Page 208 - Hydrocarbon

Field Appraisal 195

wells may provide significantly more appraisal information about reservoir
continuity than vertical wells)
a production test on a well to determine the productivity from future development
wells
coring and production testing of the water leg in a field to predict aquifer behaviour
during production, or to test for injectivity in the water leg
deepening a well to investigate possible underlying reservoirs
coring a well to determine diagenetic effects.

It is worth noting that if field development using horizontal wells is under
consideration, then horizontal appraisal wells will help to gather representative
data and determine the benefits of this technique, which is further discussed in
Section 10.3, Chapter 10.

8.4. Expressing Reduction of Uncertainty

The most informative method of expressing uncertainty in HCIIP or UR is
by use of the expectation curve, as introduced in Section 7.2, Chapter 7. The high
(H ), medium (M ) and low (L) values can be read from the expectation curve.
A mathematical representation of the uncertainty in a parameter (e.g. STOIIP) can be
defined as

H L
%Uncertainty ¼ 100%
2M

The stated objective of appraisal activity is to reduce uncertainty. The impact of
appraisal on uncertainty can be shown on an expectation curve, if an outcome is
assumed from the appraisal. The following illustrates this process.
Suppose that four wells have been drilled in a field, and the geologist has
identified three possible top sands maps based on the data available. These maps,
along with the ranges of data for the other input parameters (N/G, S o , f, B o )have
been used to generate an expectation curve for STOIIP (Figure 8.3).
If well A is oil bearing, then the low case must increase, though the high case
may not be affected. If well A is water bearing (dry), then the medium and high
cases must reduce, though the low case may remain the same. For both outcomes,
the post-appraisal expectation curve becomes steeper, and the range of uncertainty
is reduced.
Note that it is not the objective of the appraisal well to find more oil, but to
reduce the range of uncertainty in the estimate of STOIIP. Well A being dry does
not imply that it is an unsuccessful appraisal well.
The choice of the location for well A should be made on the basis of the
position which most effectively reduces the range of uncertainty. It may be for
example, that a location to the north of the existing wells would actually be more