Enhancing Value from a Mature Offshore Field: Sweet Spot Identification Leveraging Subsurface Insights and Analytical Reservoir Engineering for Optimal Infill Campaigns

Abstract S-field situated offshore the Sabah coast in East Malaysia has been producing from multi-stacked, compartmentalized reservoirs for 45 years recovering ∼45% of the STOIIP and ha s undergone 10 development campaigns. Uncertainty on identifying sweet spots through dynamic modelling has been challenging owing to limitations on data acquisition/quality/reliability and inherent issues with the modelling approach, typical for a matured field. Results from the first well of recent drilling campaign offered striking surprises, which triggered application of classical reservoir engineering practices – resulted in a successful campaign. Higher EUR per well were achieved from rest of the campaign wherein quick decision making could be achieved through focussed and integrated multi-disciplinary approach. Building on this experience, the targets for an upcoming drilling campaign were achieved through analytical reservoir engineering, complemented with robust subsurface risk assessment -seismic, geology, petrophysical and reservoir engineering considerations. Reservoir targets were identified based on meticulous analysis of well/reservoir performance, open-hole logs, contact movement and seismic bright-spots. Drainage radii per zone around existing and past drainage points were established based on the EUR. The targets were screened visually on maps lest they should interfere with the drained areas. The targets were categorized as untapped reservoirs, attic oil, sweep and low-recovery areas, which helped in summarizing risks. Targets were stringed together in an optimal way by considering the possibility of commingling reservoirs within a single infill well, thereby reducing risks, minimizing costs, and maximizing recovery. Well trajectories were planned to target the identified sweet spots through fewer platforms utilizing empty slots or abandoned wells slots. A bespoke approach was adapted to check and mitigate crossflow between zones throughout the well-life cycle through network modelling involving the material balance and vertical flow performance models. The targeted zone-level STOIIP were derived based on GRV around them, clipped for contacts and faults, which were used as inputs for material balance models. The inflow properties for each zone were benchmarked productivity of existing wells stock. The forecast for each proposed well were calibrated with liquid production, water cut, and gas-oil ratio trends observed in nearby existing wells. With growing portfolio of mature fields in the industry, the application of analytical techniques is growing leaps and bounds in identifying sweet spots. Innovative approaches/workflows like the one illustrated in this paper are often found useful. The key enablers for identifying the infill locations and optimizing them are integration of data and application of basic knowledge of sub-surface domains. This paper is a good example to illustrate the importance of such workflow and enablers to extend the life of mature field and add value to the business.