Optimization of Well Performance in a Selective Subsea Sand-Control Completion, Offshore Nigeria
- Frederic J.-B. Guinot (Addax Petroleum Corporation) | Bruno A. Stenger (Abu Dhabi Co. Onshore Oil Opn.) | Toni Ezeukwu (Addax Petroleum)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2007
- Document Type
- Journal Paper
- 250 - 257
- 2007. Society of Petroleum Engineers
- 2.2.2 Perforating, 2.2.3 Fluid Loss Control, 3.2.4 Acidising, 1.7.5 Well Control, 5.6.4 Drillstem/Well Testing, 1.3.2 Subsea Wellheads, 1.10 Drilling Equipment, 3.2.5 Produced Sand / Solids Management and Control, 2.4.3 Sand/Solids Control, 2.4.6 Frac and Pack, 4.2.3 Materials and Corrosion, 1.14 Casing and Cementing, 1.8 Formation Damage, 4.2 Pipelines, Flowlines and Risers, 4.1.5 Processing Equipment, 5.1.1 Exploration, Development, Structural Geology, 1.6 Drilling Operations, 3 Production and Well Operations, 2.7.1 Completion Fluids, 2.3 Completion Monitoring Systems/Intelligent Wells, 1.2.3 Rock properties, 5.1.2 Faults and Fracture Characterisation, 1.2.7 Geosteering / reservoir navigation, 5.1 Reservoir Characterisation, 2.4.5 Gravel pack design & evaluation, 3.2.2 Downhole intervention and remediation (including wireline and coiled tubing), 4.5.3 Floating Production Systems, 2 Well Completion, 4.1.2 Separation and Treating, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc)
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Developing the Okwori field (offshore Nigeria) required the combination of several recent technological advances. The subsea development targeted multiple oil-bearing zones in all wells with the requirement that each zone could be operated independently. Expandable sand-screen (ESS) strings were deployed within casing for up to four zones to prevent sand production from unconsolidated-sandstone reservoirs. Independent control of all zones was made possible by use of remotely operated surface-controlled sliding-sleeve units [(ROSS)trademark of Weatherford U.K.]
Losses experienced after perforating required the use of loss-control material (LCM) for well control. Initial LCM formulation appeared to yield formation damage. A special effort ensued to design optimal LCM pills to meet project requirements. This paper presents the completion issues associated with the project, the optimization of the LCM pills, and the design and placement aspect of the remedial treatment associated with this type of completion. The evolution of the completion efficiency for each zone throughout the learning curve is presented and discussed.
After discovering the Okwori field (offshore Nigeria) in 1972 (Fig. 1), several operators studied field development options but, none felt in a position to ensure sufficient economic returns. The inherent subsurface difficulties, coupled with the status of offshore technology and economic constraints, would not permit the development of such a complex project at that time. In 1998, Addax Petroleum Development (Nigeria) acquired this asset and subsequently engaged actively in studying its own development plan, which the Nigerian Petroleum Authorities sanctioned in 2002.
The highly faulted and compartmentalized nature of the Okwori reservoirs required dispersed well surface locations, hence, a subsea development with each well being tied back individually to a central floating production, storage, and offloading vessel (FPSO) (Fig. 2). To make the Okwori development economical, each well needed to intersect several oil-bearing reservoirs. The well trajectories were carefully planned to target these successive horizons. The wells were cased and cemented to isolate all reservoir sands. Local regulation stipulates that each reservoir must be able to be tested and produced individually. Therefore, an inner-completion string had to be designed to allow complete independent control of all the producing zones from surface.
In the Niger delta region, oil is produced from weak-to-unconsolidated sandstones. Because of the "interventionless?? nature of the subsea wells, a sand-exclusion system compatible with the required full selectivity was chosen for the Okwori development. ESS™ were installed inside the 95/8-in. casing. An inner completion with packers and ROSSTM was then installed before the well was tested and hooked up to the FPSO.
The Okwori development required highly productive wells to meet the set financial goals. The integrity of the wells was also an important concern because any well intervention would take place at a huge expense as a result of semisubmersible rig mobilization costs. Under these conditions, the different steps taken to minimize formation damage were key to the project success. Curing the losses after perforating without damaging the formations and restoring the original near wellbore permeability of each zone were major sandface-completion issues.
The complexity of the Okwori field can be best summarized by the large number of reservoir layers and fault-delimited compartments (Fig. 3). resulting in numerous potentially hydrocarbon-bearing pools. More than 100 of these fault-dip closures were mapped from two vintages of 3D-seismic surveys while approximately 30 pools were penetrated by six wells drilled before the start of field development
The structural complexity was explained through the development of a collapsed-crest anticline along two intersecting sets of syn- and post-sedimentary fault planes (Fig. 4). Well trajectories were designed to intersect reservoir closures parallel to fault planes. Hydrocarbon content (oil or gas) and fluid contacts were also found to be highly variable among reservoirs as well as among compartments of the same reservoir, adding to the overall field complexity. Risked oil-in-place volumes were calculated to rank reservoir targets and guide field development.
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