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The giant Schiehallion Field, West of Shetland, UK, is now entering its midlife period after nearly 10 years of production. Understanding the reservoir heterogeneity and complex flood front behaviour is now critical to future field management. The big seismically-defined pools have been accessed and the next phases of development will require ever more detailed and integrated geological, petrophysical and reservoir engineering work. Even though geophysical 4D surveys and production data have been routinely acquired in wells from field startup, several key pieces of reservoir behaviour have been difficult to capture in models. The initial production and injection data showed that reservoir compartmentalisation had been seriously underestimated and recent development wells continued to yield surprises. A new approach was needed and by 2007 this was in active planning and implementation.

Experience West of Shetland shows that the seismic can be pushed hard for defining geological envelopes, connectivity and seismic facies regions to allow hybrid seismic-geological facies modelling (Loyal and Foinaven analogues show this well). A new detailed approach was completed on Schiehallion by late 2007. More than 300 seismic-scale geological bodies were mapped out and as a first pass were populated with sophisticated seismic net pay properties. Detailed geological studies including well log interpretation, core interpretations, and a revised and outcrop-grounded facies model have combined to generate a solid understanding of the geology of the field and together with innovative modelling techniques and workflows has proved successful in understanding the key geological uncertainties. Reservoir engineering efforts are primarily focused at the long term management of the waterflood and in evaluating the benefits of additional wells in the field. A major remaining uncertainty is the future water cut development in the field as this impacts the sizing of future production and subsea infrastructure. Further development options and base management decisions need to be robust to downsides and no upside opportunities can be missed.

A full suite of deterministic models is being built to capture all main uncertainties and the static and dynamic issues worked out and analysed. The outcome is a discrete range of deterministic STOIIP models and a suite of history-matched and prediction runs that also capture the true behaviour of the field. The final crunch to getting everything together is to integrate the modelling and analysis efforts with all available data, and to this effect in addition to outcrop-based ground-truthing, there will be full close-the-loop work done on the satisfactory simulation models through 3D and 4D synthetic seismic to tie everything back together.

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