Britannia Field, UK Central North Sea: modelling heterogeneity in unusual deep-water deposits

Abstract

Britannia Field is currently the UK’s largest undeveloped gas field, containing 4.3 × 10 12 SCF gas in place and condensate reserves of 150 × 10 6 BBL. Lower Cretaceous reservoir sands of low-moderate permeability (average 50 mD) form a combination structural–stratigraphical trap. Understanding the stratigraphy is a critical factor in reservoir description, geostatistical modelling and deliverability prediction. The Britannia Formation is unique amongst known deep-water deposits in being dominated by slurry flow deposits which present challenges in reservoir interpretation and modelling. Slurry flow deposits contain most of the gas in the upper reservoir zones of UK Blocks 16/26, 16/27a and 16/27b. Many of these slurry flow sand events may be correlated over several kilometres using petrological, sedimentological and lithostratigraphical data. In contrast, clean high density turbidite (HDT) sandstones are well developed in the lower reservoir in UK Blocks 15/29a and 15/30. These sands are less extensive, and their interpretation is constrained by high resolution biostratigraphic and lithostratigraphic data. An integrated 3D modelling environment links surface modelling, reservoir characterization, and fluid flow simulation at three different scales. Firstly, the sensitivity of production to geological heterogeneity from pre-drilled wells is tested on single-zone models. Secondly, optimization of well types and completions to maximize deliverability uses sector models containing all zones. Thirdly, the sensitivity of long-term field performance predictions to large-scale structural and stratigraphical reservoir compartmentalization is tested using full-field models. The use of different scales of reservoir model for different business purposes will contribute significantly to the optimization of drilling and production strategies as the Britannia Field moves into operation during 1998.