Page 207 - Hydrocarbon Exploration and Production Second Edition

194 Appraisal Tools

The example illustrates some important steps in identifying the uncertainties and
then beginning to quantify them

consider the factors which influence the parameter being assessed
rank the factors in order of the degree of influence
consider the uncertainties in the data used to describe the factor.

The same procedure may be used to rank the parameters themselves (GRV,
N/G, f, S h , B o , RF), to indicate which has the greatest influence on the HCIIP
or UR.
The ranking process is an important part of deciding an appraisal programme,
since the activities should aim to reduce the uncertainty in those parameters which
have the most impact on the range of uncertainty in HCIIP or UR.

8.3. Appraisal Tools

The main tools used for appraisal are those which have already been discussed
for exploration, namely drilling wells and shooting 2D or 3D seismic surveys. Appraisal
activity may also include re-processing an existing old seismic survey (again, 2D or
3D) using new processing techniques to improve the definition. It is not necessary
to re-process the whole survey dataset; a sample may be re-processed to determine
whether the improvement in definition is worthwhile. In the majority of cases
where only 2D seismic is available, time and money will be better spent on shooting
a new 3D seismic survey.
Seismic surveys are traditionally an exploration and appraisal (E&A) tool.
However, 3D seismic is now being used more widely as a development tool, that
is applied for assisting in selecting well locations, and even in identifying remaining
oil in a mature field. This was discussed in Chapter 3. Seismic data acquired at the
appraisal stage of the field life is therefore likely to find further use during the
development period.
Appraisal activity should be based on the information required. The first step is
therefore to determine what uncertainties appraisal is trying to reduce, and then
what information is required to tie down those uncertainties. For example, if fluid
contacts are a major source of uncertainty, drilling wells to penetrate the contacts
is an appropriate tool; seismic data or well testing may not be. Other examples of
appraisal tools are

an interference test between two wells to determine pressure communication across
a fault
a well drilled in the flank of a field to improve the control of the dips seen on
seismic
a well drilled with a long enough horizontal section to emerge from the flanks of
the reservoir, and determine the extent of the reservoir in the flanks (horizontal