Page 69 - Well Logging and Formation Evaluation

CHAPTER 4

SATURATION/HEIGHT

ANALYSIS

The reason for putting this chapter before other advanced interpretation
techniques is that I believe it is of primary importance in correctly defin-
ing the STOIIP (stock tank oil initially in place) or GIIP (gas initially in
place) of a field. Indeed, because of the way that dynamic reservoir models
are constructed in many fields in practice, it completely supersedes any
exotic models constructed by the petrophysicist for calculating saturations.
Many times in my career I have seen petrophysical departments working
in isolation constructing fabulously complicated models to calculate satu-
rations. But when you ask the geologist what saturations have gone into the
static model, he will tell you that he is using a constant value unrelated to
the zonal weighted averages. The reservoir engineer may be using one
P c /S w table in the simulator based on just one air/mercury capillary pres-
sure measurement that he felt was representative of the reservoir in general.
I believe perhaps the most important role the petrophysicist has in a
petroleum engineering department concerns ensuring that the satura-
tion/height function being used in the static and dynamic models repre-
sent the best possible combination of core and log data, combined with
sound petrophysical judgment. In my view, such a function should have
both porosity and permeability as input variables, together with height
(which may be directly related to P c ).
There are dozens of different functions that have been used to describe
capillary behavior in rocks. I have used many of these over my career, but
I have found the Leverett J function to be the most broadly applicable. I
therefore propose to describe how such a function may be constructed for
a reservoir, using both core and log data. This function may be stated thus:

S w = S wirr + a J b (4.1)
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