The Donan, Lochranza and Balloch fields, Blocks 15/20a and 15/20b, UK North Sea

Abstract The Donan Field is a mature asset in the final phase of production, following the redevelopment facilitated by various advances in technology and subsurface understanding. The original development utilized an ingenious single-well oil production system vessel which made small hydrocarbon accumulations economic, while the use of a floating production, storage and offloading vessel to redevelop the Donan Field as the ‘Dumbarton Project’ allowed the previously stranded Lochranza and undiscovered Balloch fields to be developed. Donan and Lochranza are typical Paleocene oilfields with excellent water drive from a large regional aquifer. Balloch is an Upper Jurassic oilfield of equivalent size to Lochranza supported by a large regional aquifer but has a considerably higher recovery factor on account of excellent reservoir properties combined with a more optimal geometry to effectively sweep the reservoir. Most of the fields have exceeded pre-development expectations, particularly Balloch on account of it being developed whilst considerable subsurface uncertainties remained. Recognition of a seismic amplitude v. offset response across the Donan Field was key to redevelopment, significantly increasing the oil in place and guiding the locations of development wells. This was supplemented by the ability to geosteer the horizontal development wells in the shallowest possible reservoir sand to maximize the recoverable resources. The use of horizontal development wells facilitated the development of short, areally extensive, oil columns; while the design of the production facilities and wells to include permanent artificial lift and capacity to process large water volumes was essential.