Page 243 - Hydrocarbon Exploration and Production Second Edition

230 Estimating the Number of Development Wells

and the stabilised production rates (well initial) achieved during the production tests
on the exploration and appraisal wells.

Plateau production rate ðstb=dÞ
Number of production wells ¼
Assumed well initial ðstb=dÞ

There will be some uncertainty as to the well initial rates, since the exploration
and appraisal wells may not have been completed optimally, and their locations may
not be representative of the whole of the field. A range of well initial rates should
therefore be used to generate a range of the number of wells required. The
individual well performance will depend on the fluid flow near the wellbore, the
type of well (vertical, deviated or horizontal), the completion type and any artificial
lift techniques used. These factors will be considered in this section. For many land
and subsea developments in particular, these uncertainties can be somewhat
mitigated through phased developments.
The number of injectors required may be estimated in a similar manner, but it is
unlikely that the exploration and appraisal activities would have included injectivity
tests, of say water into the water column of the reservoir. In this case, an estimate must
be made of the injection potential, based on an assessment of reservoir quality in the
water column, which may be reduced by the effects of compaction and diagenesis.
Development plans based on water injection or natural aquifer drive often suffer from
lack of data from the water bearing part of the reservoir, since appraisal activity to
establish the reservoir properties in the water column is frequently overlooked. In the
absence of any data, a range of assumptions of injectivity should be generated, to yield
a range of number of wells required. If this range introduces large uncertainties into the
development plan, then appraisal effort to reduce this uncertainty may be justified.
The presence of faults is another element that may change the number of
injection/production wells required.
The type of development, type and number of development wells, recovery factor
and production profile are all inter-linked. Their dependency may be estimated using
the above approach, but lends itself to the techniques of reservoir simulation
introduced in Section 9.4, Chapter 9. There is never an obvious single development
plan for a field, and the optimum plan also involves the cost of the surface facilities
required and environmental considerations. The decision as to which development
plan is the best is usually based on the economic criterion of profitability. Figure 10.1

Subsurface Development Method A B C

Surface Development Plan 1 2 3 4 5 6 7 8 9

Economic Evaluation $ 1 $ 2 $ 3 $ 4 $ 5 $ 6 $ 7 $ 8 $ 9
Figure 10.1 Determining the optimum development plan.