Combining Expandable and Intelligent Completions To Deliver a Selective Multizone Sandface Completion, Subsea Nigeria

Abstract

In order to comply with local regulation, production volumes from each reservoir in the Okowri field offshore Nigeria, must be identifiable. Addax Petroleum's strategy for their Okwori subsea field development project was to produce a number of stacked reservoirs in each of the wells, using selective completions to facilitate testing and fingerprinting of individual zones. Due to the unconsolidated nature of the reservoir, downhole sand control was required for all wells. A technical study of all available technology at the time of design showed that expandable sand screen deployed inside the casedhole could deliver the combination of reservoir zonal isolation and internal diameter for integration with selective downhole control flow valves and isolation packer. This combination of expandable and intelligent completion technologies was an industry first. The initial completion design was based on casing and cementing the entire productive interval prior to perforating each of the target reservoir sands. Expandable sand screens [ESS®] were then placed across each interval spaced out with blank pipe. Isolation packers were set within the string to provide zonal isolation. Following installation of the sandface completion, downhole flow control valves were run through the expanded sand screens and the producing interval isolated, with packers set in the blank pipe run between the sections of expandable sand screen. This completion configuration delivered zonal isolation and surface control over each zone, facilitating selective production and fingerprinting. This paper describes the initial casedhole completion design, systems installation and subsequent performance as well as the engineering and project management that are required in order to deliver this combination new technology system. Further, the paper will also discuss how the original completion design has evolved with the introduction of the latest openhole zonal isolation devices to deliver a true openhole sandface completion with intelligent selectivity. Introduction The Okwori oil field [today OML 126] was discovered offshore Nigeria in 1972, approximately 50 miles southwest of the city of Port Harcourt [Fig. 1]. Average water depth in this region is 440 ft. Field appraisal revealed a complicated geological structure with fragmented hydrocarbon pools of limited extension that discouraged further development The Okwori field was acquired in 1998 by Addax Petroleum Exploration [Nigeria] Ltd. who presented a development plan that received Nigerian Petroleum Authorities approval. Okwori field development drilling started in July 2004; first oil was delivered in March 2005. The initial Okwori development phase included 8 producers and was completed by October 2006. Okwori Project Overview A challenging subsurface Away from the traditional image of an anticlinal oil trap, Okwori field is a chaotic ensemble of superimposed hydrocarbon and water bearing pools separated by faults. This structural complexity was explained through the development of a collapsed crest anticline along two intersecting sets of syn- and post-sedimentary fault planes. The convergence of multiple faults in the core of the collapsed crest and the presence of shallow gas accumulations added to the natural complexity the challenge of difficult seismic data interpretation. Today, more than 250 fault-dip closures have been identified, 59 of them with proved hydrocarbon content. Appraising while developing Given the subsurface complexity, the information available from the initial discovery and appraisal wells was very limited. The presence of hydrocarbon had been established; a well test had shown good permeabilities and pools of limited size. Side wall sample analysis established the unconsolidated nature of the reservoir sands. Development wells would have to access scattered reserves, by penetrating sand compartments of largely unknown fluid content and production potential, since even hhydrocarbon content [oil or gas] and fluid contacts differed between compartments of the same reservoir level.