Page 242 - Hydrocarbon

CHA P T E R 1 0

Well Dynamic Behaviour

Introduction and Commercial Application: Chapter 9 considered the dynamic behaviour
in the reservoir, away from the influence of the wells. However, when the fluid flow
comes under the influence of the pressure drop near the wellbore, the displacement
may be altered by the local pressure distribution, giving rise to coning or cusping.
These effects may encourage the production of unwanted fluids (e.g. water or gas
instead of oil), and must be understood so that their negative impact can be
minimised.
The wells provide the conduit for production from the reservoir to the surface,
and are the key link between the reservoir and surface facilities. This conduit
therefore needs to have the flow capacity for production (or injection) and
reliability in the face of problems such as sand production, corrosion, high pressures
or temperatures, mechanical failure and various production chemistry issues such as
waxes, scales and hydrates. The type and number of wells required for development
will dictate the drilling facilities needed, and the operating pressures, temperatures
and rates of the wells and artificial lift requirements will influence the design of the
production facilities. The application of horizontal or multilateral wells may, where
appropriate, greatly reduce the number of wells required, which will have an impact
on the cost of development. In recent years, the use of remote downhole
monitoring and control through techniques such as smart wells has further extended
the link between the facilities, the completion and the reservoir.

10.1. Estimating the Number of Development Wells

The type and number of wells required for development will influence the
surface facilities design and have a significant impact on the cost of development.
Typically the drilling expenditure for a project is between 20 and 40% of the total
capex, although for a subsea development this may be higher still. A reasonable
estimate of the number of wells required is therefore important.
When preparing feasibility studies, it is often sufficient to estimate the number of
wells by considering

the type of development (e.g. gas cap drive, water injection, natural depletion)
the production/injection potential of individual wells.

For a particular type of development, the production profile can be estimated
using the guidelines given in Section 9.6, Chapter 9. The number of producing wells
needed to attain this profile can then be estimated from the plateau production rate

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