The Maturing Alwyn Triassic Field, United Kingdom Continental Shelf: Improving Gas/Condensate Recovery from Low Net/Gross Low Permeability Sandstones

Fretwell, Nick, Pierre Baux, Total E&P UK PLC, Aberdeen, United Kingdom

The Alwyn North Field is located on the western side of the Viking Graben of the Northern North Sea and is one of the N-S elongated westerly-dipping fault blocks of the Brent province. Current production is from 3 reservoirs. Initial Brent oil and Statfjord gas produc­tion was later supplemented by Triassic gas condensate recovery after successful appraisal in 1995. A further 12 dedicated Triassic producers have since been drilled as conventional vertical wells.

The Carnian to Rhaetian Upper Lunde gross succession is up to 800m thick. The Alwyn reservoirs are 2-10m thick fluvial sandstones deposited as ephemeral and perennial chan­nels with splay and lake margin deposits within a dominant floodplain paleosol environment. The sands are heavily and variably cemented with calcite and chlorite and most permeabili­ty results from secondary porosity generation in the burial environment. Production from thin sands comes from over 10 stratigraphic intervals spanning over 400m thickness of sequence, but is constrained by reduced reservoir quality at depth. Most production to date is from two intervals where channels are preferentially laterally and vertically connected sug­gesting constrained valley-fill deposition following periods of overall incision. These inter­vals are significantly depleted even in downdip locations over 2km from the main crestal producers.

To assess remaining recoverable reserves, our approach uses numerous 3D geomodel realisations that have been constrained and fully conditioned by a rigorous analysis of his­torical well performance data. These models are used to assess incremental reserves bene­fits from multilateral well designs and fracturation techniques, to prolong Alwyn Field life.