Page 108 - Well Logging and Formation Evaluation

98 Well Logging and Formation Evaluation

assumed that any inaccuracy in the GR reading itself is incorporated
within the range we assumed for GR co .
The next step is to calculate the uncertainty in porosity. Consider the
equation:

r - density r - ).
f = ( m ) ( m r f
We will do exactly the same as we have done with the net/gross, i.e.,
import the density readings into column A of a spreadsheet. At this stage
it is advisable to remove all the intervals that are designated as non-
reservoir. Determine allowable ranges for r m and r f and at the top of each
column determine the values to be used in each run. If the allowable range
for r m is, say, 2.65–2.67g/cc, the equation would look like:

+
.
= 2.66 001* (0 5. -rand ) ) (denote this as RHOM, and likewise for RHOF ).
(
Down column B insert the equation:

-
= (RHOM - Ax ) (RHOM RHOF ).
$
Copy column B 50 times across the spreadsheet.
Average each column at the bottom and then take the mean and
standard deviation of the distribution of average porosities as you did
for the net/gross. The uncertainty in the average porosity may be taken as
two standard deviations. Since the porosity equation is linear, the mean
porosity should be the same as that calculated through fixed fluid and
matrix densities. Finally we will deal with saturation in an identical
manner, although there are a few complexities. From Archie:

-
n
R t = R w *f - m * S w .
However, we wish to finally derive a porosity-weighted saturation, so it
is better to use the equation:

}
f r
*
S wpor = ( ( r m - density ) (r m - )) { R t ( * (r m - density ) (r m - )) mRw ( -1 n) .
f r
(5.8.1)
Note that this equation is equivalent to S wpor =f* S w, substituting the
porosity equation in Archie.