Subsea Development of Okwori and Nda Oil Fields, Niger Delta

Abstract

Summary 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. Introduction 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 development plan. 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 gas accumulations. 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.