Page 74 - Well Logging and Formation Evaluation

64 Well Logging and Formation Evaluation

If you have a gas column above an oil column in the reservoir, gener-
ate two set of tables, one for an oil/water system and one for a gas/water
system using the appropriate s, q, rho h, and rho w values.

Exercise 4.1. Core-Derived J Function

1. Based on the core capillary pressure measurements presented in
Appendix 2, derive a J function.
2. Recalculate the sums and averages using the J function derived from
these core data.

4.2 LOG-DERIVED FUNCTIONS

The methodology applied to core data can also be applied to the logs
to derive an independent J function, provided that the position of the FWL
is known. This function is useful for comparison with the core-derived
function and may be used if no core data are available.
A secondary benefit is that it is possible during the fitting procedure to
eliminate thin bed and invasion effects and provide a means of generat-
ing a high-resolution saturation log that depends on only the density log.
In order to fit a J function, follow these steps:

1. Use the depth of the FWL and the true vertical depth subsea (TVDss)
to generate a curve for the height above FWL.
2. Use equation 4.3 to derive a curve for P c .
3. Use equation 4.2 to derive a curve for J. It is assumed that you already
have some kind of poroperm relationship that can be used to relate k
to the porosity.
4. Use equation 4.4 to derive a curve for S wr . Set S wirr to be 0.01 less than
the lowest S w seen on the logs.
5. Make a crossplot of Log(J) vs. Log(S wr ).

You will find a cloud of points. Note that shoulder-bed effects and inva-
sion of water-based mud will always pull points to higher S wr values com-
pared with points originating from thick beds and for which less invasion
is occurring. Therefore, by fitting the constants a and b so that the model
follows the leading edge of the cloud of points, you will effectively be
correcting for thin bed and invasion effects.