Page 418 - Hydrocarbon Exploration and Production Second Edition

CHA P T E R 1 7

Managing Decline

Introduction and Commercial Application: The production decline period for a field is
usually defined as starting once the field production rate falls from its plateau rate.
Individual well rates may, however, drop long before field output falls. This section
introduces some of the options that may be available, initially to arrest production
decline, and subsequently to manage decline in the most cost-effective manner.
The field may enter into an economic decline when either income is falling
(production decline) or costs are rising, and in many cases both are happening.
Whilst there may be scope for further investment in a field in economic decline, it
should not tie up funds that can be used more effectively in new projects. A mature
development must continue to generate a positive net cashflow and compete with
other projects for funds. The options that are discussed in this chapter give some
idea of the alternatives that may be available to manage the inevitable process of
economic decline, and to extend reservoir and facility life.

17.1. Infill Drilling

Oil and gas reservoirs are rarely as simple as early maps and sections imply.
Although this is often recognised, development proceeds with the limited data
coverage available. As more wells are drilled and production information is
generated, early geological models become more detailed and the reservoir becomes
better understood. It may become possible to identify reserves which are not being
drained effectively and which are therefore potential candidates for infill drilling.
Infill drilling means drilling additional wells, often between the original
development wells. Their objective is to produce yet unrecovered oil (Figure 17.1).
Hydrocarbons can remain undrained for a number of reasons:

attic/cellar oil may be left behind above (or below) production wells
oil or gas may be trapped in isolated fault blocks or layers
oil may be bypassed by water or gas flood
wells may be too far apart to access all reserves.

In the case of attic/cellar oil and isolated fault blocks or layers, it is clear that
hydrocarbon reserves will not be recovered unless accessed by a well. The economics
of the incremental infill well may be very straightforward; a simple comparison of well
costs (including maintenance) against income from the incremental reserves. Reserves
which have been bypassed by a flood front are more difficult to recover. Water will
take the easiest route it can find through a reservoir. In an inhomogeneous sand,
injected water or gas may reach producing wells via high-permeability layers without
sweeping poorer sections. In time, a proportion of the oil in the bypassed sections

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