was selected to determine porosity, matrix permeability,
and fluid saturations. Tables 4.20 and 4.21 give the values of permeability
and porosity, respectively. A semilog plot of permeability vs. porosity
showed a considerable scattering of data points. Nevertheless, a best
least-squares fit line is placed along the relative general trend of these
points and the corresponding equation is derived:

log k = 0.285$ - 2.98 (4.159)

During the second phase, 162 unprocessed core samples, representative
of the pay intervals in six of the ten cored wells, were examined,
and lithology, texture, and type of porosity were determined. Also,
32 core samples representing the expected range of rock parameters,
as determined from a preliminary computer correlation of whole-core
and log data, were selected for special plug-core analysis. Samples with
extremely large vugs were excluded from special core analysis in an
attempt to determine more accurately the properties of the matrix.
Following routine cleaning and drying procedures as described in
Appendix A, air permeability and porosity (using Boyle's law porosim-
eter) were determined. Figures 4.50 and 4.51 show a comparison of,
respectively, whole-core permeability vs. plug permeability and whole-
core porosity vs. plug porosity. It is evident from these figures that there
are very low coefficients of correlation. "his emphasizes the problem
of obtaining representative core samples in carbonate reservoirs. Based
on this limited (or lack of) agreement, it was concluded that the
requirement of permeability and porosity agreement between the plug-
and whole-core values on the sample-by-sample basis should not be the
criterion for selecting samples for plug-core analysis.
During the third phase, resistivity measurements were made on 32 core
plugs and formation resistivity factors were calculated using Equation
4.5. Figure 4.52 is a log-log plot of F vs. 0, and the equation of the best-fit
line is:

1
FR = - (4.160)
b2.1

Thus, the cementation factor m of this carbonate formation is 2.1.
Similarly, resistivity index calculations were made and the results