Deepwater Development: Wet or Dry Tree?
Authors: Reid David , Nunez Daniel , Dekker Martijn 
Published: June 2014

This paper explores and discusses the interplay between field development and lifecycle reservoir management and the selection and operation of dry or wet tree host systems for the development and production of deepwater oil and gas fields. Drawing on insights from recent Shell deepwater projects, the selection criteria related to development and reservoir management are highlighted and discussed to portrait the advantages and limitation of the two different host concepts. The paper does not attempt to expand on the engineering and construction elements of either option.

Introduction

Globally production from deepwater fields contributes over 5mln barrels of oil per day of total production which is expected to significantly grow in the coming years. By the end of decade production from recent discoveries, in-appraisal projects and those projects currently in the construction phase are expected to grow global deepwater production to 8 - 10mln barrels of oil per day. Over the next five years it is anticipated that the global deepwater industry will spend in excess of USD$250 billion in capital investments.

In deepwater it is common for approximately one third of the initial capital investment to be related to the host system. The host serves among other purposes as the key well/subsea system tie-in point, support structure, production processing system, (in some cases) drilling operations and accommodation for crew. Selection of the host concept is critical and is dependent on a robust understanding of the many upfront development drivers. The host is ultimately the key linchpin in the overall development system and is critical to deliver a solution which maximizes economic value over the life of the project, provides a degree of mitigation against key reservoir and production uncertainties, which are inherent in deepwater developments, along with providing flexibility to capture any potential future production upsides that have been identified in the proximal development location.

On this basis, perhaps the most critical and important analysis required during the initial phase of a deepwater oil and gas appraisal/development project is the identification of those aspects of the opportunity which drive the project scope and the project value proposition. Correctly identifying these " key drivers" upfront lays a solid foundation from which subsequent major project decisions related to (a) selection of an integrated development concept, (b) approach to the project definition phase and (c) project execution model, can be taken. Successful developments identify both the " key drivers" and their " interplay" early in the development process and continue to revisit this framework at key decision points as the project matures in order to test both validity and potential impacts on these decisions. Both the identification of key drivers and the subsequent project decisions they impact play firmly into the eventual approach adopted for production operations - the phase of the project where the production value is actually realized. Failing to develop a robust understanding of the key development drivers during the appraisal and feasibility phases of a project can result in major recycle and project delays, or worse poor concept selection decisions which inhibit the delivery of production and ultimately cash flow/value from the asset in the future.

One of the key, if not most important, decisions related to deepwater host selection is whether to adopt a dry or wet tree system. Both systems offer distinct advantages and disadvantages depending on the context of their application in developing deepwater petroleum resources. Clearly, the decision on a dry versus wet tree system cannot be taken in isolation and should be taken in the context of the overall development system evaluation from the reservoir in the subsurface, preferred recovery mechanism, reservoir location and well placement preferences, well design, subsea equipment, processing requirements, full life reservoir management requirements and metocean/structural design considerations of both host and riser systems.