Located in remote interior South of Oman, Field X historically comprised of multi-stacked clastic reservoirs. Since its production in 1980s, production had declined significantly that the field was identified as a potential abandonment candidate. In 2020, Field X integrated team successfully discovered commercial oil accumulation in overburden, naturally fractured carbonates reservoir Y; first carbonates discovery in the area. This case study focused on how the team embarked on the challenge to accelerate production from this new reservoir. Following this success, short-term opportunities were identified whereby existing vertical well stocks were re-perforated to evacuate production from reservoir Y.

Key decisions that needed to be answered for full field development concept were optimum number of wells, well type and recovery mechanism. Analogues study was done and horizontal appraisal wells were identified and drilled. Acquisition and interpretation of static (including image logs) and dynamic (including pressure transient analysis) data in reservoir Y provided somewhat fractures characterization. A model framing event was conducted to identify complexity level of dynamic modeling required to answer above key decisions. History matching and sensitivity analysis using experimental design was performed to identify the most impacting key parameters to the development. Reperforation of existing vertical well stocks and 2021 horizontal well appraisal drilling not only generated early oil, but also valuable data gathering which helped in determining uncertainties ranges. Sensitivity analysis concluded that key uncertainties to the project were fracture characterization and its distribution across the field and oil water contact. Decision-based modeling approach was carried out. This means rather than building complex 3D-discrete fracture network model, a relatively simple full-field 3D-Dual Permeability simulation was deemed sufficient to answer decisions to be made whilst honoring uncertainty ranges. This was not only efficient but also at the same time brought early insights to key uncertainties in the development and therefore the team was able to identify what additional data acquisitions are relevant for the project moving forward. Three deterministic realisations were selected from probabilistic runs, and forecast results were also benchmarked to analogous fields for realism checks.

The development will be phased, with high confidence wells drilling scheduled to start in early 2023, to balance between early oil recovery (first key value driver of the project), lower UTC and maximising ultimate recovery. The total development of reservoir Y, up to 5 wells drilling campaign, was estimated to bring total resources volume double of that of NFA's (no further activities) remaining reserves. This leads to growth for Field X with attractive UTC and NPV and ultimately maximizing its value.

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