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Hydrogeology Journal

, Volume 11, Issue 1, pp 142–161 | Cite as

Fully coupled modeling of seabed subsidence and reservoir compaction of North Sea oil fields

  • Roland W. Lewis
  • Axel Makurat
  • William K. S. Pao
Paper

Abstract

This paper focuses on the aspects of fully coupled continuum modeling of multiphase poroelasticity applied to the three-dimensional numerical simulations of the Ekofisk oil reservoir in the North Sea (56°29′–34′N, 03°10′–14′E). A systematic presentation is chosen to present the methodology behind fully coupled, continuum modeling. First, a historical review of the subsidence phenomena above an oil and gas reservoir is given. This will serve as a background against which the relevance of the present approach to compaction and subsidence modeling will be demonstrated. Following this, the governing equations for a multiphase poroelasticity model are briefly presented. Particular attention is paid to the analysis of the pore-compressibility term usually used in an uncoupled approach for characterising the host-rock deformation. A comparative numerical analysis is carried out to contrast and highlight the difference between coupled and uncoupled reservoir simulators. Finally, a finite-element numerical model of the Ekofisk field is presented and a significant result is a contour map of seabed subsidence which is in general agreement with the shape of the subsidence contours based on past bathymetric surveys. Analysis of the simulation reveals that, due to the downward movement of the overburden, oil migration occurs from the crest of the anticline in which the field is situated, towards the flank. The pore-pressure depletion in the reservoir is significantly delayed due to the replenishment of the reservoir energy via the formational compaction. Horizontal movement in the reservoir, which is neglected in traditional modeling, can be significant and comparable in magnitude to the vertical subsidence.

Keywords

Multiphase flow Poroelasticity Ekofisk oil reservoir Finite-element analysis North Sea 

Résumé

Cet article concerne les aspects de la modélisation de la poro-élasticité multiphase complètement couplée d'un continuum, appliquée à des simulations numériques en trois dimensions du réservoir pétrolier Ekofisk, en Mer du Nord (56°29′–34′N, 3°10′–14′E). Une présentation systématique est choisie pour donner la méthodologie liée à la modélisation complètement couplée d'un continuum. Tout d'abord, une revue historique des phénomènes de subsidence au-dessus d'un réservoir de pétrole et de gaz est donnée. Ceci servira comme base par rapport à laquelle on démontrera le bien-fondé de l'approche utilisée pour la modélisation de la compaction et de la subsidence. Ensuite les équations de base d'un modèle de poro-élasticité multiphase sont rapidement présentées. Une attention particulière est apportée à l'analyse du terme de compressibilité porale habituellement utilisé dans une approche non couplée pour caractériser la déformation de la roche magasin. Une analyse numérique comparative est menée pour mettre en évidence la différence entre des simulateurs de réservoirs couplés et non couplés. Finalement, un modèle numérique aux éléments finis du champ pétrolier d'Ekofisk est présenté; un résultat significatif est une carte des contours de la subsidence du fond marin qui est dans l'ensemble cohérente avec la forme des contours de la subsidence fournie par les relevés bathymétriques passés. L'analyse du calcul montre que, du fait du mouvement d'affaissement de la couverture, la migration du pétrole s'effectue depuis la crête de l'anticlinal dans lequel est situé le gisement vers le flanc. La baisse de la pression porale dans le réservoir est significativement retardée du fait de la reconstitution de l'énergie du réservoir par la compaction de la formation. Le mouvement horizontal dans le réservoir, qui est négligé dans la modélisation traditionnelle, peut être d'un ordre de grandeur significatif et comparable à la subsidence verticale.

Resumen

El presente artículo se centra en los aspectos de un modelo continuo completamente acoplado de poroelasticidad multifase que ha sido aplicado en simulaciones numéricas tridimensionales del reservorio petrolífero de Ekofisk, en el Mar del Norte (56°29′–34′N, 03°10′–14′E). Se ha elegido una presentación sistemática para exponer la metodología utilizada para el modelo continuo completamente acoplado. En primer lugar, se hace una revisión bibliográfica de los fenómenos de subsidencia en un reservorio de petróleo y gas. Esto sirve como argumentación para justificar la relevancia del presente enfoque para modelar la compactación y la subsidencia. A continuación, se presentan brevemente las ecuaciones que gobiernan el modelo de poroelasticidad multifase. Se enfatiza en el análisis del término de compresibilidad de los poros, que se utiliza de forma habitual en el enfoque desacoplado para caracterizar la deformación de la roca madre. Se hace después un análisis numérico comparado con el fin de contrastar y resaltar las diferencias entre los simuladores acoplados y los desacoplados. Finalmente, se presenta el modelo numérico en elementos finitos del campo Ekofisk, destacando el mapa de contornos de la subsidencia del fondo marino, en buena concordancia con la forma real de la subsidencia inferida mediante registros de batimetrías. El análisis de los resultados revela que, debido al movimiento descendente del terreno situado por encima, se produce la migración del petróleo desde la cresta del anticlinal en el que se halla el campo hacia los laterales. El descenso de la presión de líquido en el reservorio se ve significativamente retrasado por la recuperación de la energía del reservorio vía la compactación de la formación. El movimiento horizontal dentro del reservorio, que es menospreciado por los modelos tradicionales, puede ser significativo y comparable en magnitud a la subsidencia vertical.

Notes

Acknowledgements

The authors would like to thank Dr. Hamid R. Ghafouri for doing parts of the simulations for the Ekofisk reservoir.

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Copyright information

© Springer-Verlag 2003

Authors and Affiliations

  • Roland W. Lewis
    • 1
  • Axel Makurat
    • 2
  • William K. S. Pao
    • 1
  1. 1.Mechanical Engineering DepartmentUniversity of Wales SwanseaSwanseaUK
  2. 2.Shell International Exploration and Production B.V.RijswijkThe Netherlands

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