Addax Petroleum’s operated Okwori oil field, offshore Nigeria, illustrated
the benefits of reviving shelved projects, because of an insufficient return on
investment using more traditional approaches, by applying more recent technical
and contractual solutions.
The Okwori project demonstrated the feasibility of developing small and
geologically complex offshore oil fields in medium water depth of 440 ft with
subsea technologies traditionally used for large fields.
In the subsurface, the Okwori wells combined multiple selective completions
hydraulically controlled from the surface with expandable sand screens as the
downhole sand exclusion solution. This combination of equipment in subsea wells
used to fully develop a small offshore oil field marked another technological
first in Nigeria.
Far away from pre-existing facilities and with less than 50 million bbl of
median technical reserves at the time of project sanction, the Okwori oilfield
development went a step further than the more usual subsea tieback to a
pre-existing offshore production facility. The Okwori development plan was
based on horizontal subsea trees flowing to a leased spread-moored floating
production storage and offloading (FPSO) vessel by means of (6-in.) flexible
subsea flowlines and risers.
The Okwori leased production facilities had a built-in capability for
additional tiebacks such as the Nda oil field, whose development was completed
in October 2006, or for later redeployment in other parts of the acreage
depending on further exploration and appraisal drilling.
A review of the field operations to date highlighted a steep learning curve
in the formulation of completion design, completion fluids, stimulation,
downhole sand exclusion systems, and bean-up/bean-down procedures.
The Okwori oil field (OML 126) was discovered offshore Nigeria in 1972,
approximately 50 miles southwest of the city of Port Harcourt (Fig. 1). Despite
a prolific initial well test, subsequent field appraisal revealed a complicated
geological structure and fluid distribution with fragmented hydrocarbon pools
of limited extension. The Okwori field therefore remained dormant until Addax
Petroleum Exploration (Nigeria) Ltd. acquired the asset in 1998 and provided a
Okwori field development drilling started in July 2004 after drilling ND-1,
the Nda oilfield discovery well located due west of Okwori. Okwori first oil
was delivered in March 2005 as planned.
Subsurface Critical Success Factors
In the subsurface, Okwori’s main challenge was the large number of reservoir
layers and fault-delimited compartments resulting in numerous potentially
hydrocarbon-bearing pools. More than 100 of those pools were mapped from two
vintages of 3D seismic surveys; before field development, six wells appraised
30 such pools (Fig. 2).
The Okwori structure resulted from a collapsed crest anticline with two
intersecting sets of syn- and post-sedimentary fault planes (Fig. 3). It was
noted that seismic imaging was of poor quality owing to the convergence of
multiple faults in the core of the collapsed crest and the presence of shallow
Appraisal well trajectories were designed to scoop reservoir closures
against fault planes. Hydrocarbon content (oil or gas) and fluid contacts
differed between compartments of the same reservoir level, adding another level
of complexity to the development. Risked oil-in-place volumes were computed to
rank reservoir targets and guide the field development. Each development well
was considered as an appraisal well for which the decision to complete any
reservoir level would be taken after drilling and logging the well. It was also
clear that the size of these hydrocarbon pools would seldom justify more than a
single producer per pool. Nigeria petroleum law specified a minimum distance of
800 m between two drainage points in the same hydrocarbon pool, which in Okwori
meant only a single possible completion per pool.
Pressure maintenance through water or gas injection would require additional
wells, a situation neither financially attractive nor technically desirable
because of the small dimensions and compartmentalization of the oil rims.
Finally, the Okwori reservoirs were made of unconsolidated sandstones from
the Niger Delta that required some form of sand control.
© 2007. Society of Petroleum Engineers
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- Original manuscript received:
28 June 2006
- Revised manuscript received:
5 September 2006
- Manuscript approved:
14 November 2006
- Version of record:
20 March 2007