Integrated Subsurface Description of Complex Estuarine and Deltaic Reservoirs, Wara Reservoir SW Burgan Field
Authors: Bowman A.. , Wells M.. , Al-Enezi B.. , Maraj P.. 
Published: October 2017

Objectives/Scope: This study focusses on the Wara Formation reservoir in the Greater Burgan Field. The study rapidly assessed the sedimentological and stratigraphic controls on production and water-flood behaviour in the southwest portion of the Wara reservoir in the Burgan Field, which is in the early stages of water-flood development.

Methods, Procedures, Process: This eight-week long study delivered a refreshed Wara reservoir zonation scheme in the southwest portion of the Burgan field and new Reservoir Depositional Element Maps. Integration of production data (datum pressures. PLT's-ILT's and tracers), geological data (core, well-logs, analogues) enabled the identification of flow units bounded by significant ‘flooding surface shales’. It also showed that communication between these flow units is highly baffled because of the extensive bounding mudstone layers and complex internal flow unit sand and shale body architecture.

Results, Observations, Conclusions: Two main Reservoir Management Units, which are bounded by laterally extensive mudstones that can be correlated across the study area, were identified. The oldest of these is developed in the Lower Wara and comprises multi-storey fluvial/tide-influenced deposits deposited within an incised valley. A prominent flooding surface separates the Lower Wara Formation from the Upper Wara, which comprises tide-influenced proximal delta-front and delta-top deposits. Integration of reservoir geology, pressure and dynamic production data suggests that there are at least two higher-resolution units within the Upper Wara. Recognition of this reservoir architecture has implications for zonal perforation strategies and injector-producer conformance; for example, water injection into the Lower Wara flow unit is highly unlikely to provide pressure support or sweep to either of the Middle Wara flow units. Lateral communication within a given flow unit is expected to be better, but still limited, due to multiple levels of geological heterogeneities. Whilst injectors may provide pressure support over relatively long-distances (-5 km), fluid flow-paths are expected to be complex and tortuous.

Novel/Additive Information: The study highlights the value of rapidly integrating diverse subsurface data sets, including core, electric-logs, pressure and production data to assess reservoir layering and sand body architecture, reservoir description uncertainties and their potential impact on production. New subsurface products delivered by the study illustrate how reservoir layering and reservoir architecture strongly impact oil-water contact movement due to production. Coning from a bottom drive aquifer, which occurs in relatively homogenous high net-to-gross reservoirs such as the Burgan Formation, is not a significant issue in the Wara Formation, where there are multiple flow units characterized by internal baffles and barriers. Phenomena such as multiple moved oil water contacts and cusping lead to uneven flood fronts and reduced sweep efficiency.