Page 155 - Well Logging and Formation Evaluation

Production Geology Issues 145

volume of the GBV of the oil column. To convert this area into a volume:
Draw a square of side 1cm on the graph paper. Along one horizontal
side, indicate how many square meters are represented by 1cm. Like-
wise, on the vertical side indicate how many meters in depth are repre-
sented by 1cm. Taking the product of these two conversion factors will
yield the conversion factor to convert square centimeters into cubic
meters of rock.
In the example shown in Figure 10.1.3, if the area measured from the
2
graph were 10cm , the GBV of the oil column would be given by:
5
7
3
=
GBV 10*10 *50 = 5*10 m .
The technique illustrates well why there are problems with using
net/gross when reservoir quality varies in depth in a reservoir. This can
be seen as follows. Imagine that the horizon shown in the example con-
sists of a very good quality sand overlying a nonproducible shale. Say the
porosity in the sand is 30% and S w = 0.10. However, taking the whole
interval into consideration, the net/gross is only 30%. One would estimate
the STOIIP (stock tank oil initially in place) to be:

STOIIP = GBV* 0.3 * 0.9 * 0.30) B o
) (
(
) (
where B o is the oil formation volume factor. Now consider what would
have happened if the base of the good sand were mapped instead of the
entire package. A new GBV (GBV¢) would be determined by planimeter-
ing the new area-depth graph. The net/gross corresponding to GBV¢ is
now 1.0 instead of 0.30, but of course GBV¢ is less than GBV. Another
estimate of the STOIIP is then given by:

STOIIP¢ = GBV * 0.3 * 0.9 * 1.0) B o .
¢ (
) (
) (
Because of the nature of the way that the OWC cuts across the struc-
ture, it is certainly not the case that STOIIP = STOIIP¢. In essence, because
the poor-quality rock falls preferentially below the OWC, one is under-
estimating the STOIIP by using the coarser interval and applying a
net/gross. Conversely, if the good-quality rock had been located at the
base of the sequence instead of at the top, the STOIIP would have been
overestimated through using net/gross.
The concept of area-depth mapping may also be applied to other param-
eters. For instance, if a map is made of the EHC (net*porosity*hydro-