Page 404 - Hydrocarbon Exploration and Production Second Edition

Managing the Producing Field 391

production of the hydrocarbons from the reservoir. The well constraints which may
limit the reservoir potential may be split into two categories; the completion interval
and the production tubing.
The following table indicates some of the constraints

Completion Interval Constraints ProductionTubing Constraints
Damage skin Tubing string design
Geometric skin – size
Sand production – restrictions to flow
Scale formation Artificial lift optimisation
Emulsion formation Sand production
Asphaltene drop-out Scale formation
Producing unwanted fluids Choke size

To achieve the potential of the reservoir, these well constraints should be
reduced where economically justified. For example, damage skin may be reduced by
acidising, while geometric skin is reduced by adding more perforations, as described
in Section 10.2, Chapter 10. Scale formation may occur when injection water and
formation water mix together, and can be precipitated in the reservoir as well as on
the inside of the production tubing; this could be removed from the reservoir and
tubing chemically or mechanically scraped off the tubing.
Unwanted fluids are those fluids with no commercial value, such as water, and
non-commercial amounts of gas in an oil field development. In layered reservoirs
with contrasting permeabilities in the layers, the unwanted fluids are often produced
firstly from the most permeable layers, in which the displacement is fastest.
This reduces the actual oil production, and depletes the reservoir pressure. Layers
which are shown by the PLTor TDT tools to be producing unwanted fluids may be
‘shut-off ’by recompleting the wells. The following diagrams show how layers which
start to produce unwanted fluids may be shut-off. An underlying water zone may be
isolated by setting a bridge plug above the water bearing zone; this may be done
without removing the tubing by running an inflatable through-tubing bridge plug.
An overlying gas producing layer may be shut-off by squeezing cement across
the perforations or by isolating the layer with a casing patch called a scab liner, an
operation in which the tubing would firstly have to be removed. This would be
termed a workover of the well and would require a rig or at least a hoist, for shallower
wells with simple completions (Figures 16.6 and 16.7).
Workovers may be performed to repair downhole equipment or surface valves
and flowlines, and involve shutting in production from the well, and possibly
retrieving and re-running the tubing. Since this is always undesirable from a
production point of view, workovers are usually scheduled to perform a series of
tasks simultaneously, for example renewing the tubing at the same time as changing
the producing interval.
Tubing corrosion due to H S (sour corrosion) or CO 2 (sweet corrosion) may
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become so severe that the tubing leaks. This would certainly require a workover.