Benefits of Adding Energy to the Wellstream at the Mudline
Authors: Johansen Oddgeir , Leporcher Emile , Lezeau Patrick 
Published: October 2008

This paper investigates the benefits of adding energy or performing partial processing to a subsea production system. An hydrodynamic transport study has formed the basis for a subsea process and power demand analysis. It was followed by a technology review to identify current technology gaps for realizing the studied alternatives. A generic field case has been selected as work basis. A spreadsheet model has been developed for year-by year analysis of various production profiles and to combine the results of the different parts of the study. This paper will focus on the study summary tool with emphasis on three scenarios where the reservoir depletion strategy varies from fixed liquid production with pressure maintenance by water injection, maximum acceleration of production using the available installed power and reduced reservoir pressure maintenance. The results are discussed based on an economic NPV analysis where added oil sales value, CAPEX and OPEX are input. This paper will not focus on technology gaps, feasibility or state of the art. However the study has covered these aspects. The CAPEX figures do not cover savings compared to topside solutions even though these savings might be substantial.

The study has demonstrated that there are great potential benefits from boosting at the mudline level in deep offshore conditions.For high productive wells, mudline boosting technology may compensate for high riser pressure drop at high watercut.For wells with lower productivity index, multiphase pumps or gas/liquid separation with liquids pumping might lower the wellhead pressure and accelerate the production.For reservoirs where water injection is not effective or not recommended, multiphase pumps or gas/liquid separation and pumping might be used to lower the bottom hole pressure. However this requires high power demands and possibly multiple pumps.The separation alternatives are not competitive unless topside savings are obtained, but if the technology becomes commercial, it has a large savings potential.

Introduction.

Producing hydrocarbons from deep offshore fields involves many technical challenges, and it is generally assumed that the challenges get tougher with larger water depths. For several aspects of the installation, maintenance and operation of subsea equipment, this assumption is true. However, there are also benefits linked to deep water production, benefits that are increasing with increasing depth. These benefits are linked both to the environment of the installation and to process conditions.

Our industry is bound to develop deep water technology if we want to produce from the deep water fields. However, when doing so we must keep in mind the positive aspect of the subsea environment and maintain focus on the total field economy, using the advantages of the deep waters to reduce cost pr. barrel and stay competitive even at low oil price. We will in this paper look at the benefits of deep water production, compare some new concepts and evaluate the economy resulting from life cycle cost and income profiles.