A Case History: Different Paths, Same Destination – Two Modelling Approaches for the Corrib Gas Field

Abstract This paper compares two independent subsurface models developed by the Corrib JV partners, Vermilion Energy Ireland (operator) and Nephin Energy (non-operating partner). The models were built to investigate a static-dynamic GIIP discrepancy of 0.1–0.4 Tcf for development opportunities and long-term field optimization. Corrib is located 70 km west of Ireland at 350 m water depth. It is a faulted anticline with four-way dip closure, with the top structure crest at 3,300 m subsea. The Triassic Sherwood Sandstone reservoir is developed by six subsea wells tied back to Bellanaboy Bridge onshore gas terminal via a 95km pipeline. Production was brought onstream in 2015, reaching a plateau rate of 350 mmscf/d before declining to approximately 100 mmscf/d today. The static GIIP range is 1.1–1.4 Tcf. Nine years of production data demonstrate a dynamic connected GIIP of 1 Tcf. In 2021, ocean bottom cable (OBC) seismic was reprocessed for image quality in the Sherwood and Carboniferous horizons ahead of a series of major gas plant upgrades and possible infill and deeper horizon drilling. The significant improvement in seismic image quality gave a new top structure map and allowed for detailed fault mapping. The Sherwood reservoir consists of a high net-to-gross sequence of low-sinuosity braided river bars, playa, and sandflat facies in a dryland fluvial environment. Permeability varies significantly, with upper A/B/C sands (1–100 mD) exhibiting better reservoir quality than the deeper D to I sands (0.1–10 mD). Dynamic data has proven global communication between all wells. Reservoir performance is monitored using wellhead/downhole pressure data, pressure transient surveys, and well tests. Each JV partner's 3D static model adopted a "back to basics" approach, incorporating a core and well data-driven static model. Vermilion's model includes additional concepts such as paleo-flow and paleo-gas below the GWC. At high level, both partners estimate similar static GIIP range above the GWC with some differences in how GIIP, permeability and vertical connectivity are distributed. History-matched reservoir simulation models inform performance analysis and forecasting. Nephin's history match requires pore volume reduction in poorer E+ sands, suggesting limited contribution from these facies. In Vermilion's history match the paleo-gas is immobile and gas contribution from the D+ sands is reduced by reduction in pore volume, permeability and vertical connectivity. Both models come to a similar dynamic connected GIIP. There has been no water breakthrough after 9 years production. In Vermilion's model this is represented by the immobile paleo-gas preventing aquifer movement. Nephin's model uses a degraded aquifer. Despite the differences, both models highlight similar reservoir risks and yield comparable production forecasts. The paper demonstrates how integrated geological and dynamic modelling by two separate subsurface teams supports a robust understanding of Corrib's long-term production potential.