Abstract The Bacchus Field, discovered in 2004, is a small borderline high-pressure–high-temperature (HPHT) oil field 6.8 km east of the Forties Alpha Platform. The reservoir is Fulmar Sandstone with a rotated fault-block trap. The reservoir is typically thin (10–50 m) and difficult to image seismically. Compartmentalization was anticipated due to significant in-field faulting. The Bacchus development decision was made when considerable geological uncertainty remained. The key risk-mitigation strategies employed during the development of Bacchus were to drill long horizontal wells, contacting multiple reservoir compartments, while maintaining a flexible development plan. The ability to react to unexpected results was facilitated by optimizing the development data-acquisition programme. Drilling risk and cost were minimized by exploiting existing well control for landing development wells, combined with pilot drilling in untested parts of the reservoir. Development wells were designed to be geometrically robust, minimizing the requirement for geo-steering. This ensured low wellbore tortuosity that did not compromise the completions. Bacchus was successfully developed despite the final distribution of reserves being radically different from the pre-development perception. It is argued that maintaining a flexible development plan was far more effective in maximizing the value of the Bacchus development than more extensive pre-development appraisal or modelling.

Abstract The Forties Field, discovered by BP in 1970, is the largest oilfield on the UK Continental Shelf. It is trapped in a simple four-way dip closure, with a Paleocene turbidite sandstone reservoir. The Forties Field originally contained between 4.2 and 5 billion bbl of oil, with 2.75 billion bbl produced to June 2017. Production has been supported by water injection and the influx of a regional aquifer. The original development contained equally spaced producers with peripheral injectors. As the field matured, production was concentrated in the crestal parts of the field with injectors tending to be moved upflank. With the development of seismic lithology prediction and fluid detection, together with 4D seismic technology, it became possible in the late 1990s to target bypassed oil in unexpected locations throughout the field. In 2003, BP sold the field to Apache who were able to rejuvenate production, adding over 170 MMbbl oil reserves, with an extended drilling campaign targeting bypassed pay identified using seismic technologies. Production at the Forties facility has been further enhanced by the development of four satellite oilfields, Bacchus (Jurassic reservoir), Brimmond, Maule and Tonto (Eocene reservoirs), together with Aviat (Pleistocene reservoir) produced for fuel gas supply.