Summary
Several downhole-sand-control failures in Okwori subsea oil producers
triggered a significant overhaul of the downhole-sand-control method and a
review of topside sand management. Because the eventuality of further
downhole-sand-control failures in existing producers could not be ignored, a
surface-sand-handling and -management facility was designed. For the future
development producers, the downhole-sand-control method was re-engineered to
increase its reliability. The initial multizone, fully selective
sand-control-completion system, with expanded sand screens inside the casing,
was abandoned. A revised openhole sand control was designed with similar
completion selectivity. The new completion system implied a change in drilling
practices, in reservoir drill-in fluid, and in filter-cake-cleanup techniques
that required simultaneous engineering. These revisions were validated and
implemented only a few months after encountering the first major sand-control
failure. The Okwori floating production, storage, and offloading (FPSO) vessel
was not expected to deal with sand production. Temporary sand traps were
installed to minimize production downtime and to determine flow regimes that
minimized sand production. Severe-service adjustable chokes were introduced to
avoid valve erosion and stabilize production rates. Finally, a long-term
topside-sand-management solution was designed and recommended for
implementation.
Introduction
With a rig-intervention cost of USD several million and high initial capital
expenditure in the range of USD 25 to 35 million, Okwori subsea-development
producers were designed to be "interventionless" as defined in Powers
et al. (2006). Because of the very high capital expenditure, each Okwori
completion must access enough reserves to minimize payback duration and
optimize net present value. In the stacked and compartmentalized Okwori
reservoirs (Figs. 1 and 2), accessing sufficient reserves per completion
required carefully crafted well trajectories penetrating multiple targets like
"beads threaded on a necklace." Because selective production from
different reservoirs and sand exclusion were mandatory, the Okwori producers
required a combination of expensive downhole equipment. "Murphy’s law"
stands as a warning that the more equipment run in a hole, the more that can
and will go wrong. The quest for interventionless Okwori wells translated,
therefore, into a risk-minimizing search based on best-engineering practices,
proven technologies, and best-implementation practices, with the additional
constraint of challenging project economics. Risks that could affect the
project adversely were mitigated, and technical solutions were selected so that
the project would remain economical throughout its life. Early
downhole-sand-control failure on two Okwori producers during the first stage of
the development phase prompted the producer and service companies alike to
tackle these issues within a very tight schedule.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
22 February 2007
- Meeting paper published:
30 May 2007
- Revised manuscript received:
4 July 2008
- Manuscript approved:
21 July 2008
- Published online:
16 March 2009
- Version of record:
1 March 2009
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