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commercial, and drilling near the limits so indicated. The appraisal wells may
therefore be some kilometres from the discovery well.
With the drilling of the appraisal wells, the nature of the accumulation
begins to emerge. With each well, the information gained is used to refine the
model of the accumulation until the point is reached when it can be stated
with some confidence that the accumulation is sufficiently large and contains
sufficient recoverable oil or gas to make the development of the field com-
mercially sensible.
In a simple onshore structure, these appraisal wells may be designed so
that they can produce petroleum eventually; but their real purpose is to ob-
tain more information. Offshore, the appraisal wells will not be capable of
more than limited testing of reservoirs because the economics of offshore
production requires production platforms that will be used first to drill a
number of deviated wells to develop part of the field, then to produce them
to a central facility for preliminary processing (such as gas separation) and
transport through pipelines to the shore facilities. More than 20 wells are
usually drilled and produced to one platform, so the appraisal programme
must be sufficiently comprehensive offshore to permit planning of these wells
and the platforms.
It is in the nature of petroleum fields that the decision to develop is made
without full knowledge of the accumulation. Full information is available
only when it is no longer needed. The appraisal phase ends with the decision
either to abandon the prospect as non-commercial, or to develop it as an oil
or gas field.
After the decision to develop is made, development wells are drilled for
the production of oil (or gas, as the case may be); and their siting is such that
each reservoir will be developed as economically as possible (which is not
usually the same as as cheaply as possible) on the model that has evolved
with the added information, which may include additional seismic data as
well as the borehole data. This is an exercise in engineering economics based
on geology that must also take transport of the production into account. It
will normally take at least five years from discovery to put a new field onto
production, and during this time there is no income from the field to offset
the expenses incurred in its development.
The rate at which a new field can be produced economically is part of the
nature of an oil field. It depends on the number of reservoirs, their size, their
productivity and other factors that will be discussed later. The rule of thumb
is that the faster the rate of production, the smaller the ultimate yield of the
field; but the rate must be fast enough to generate sufficient income from
the investment. So the production rate chosen is a balance between these op-
posing interests such that the development as a whole is as commercially
attractive as possible within the constraints imposed by Nature and the govern-
ment regulating such activities.
In the course of development there will be some surprises, and these will