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Akpo – A Giant Deep Offshore Development
Figure 2: Floating Production, Storage and Offloading at
maximisation of liquid production depends on the control of the GLR
Port Harcourt, Nigeria
of the wells, which may vary with the reservoirs and the position of the
producing interval within the hydrocarbon column.
A Reservoir-driven Subsea Layout
The subsea network design is a critical part of the development
scheme (see Figure 3). It must be built while taking into account
different technical rules (i.e. flow assurance, drilling feasibility,
installation constraints, etc.), while at the same time including
flexibility for reservoir model evolution or actual reservoir behaviour
after the start of production.
In the early stages, the subsea layout is one of the main evolving
concerns due to changes in the reservoir interpretation throughout the
project’s life. It is particularly true for the flowline trajectories, which
must be laid out in such a way that it is possible to reach all of the well
Figure 3: Artist’s Impression of the Akpo Development targets for the producers and the injectors. It is also true for the flow-
assurance constraints, which are dependent on well by well production.
Reservoir Models
The Akpo Reservoir Model was derived from the geological model,
seismic interpretation, well data and regional context, and it leads to
a definition of the well targets. In turn, this allows the production
profiles to be estimated and a subsea layout to be initiated. An
iterative process of cost estimating and refining takes place until an
optimised solution is achieved.
In the case of Akpo, the initial conceptual development plan was
based on full gas re-injection, which then evolved into a hybrid gas
export scheme at project sanction. Reservoir studies demonstrated
that the reduced injection rate of the hybrid option in fact increased
liquids recovery while continuing to enable substantial gas export.
API) with a high GLR (1,600–7,300scf/bbl). Critical fluids are now Effects on the Subsea Layout
better understood and have been the subject of recent The subsea layout design must respect certain rules and constraints:
developments in the field, but this is the first time such a
development has been conducted in a deep offshore location. The safety (FPSO layout of safe and hazardous areas, riser/umbilical
specifics of this fluid type are: interaction due to motion);
environment (soil conditions for equipment installation, mooring
complex oil column, which requires special attention from the reservoir footprint exclusion zones, water depth, seabed topography);
management strategy to avoid any loss of liquid recovery; and flow lines (up-sloping criterion to minimise pockets, line length,
high GLR, which has an influence on the type of development. line stability);
Akpo is not only a giant condensate field with an estimated operations (maximum and minimum flow rates in flow lines and
620Mbbls reserve, but also a gas field with 1tcf planned export gas. jumpers, line preservation during shutdowns to prevent hydrates,
well-testing requirements);
In the case of Akpo, specific attention is given to pressure well trajectories (shallow gas hazards, drilling feasibility, target
maintenance, as models indicate that a low pressure (LP) drop would azimuth change, stand-off distance);
induce a change to a dysphasic context and jeopardise the liquid installation (flow-line geometry, spacing of water injection in-line
recovery. This pressure maintenance can be ensured with water and tee assemblies, minimum distance between lines); and
gas injections and necessitates the availability of sufficient injection control system (number of wells per umbilical core).
capacity at start of production. As the hydrocarbon volume occupied
in the reservoir conditions is much larger than the volume of liquid These rules and constraints dictated the framework to design the Akpo
produced (formation volume factor 2–5), the producer/injector ratio is subsea layout. In addition, the subsea layout is influenced by surface
higher than usual and reaches one injector for one producer. installation constraints, such as the overall maximum flow rates
(production and injection), delivered pressure and temperature limits.
Gas production is a crucial part of the reservoir management as it is Flow-assurance concerns are critical for any deepwater
limited by the capacity of the FPSO and, in particular, the emergency development. The static head at seabed temperature invariably puts
flaring system and the export pipeline capacity. Thus, the the system in the hydrate regime. The Akpo solution is to have a
EXPLORATION & PRODUCTION – VOLUME 7 ISSUE 2
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