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Results
Strategies and Best Practices for Selecting Reservoir Sector Schemes to Manage Offtake in a Waterflood
Obeta, Chukwudi (ZADCO) | Al Dhanhani, Omar (ZADCO) | Tee, Aaron (ZADCO) | Al Shehhi, Budoor Hasan (ZADCO) | Qi, Zili (ZADCO) | Li, Dachang (ZADCO) | Brantferger, Kenneth (ZADCO)
Abstract Most reservoirs, especially in carbonate systems, are characterized by heterogeneities in pore systems, permeability, diagenetic imprints, faults and lineaments, fractures, fluid systems and displacement parameters. These variable set of conditions may result in significant differences in in-place volumes as well as life cycle production behavior from different regions of the reservoir when subjected to the same depletion scheme. To guide and control the reservoir life cycle development and management, and to leverage the full benefit of tailoring specific depletion designs suitable for specific areas of the reservoir, the asset owner has instituted a new set of guidelines on reservoir sectorization to govern all operated assets. Reservoir Sectorization could be defined as the process of outlining regions of the reservoir defined by prominent geologic features (structural, stratigraphic, diagenetic, fluid contacts, etc.) that control reservoir and well performance within which production targets can be defined, analyses conducted and surveillance plans designed. A large offshore carbonate reservoir operated by ZADCO, which is the subject of this paper, is impacted in several ways by the aforementioned geological complexities and was deployed as a pilot to test and fully implement the new guidelines on reservoir sectorization. Previously, the existing sector pattern for the candidate reservoir was based on a five-spot flood scheme which is no longer suitable in view of the change to the current field development concept of artificial islands and long MRC wells. Therefore, in addition to compliance with the new guidelines, the key business driver for re-sectorization is to ensure sector geometries that support a line drive system, avoid wells intersecting sector boundaries and provide flexibility to adapt well placement to evolving field development challenges. Advanced integrated workflows (analytical and simulation) were implemented and several sectorization schemes designed (1 or "no" sector, 3 sector, 4 sector, 4 quad, and 5 sector-patterns), assessed and benchmarked against defined criteria for success. Further optimizations through numerous dynamic model sensitivities led to adoption of a particular scheme that best complies with new guidelines on sectorization as well as meet the life cycle production target for the reservoir. This paper highlights the workflows implemented, the challenges encountered and the proposed best practices in working and adopting a reservoir sectorization scheme that best enables sustainable production and maximum recovery within the constraints of the asset owners' guidelines.
Assessment of Simultaneous Water and Gas Injection SWAG Pilot in a Giant Offshore Carbonate Reservoir
Chen, Peng (Zakum Development Company) | Al Sowaidi, Alunood K. (Zakum Development Company) | Patel, Harshad (Zakum Development Company) | Brantferger, Kenneth (Zakum Development Company) | Bin Buang, Kamarul Ariffin (Zakum Development Company) | Syed, Fahad I. (Zakum Development Company) | Shehhi, Rashed Al (Zakum Development Company)
Abstract Gas injection has been widely used by industry for improving oil recovery. A common variation of gas injection is to have water-alternating-gas (WAG) injection which attempts to overcome gas override in an oil reservoir. In this paper, a simultaneous water and gas injection pilot is discussed. Water and gas are injected simultaneously using two strings in the same well with water injected into an upper zone and immiscible hydrocarbon lean gas injected into a lower zone. This is referred as SWAG in the paper. The subject reservoir is part of a large field located offshore in the Middle East. Water flood started in this under-saturated oil reservoir since 1980’s. Five immiscible gas injection pilots (GIPs) including the SWAG pilot were initiated in late 2001 in this reservoir to assess the benefits of immiscible gas injection in improving oil recovery. This paper describes the performance analysis from field surveillance data of the SWAG pilot which is also aided by a sector simulation model. Performance comparisons of the SWAG pilot versus the earlier completed gas injection pilots is also demonstrated. The history matched model successfully predicts gas arrival timing in the pattern producer. The model results are used to infer the swelling effect of the injected gas as well as to estimate gas sweep efficiency. Both RST log and simulation results show that simultaneous water injection introduced in the upper zone is not able to mitigate gas override. The model analysis, coupled with results from special core analysis, suggests the immiscible gas is not as effective as water injection in maximizing oil recovery from this reservoir. The volumetric sweep efficiency of the injected immiscible gas is estimated to be less than 10% pore volume (PV) and the swelling benefit is less than 1% OOIP at the time of gas breakthrough.