Abstract The need for a new Schiehallion full field reservoir simulation model was driven by the requirement to re-evaluate the reserves in the field: the existing model indicated that the modelled volumes were potentially too conservative. This, coupled with a 50% increase in the wells database through ongoing development drilling, was the main reason for building the new model. An integrated multidisciplinary team consisting of BP and Shell staff was set up to build a new full-field reservoir simulation model for reserves re-evaluation. The paper outlines the workflow employed in building the new model, FFM2003, and describes elements of this workflow in more detail, concentrating on lessons learned during the process.

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.

Abstract The Schiehallion subsea development comprises two fields, Schiehallion and Loyal, which are located approximately 200 km to the west of the Shetland Islands in the UK Continental Shelf. The Schiehallion and Loyal fields were discovered in late 1993 and 1994, respectively, with a combined oil-in-place of more than 2.3 Bbbl. The fields are developed under waterflood and were on production from 1998 to 2013. After an extended shut-in, the fields were brought back on line in 2017, through new floating production facilities. Most of the production to date has been from the Paleocene Vaila Formation deep-water turbidite, in the T31 and T34 reservoir intervals. The ongoing Quad 204 redevelopment drilling programme commenced in April 2015, has drilled and completed 21 wells to date, and is expected to continue for several more years. The campaign includes new producer–injector pairs and stand-alone wells to support existing well stock, targeting stacked turbidite reservoir intervals, including the youngest T35–T34 interval, the main T31 interval and the previously under-developed T28–T25 fairway. In addition to an active drilling programme, a 4D seismic survey was acquired and processed in 2018, and its interpretation is key to unlocking further potential sources of value in this mature field.

Abstract John Playfair first visited the Scottish mountain, Schiehallion, during Nevil Maskelyne’s 1774 plumbline deflection experiment, which was conducted to measure the density of the Earth. The mathematician Charles Hutton analysed the survey data from the experiment, reporting the mean Earth specific gravity as 4.5 in 1778. Playfair undertook a lithological mapping exercise in 1801, to improve the accuracy of Hutton’s estimate, and reported a range of 4.56–4.87 in 1811. The computation of the gravitational effect of topography with variable subsurface density effectively made him the creator of the first geophysical model. As such, not only was Playfair’s Schiehallion contribution pioneering in itself, but it was representative of his more significant works in both mathematics and geology, in that he built on existing benchmark work with novel and valuable additions of his own. Although Playfair’s map of the extent of the Schiehallion quartzite was quite accurate, the Society’s fourth President, John MacCulloch, having visited Schiehallion, was dismissive of Playfair’s representation of the subsurface density variation. MacCulloch spent several years searching Scotland for a more favourable site for a plumbline experiment, travels that allowed him to compile the data for his 1836 geological map of Scotland.