The Application of Unstructured-Gridding Techniques for Full-Field Simulation of a Giant Carbonate Reservoir Developed With Long Horizontal Wells
- Helle Vestergaard (Maersk Oil North Sea UK Ltd) | Henrik Olsen (Maersk Olie og Gas AS) | Ali S. Sikandar (Centrica plc) | Ismail A. Abdulla (Qatar Petroleum) | Rashed Noman (Qatar Petroleum)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- December 2008
- Document Type
- Journal Paper
- 958 - 967
- 2008. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 5.5 Reservoir Simulation, 5.4.1 Waterflooding, 5.2 Reservoir Fluid Dynamics, 4.1.2 Separation and Treating, 5.5.8 History Matching, 5.1.5 Geologic Modeling, 1.2.7 Geosteering / reservoir navigation, 5.5.2 Core Analysis, 1.6 Drilling Operations, 6.5.2 Water use, produced water discharge and disposal, 4.1.5 Processing Equipment, 5.1.2 Faults and Fracture Characterisation, 5.1 Reservoir Characterisation, 5.6.2 Core Analysis, 4.3.4 Scale, 5.8.7 Carbonate Reservoir
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This paper presents a gridding study relating to reservoir simulation of a giant, complex, low-permeability carbonate reservoir developed with 75 ultra long horizontal wells in a densely spaced alternating injector/producer pattern.
The lateral magnitude of the Al Shaheen field in Qatar and the radial layout of the multiple ultra long horizontal wells in the field posed a challenge in modeling of individual well performance using a manageable grid size with an acceptable run time for history matching. Reservoir modeling was complicated further by the complex reservoir characteristics with a tilting free-water level (FWL), separate gas caps, large lateral variations in oil properties, and wettability-dependent flow characteristics. These features had to be incorporated into the initialization and dynamic modeling of the reservoir, which added further to the memory requirements of the simulation model.
This paper describes the process of selecting a suitable simulation grid for history matching the performance of this reservoir on a full-field basis. Conventional Cartesian gridding techniques, including the use of local grid refinements (LGRs) in areas of interest, were pursued initially but were shown to be inadequate for full-field modeling of this complex reservoir. The gridding problem was solved by the use of 2.5D perpendicular-bisector (PEBI) grids around each of the horizontal wells in the field. This allowed for sufficient resolution between wells and also aligned the grid with the well paths, thereby avoiding grid nonorthogonality issues.
The efficiency of the PEBI model was also demonstrated by the comparison of CPU performances. Run times for the full-field PEBI model were equivalent to that of a conventional Cartesian model with suitable local grids covering only 20% of the wells. Both models had approximately 700,000 active cells and required 3-4 GB of memory. A full-field model relying on conventional LGRs around all wells was not built because it would involve significantly more grid cells and, therefore, would become considerably slower and require more memory.
The Kharaib B reservoir is a thin, widespread, low-permeability reservoir with an aspect ratio of approximately 1:1,000. The reservoir is a major carbonate reservoir in the Al Shaheen field (Thomasen et al. 2005) in Block 5 offshore Qatar, covering an area of approximately 514,000 acres (2,080 km2). The Kharaib formation forms part of the Lower Cretaceous Thamama group that is widely present in the southern Arabian Gulf.
At the time of this study, the field was developed with 75 very long horizontal wells in an irregular well pattern designed for waterflooding, which relies on close well spacings because of the low permeability of the carbonate.
Continuing evaluation of reservoir performance and assessment of further development potential of the field prompted the need for detailed history matching on a full-field basis. The modeling issues discussed in this paper were part of a history-matching study completed in 2004 that formed the basis to develop the reservoir further with 89 additional wells to infill existing well patterns and to develop the thinner oil column toward the flank of the reservoir, which is currently ongoing.
The complex nonequilibrium conditions prevailing in the low-permeability reservoir are first described, together with the wettability-dependent flow characteristics. The changes in fluid properties, for example, along the long horizontal wells are substantial in some instances. The described complexities also add to the memory requirements of a simulation model. The practical implementation in the reservoir-simulation model is described in Appendices A, B, and C.
The paper then illustrates how conventional Cartesian gridding techniques were first used to model the reservoir, giving examples of the inadequacies associated with this gridding technique. Further refinement of the grid using LGR around wells to compensate for the lack of grid resolution between the densely spaced wells was also attempted, but the required model size became impractically large. It is then shown how PEBI gridding along the long horizontal wells is fit for modeling this reservoir. It facilitates accurate representation of interwell distances and provides sufficient resolution between water injectors and producers, while ensuring realistic modeling of water movement away from a water-injection well.
Finally, the comparison of CPU performances for conventional- and PEBI-grid models supports the selection of the PEBI technique as an efficient solution for the full-field gridding requirements for the giant Kharaib B field.
The subsequent history-matching process was facilitated dramatically by use of PEBI grids, and significant time savings were realized.
|File Size||9 MB||Number of Pages||10|
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