Production Technology Challenges in Deepwater Subsea Tie-Back Developments By
E.A. Ageh, SPE, O.J. Uzoh, SPE, and I. Ituah, Shell Nigeria E&P
A
bstract
The era of easy oil is coming to an end, a lot of the major reserve finds these days are located in very challenging operating environments such as deep and ultra-deep water. Subsea tie-back systems over the years have evolved as a solution to the challenge of harnessing these reserves in a cost effective manner. The challenges for subsea type developments are not only limited to the cost of drilling and infrastructure or the complexity of the subsea layouts but also the technology of assessing and producing the volumes to surface poses a great challenge. Production technology challenges include multi-phase fluid flow, completion design, flow assurance (hydrate mitigation & management), well intervention and long term well monitoring. Of particular concern are the issues of waterflood management, intelligent completion & production systems which are core to achieving increased ultimate reservoir recovery and production volumes required for cost effectiveness. This article highlights the major production technology challenges articulated for a typical long subsea tie-back development and discusses how these could be managed.
Introduction
For the sake of this review we considered a deepwater field located in the Gulf of Guinea. The field is comprised of a combination of stratigraphically and structurally trapped hydrocarbon accumulations in middle miocene turbidite reservoirs. Six oil-bearing reservoirs (A, B, C, D, E & F) have been targeted for development as shown in Figure 1, the reservoir bodies are stacked with distinct NNE to SSW orientation with the exception of “A” reservoir which has an east to west orientation. The proposed development concept is a tieback to a host
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- February 2017
facility (an FPSO) that is about 18 kilometers away and is the most optimal concept for economic development of the opportunity. The development philosophy is to fill ullage on the FPSO as host production declines and also to utilize existing infrastructure as much as possible. The expected full field development will consist of 32 subsea wells (17 production and 15 water injection) including dual zones completions (Smart wells).
Background
The base case development scheme includes: (Figure 2) Ó - Drilling of 25 wells in the first phase including 13 production wells and 12 injection wells. Ó - Connection of the production wells to four production manifolds and the injection wells to three injection manifolds. Each of manifolds has slots for six wells Ó - Laying of 10 in. subsea pipe-in-pipe single loop production flowline and injection flowline connecting the field to the host. Ó - Gaslift lines for riser-base gaslift Ó - Installation of risers at the starboard side of the FPSO to ensure connection of the subsea lines to topsides Ó - System of umbilicals and termination assemblies for controls and chemical injection.
Technical Challenges
One of the key technical challenge is how to design a robust production system to economically transport the production fluids from subsea wells to topsides and to be able to sufficiently mitigate all the flow assurance and operational risks. Flow assurance risks are crucial due to existing high pressures and low temperatures. The subsea system design and well completion design for this tie-back field was greatly influenced by flow assurance
