Evaluation of Development Options for Marginal Offshore Green Fields in Angola
Authors: De Carvalho Nilton Augusto Barreto , Pinheiro Nilton Claudio , Grammer Sunday , Mahbou Eric Ndonwie , Chaus Konstantin S. 
Published: October 2011

The first integrated development study of a green offshore field in Angola comprising of two adjacent marginal oil reservoirs is evaluated as single entity and enables decisions to be made with a view of the bigger picture. The field under consideration comprises of two small reservoirs (Reservoir 1 and Reservoir 2) located in deep-water offshore, 85 Km off the coast of Angola and operated by Sonangol P&P (the Angolan National Oil Company).

The main challenge was to devise development strategies for these small reservoirs and produce them into common facilities from a nearby marginal field without jeopardizing current production profiles. Traditionally evaluation of oil and gas FDPs involves several teams evaluating various numerous independent simulation results of models representing sections of the entire system. Reservoir and network simulations would normally be executed separately and this separate analysis could result in biased development decisions being taken; and often lead to over or under designed production and process facility systems.

At a time of increasing demand for oil and gas, Sonangol is also working to squeeze assets for maximum, efficient recovery and optimum production while reducing operating and investment costs. Integrated asset modelling has helped to achieve these objectives.

Integrated asset modelling, couples the reservoir, surface gathering network and process facility models allowing the field-wide modelling and simulation of proposed technical developments and enhancement solutions to be evaluated as one complete system. Furthermore an economic model can also be included to evaluate the monetary impact of the solution for the entire asset. Work-overs and other remedial solutions can also be evaluated prior to taking decisions for implementation.

With the use of this technology during the FDP evaluation, relatively fast analysis was performed on the entire asset, coupling these two sub-surface reservoir models to a new proposed surface gathering network model which finally links to an existing FPSO.

This paper presents processes and workflows applied in the asset modelling of this green field in order to arrive at optimized, fit for purpose and cost effective FDP. The work presented here covers the construction of the well models, surface gathering network; quality control checks of two subsurface reservoir models; simulation and analysis of the results of the integrated asset model.

The challenges encountered in studying this green field as an entire system, range from ascertaining reservoir global and well constraints through to network optimum sizing of down hole equipment, back pressure effects, de-bottlenecking and gas lift considerations with some insight into slugging at the riser base. Results and analysis used in the decision making process are also presented. Finally, a discussion on the impact this technology made to the entire FDP process is highlighted.

Introduction

With an integrated asset model, one is able to model and simulate the reservoir, gathering/production network and facilities in a collaborative manner - following fluid flow from subsurface to surface i.e. each model positioned so that the output of one model is the input to the next. Such a setup of coupled models allows asset teams to perform analysis and what-if scenarios and see the results across the entire asset.