Akpo: A Giant Deep Offshore Development
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 18816, "Akpo: A Giant Deep Offshore Development," by F. Rafin, A. Laîne, and B. Ludot, Total, prepared for the 2007 Offshore Technology Conference, Houston, 30 April–3 May.

Deep offshore developments are difficult to implement and often are beyond the limits of proven technologies at their inception. At plateau production, Akpo field, 200 km offshore Nigeria in 1400 m water depth, will produce and export 175,000 B/D of condensate and will export at startup 320 MMscf/D of gas to the Bonny liquefied-natural-gas (LNG) plant, onshore Nigeria.

Introduction

The Akpo development experienced several challenges. A hybrid condensate-production/gas-export development scheme maximized hydrocarbon recovery. Reservoir management required massive pressure-maintenance facilities, extensive use of intelligent and selective completions, and subsea multiphase-flow measurements. A drilling strategy and well architecture had to be developed that maximized reliability and availability of the subsea layout. An extensive qualification and testing program of equipment was needed to meet the reservoir conditions. Also, the FPSO concept pushed the limits for handling the high volume of high-pressure fluids and gas inventory.

Akpo Development

Wells are in water depths ranging from 1300 to 1440 m across the block, and there are areas where the subsurface geology challenges drilling and completion capabilities. A total of 44 subsea wells (22 production, 20 water injection, and 2 gas injection) will be drilled in dispersed clustered groups, as shown in Fig. 1. The production wells will maximize the duration of the condensate-production plateau. Reservoir management will be applied to produce condensate from low gas/liquid ratio (GLR) reservoirs initially and control the arrival of water in the wells. In addition to reservoir constraints, well-target locations for the field were derived, taking into account such factors as drilling and completions, seabed topography, shallow gas, flow assurance, and flow-line laying and stability.

The field is developed with a complex subsea infrastructure, having a network of flowlines, umbilicals, and production facilities. The 110 km of subsea flowlines are connected to the surface production facilities by 14 steel catenary risers hanging off both sides of the floating production, storage, and offloading (FPSO) vessel. The subsea network comprises four production loops, one gas-injection line, four water-injection lines, one gas-export line, and 65 km of static and dynamic umbilicals.

The subsea production and gas-injection wells are connected from 10 manifolds (nine production, one gas-injection) installed on the seabed. Condensate production will start in 2008, and plateau production of 175,000 BOPD will be reached within 90 days. In addition, up to 320 MMscf/D of gas will be delivered to the Bonny LNG plant, onshore Nigeria.

Complex Reservoir Fluid

The reservoir fluid in Akpo is a critical fluid; therefore, whether it is liquid or gas depends on pressure, temperature, and depth. Consequently, no sharp gas/liquid contact exists within the hydrocarbon leg. The produced liquid is light oil and condensate (42 to 53°API gravity), with a high GLR of 1,600 to 7,300 scf/bbl).