Managing reservoir uncertainty after five years of field life: Britannia Field

Abstract

The Britannia Field, located 210 km northeast of Aberdeen, is the largest Lower Cretaceous condensate gas accumulation in the North Sea. The field spans four UK blocks (250km2) and is a combination structural/stratigraphic trap. Assessing the remaining uncertainty in a field the size of Britannia is crucial to future development planning scenarios. After five years of field life, it has proved vital to take the ‘reservoir pulse’ and to evaluate the key reservoir parameters that impact the future development of the field.The Britannia reservoir consists of deep-water mass-flow sandstones, ranging from high density ‘clean’ turbidites to non-reservoir facies consisting of ‘mixed-slurry’ and debris flows. Post-depositional remobilization and slumping have modified the original depositional fabric; however, a wealth of core data (over 20000 ft), along with 63 logged wells and dynamic production data, have allowed refining of the flow unit definition within the reservoir. A rigorous re-correlation study based on the integration of biostratigraphy, chemostratigraphy and pressure data has defined an overall sheet-like reservoir architecture, with some units exhibiting a more complex channel-like geometry (Zones 30/40).The Britannia reservoir teamhas capitalized on recent advances in reservoir modelling technology, including a more rigorous approach to uncertainty analysis in both the static and dynamic realm. The results from a dynamic uncertainty study have provided key information on the range and magnitude of key petrophysical parameters and their impact on reservoir performance and ultimate recovery. Weighing the impact of a deterministic versus stochastic approach has also been a critical factor in the facies distribution. The major uncertainties in the dynamic reservoir model that impact reserves and recovery factor are (1) original gas in place, (2) fault and/or stratigraphic transmissibility ‘baffles’, (3) effective permeability, and, (4) condensate banking. In the static geological model, Jacta – a Gocad uncertainty module – has led to over 300 realizations of the full field model. In the dynamic realm, experimental design was used to monitor reservoir performance by capturing the variables having largest impact on dynamic flow behaviour and gas recovery.