The Role of 4-D Seismic Interpretation and Dynamic Modeling in the Successful Drilling of a Late Stage Development Well in the West Brae Field, North Sea, UK

Bacciotti, Florence ¹; D'Amore, Karine ¹; Seguin, Julien ¹
(1)Subsurface, Marathon Oil UK Ltd, Aberdeen, United Kingdom.

West Brae is a Paleocene turbidite field composed of two reservoirs: the Balder and the Flugga sandstones. Production started in 1997. A 4D seismic volume was generated over the field (using 1993 baseline and 2007 monitor surveys) to aid the identification of potential late stage development targets.

In the Flugga reservoir, the main 4D response is due to a water rise caused by the producing wells. In the northwest area of the field, the 4D seismic did not exhibit a water response despite the presence of a production well since 2005. A water cone developed at the heel of the well but not at the toe, although the reservoir properties remain equally good along the well path. To the west of this area, a linear 4D water response feature is present along the edge of the reservoir. These observations indicate the potential presence of a compartment that is inefficiently swept or un-drained. A pilot hole drilled in 2005 in the northwest also showed that the oil-water contact had not risen here, despite 8 years of production.

The integration of all the data into the dynamic model during the history matching process implied that extensive, but sub-seismic barriers (faults, shales or both) are necessary to produce an effective baffle to fluid flow and pressure transmission. Numerous iterations were carried out to determine which geometries best reproduced the observed 4D signal while also honoring the production and pressure data. The base case model incorporates a shale barrier in the form of an anticline. In this model, all Flugga producers are located above the anticline whereas the 2005 pilot penetrates the compartment below the barrier. The barrier therefore protects the area without a 4D water response from being swept by the producers. The linear feature results from an edge drive mechanism induced by the producing wells as water overrides the shale barrier.

Integration of 4D seismic interpretation and multiple alternative history-matched dynamic models led to the definition of an economic infill target in the compartment shown to be un-swept. A pilot well and an oil producer were successfully drilled at the end of 2010. Both wells confirmed the model of water overriding a shale barrier and the presence of an un-swept compartment underneath.

The 4D seismic has been a key tool for West Brae reservoir management. Integration of the 4D data changed how the reservoir was viewed and aided identification of a target that would not have been drilled otherwise.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.