Marginal Fields: Technology enables profitability/Marginal Fields and their Challenges
Authors: Husy Samuel 
Published: May 2011

The number of small and widely dispersed reservoirs is increasing. Development architectures and technologies traditionally applied to "elephants" may not be economical in the case of marginal fields.

Marginal fields can be produced either by tie-back to existing facilities, or in a stand-alone scheme in which case joint development of several marginal fields could be considered. The tie-back option reduces the initial investment but the maximum tie-back distance is constrained by the Reservoir and Fluid properties, implying flow assurance difficulties on long distances. Typical distances are up to 50 km for oil and more for gas. Standardization and minimization of the amount of equipment and flowlines are required to reach the cost target. For the stand-alone oil case, it must be assumed that tie-back to existing facilities is not feasible: on grounds of too long distance not compatible with flow-assurance constraints or lack of spare capacity at the host and no possibility of debottlenecking.

The main technical challenges for tie-back of marginal fields will be: flow assurance, wax/hydrate/sand management and appropriate artificial lift to boost the production. Preservation, start-up and sometimes pigging are also key issues.

New technologies may offer answers to such challenges:electrical heating will enable long distance single line tie-backsgas/liquid separation facilitates hydrate and slug management and topside debottlenecking, while accelerating the flow ratessubsea pumping allows longer tie-backs and increases production rateall-electric subsea systems reduce umbilical costs.

This paper will first review the limits of these technologies and examine the influence of the key parameters on the overall cost compared to standard architecture. It will suggest some rules for selecting subsea architecture and look at how new technologies could address some of the challenges mentioned above. The challenges for stand-alone case are completely different than for an extended tie-back and will not be covered in this paper.

Introduction

While the large reservoirs in the deep offshore are being produced or developed, the number of smaller and widely dispersed reservoirs is increasing. In West Africa and the Gulf of Mexico, many developments are planned over the next 4 or 5 years in waters deeper than 1000 meters. Smaller reservoirs raise new technical challenges, as their reserves often do not justify a development on a stand-alone basis. Development architectures and technologies traditionally applied to "elephants" may not be economical for marginal fields.

Marginal fields can be produced either as a tie-back to existing facilities, or combined with others in a joint (stand-alone) development. The tie-back option reduces the initial investment but its maximum distance is constrained by the Reservoir and Fluid properties, implying flow assurance difficulties on long tie-backs. The constraints with the extended tie-backs are: more demanding thermal requirements, extra boosting requirements, flow control issues and long transients operations.