Integrating 4D Seismic Data with Production Related Effects at Enfield, North West Shelf, Australia

Abstract

Abstract The Enfield field has a 160 m oil column located between a medium sized gas cap and a water/leg aquifer system. Enfield is undergoing an active water-flood utilizing both up-dip and down-dip water injection. The water-flood reservoir management of such a field requires timely information concerning reservoir pressures, water-flood sweep and movement of gas and water contacts. Conventional reservoir monitoring practice obtains this information by monitoring at the wellbore. Such approaches require significant time and water-cut development to determine how the reservoir and water-flood is performing and provide little spatial information as to how the water-flood is affected by faults, preferential pathways and structural variation. 4D seismic methods represent a powerful tool to assist reservoir management. This work describes the planning, implementation of an early 4D program for the Enfield water-flood and history matching process. Pre-development feasibility work indicated that Enfield had rock properties favourable for 4D monitoring as reservoir sands are acoustically soft and identifiable with seismic amplitudes. Post-production, early 4D monitoring has provided unique and timely insight into water movement both within the reservoir and through active fault networks that wellhead data alone would not provide. This work has shown the benefit of 4D in the following areas; tuning of injected water flows to a northern fault/aquifer system, locating new injector producer pairings, improved utilization of geologic and seismic barrier and baffle features into the history matching process and finally showing how the seismic response to up-dip water injection, around a key injector, was more subtle and supported the choice of imbibition relative permeability relationships and trapped gas saturations. Validation of insights was provided by the use of synthetic seismic modeling of simulation results. Introduction The Enfield field is an active water-flood project located in production permit WA-28-L, some 40km north of Exmouth, offshore Western Australia and is jointly owned by Woodside Energy Ltd (60%, Operator) and Mitsui Australia E&P Ltd (40%). Water depth across the field varies from approximately 325m in the east to 550m in the west. The Enfield oil reservoirs are within the Upper Jurassic to Lower Cretaceous age Macedon Member, comprising generally clean, high permeability, unconsolidated sandstones contained within the crest of a north-easterly plunging fault bounded terrace that covers approximately 16 km2 with a total relief of 350 m. The Enfield trap was created by cross fault juxtaposition of the relatively thin sandstone intervals encased in a thick marine shale sequence. The trap is characterised by structurally conformable high seismic amplitudes associated with the hydrocarbon-bearing reservoir interval. Enfield is highly faulted with a gentle structural dip of 3°.