Impact of Grid Selection on Reservoir Simulation
- M.K. Abdou (ADCO Producing Co. Inc.) | H.D. Pham (ADCO Producing Co. Inc.) | A.S. Al-Aqeell (ADCO Producing Co. Inc.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- July 1993
- Document Type
- Journal Paper
- 664 - 669
- 1993. Society of Petroleum Engineers
- 5.1.2 Faults and Fracture Characterisation, 5.1.1 Exploration, Development, Structural Geology, 4.3.4 Scale, 6.5.2 Water use, produced water discharge and disposal, 6.6.2 Environmental and Social Impact Assessments, 5.5.8 History Matching, 5.4.1 Waterflooding, 4.1.2 Separation and Treating, 1.2.3 Rock properties, 5.5 Reservoir Simulation, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 5.1 Reservoir Characterisation, 4.1.5 Processing Equipment
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This paper presents a field case study of a faulted, multilayer carbonatereservoir to quantify the impact of grid selection on the reservoir historymatch. Comparison of the history match using both orthogonal and nonorthogonalgrids revealed that the simple five-point scheme can lead to significant errorin the history match and in the different representations of the reservoir.
A simulation study of the Thamama reservoir, a faulted carbonate reservoirin Abu Dhabi, investigated flow communication across faults and evaluateddevelopment options for the field. The field is an elongated anticline thatcovers 62 sq miles. Geophysical and geological data and early reservoirperformance indicated that the field was separated into several fault blocks bypartially sealing and nonsealing faults.
Two full-field simulation models were constructed that used two differentapproaches to represent faults: (1) an orthogonal (regular) grid model withfaults defined at block boundaries and (2) a nonorthogonal (irregular) gridmodel with block boundaries defined at fault traces. The first approach resultsin a greater number of cells and presents difficulties in representing thefault geometry properly. However, finite-difference techniques can be used fornumerical calculations. The second approach uses fewer gridblocks and providesan accurate description of the fault pattern, but transmissibility calculationscould lead to significant errors because the grid is locally distorted.
To quantify the impact of grid selection on the reservoir history match,both models were used and the results compared in the simulation study. Historymatching was performed first with the orthogonal grid model. Then, a historymatch using the same matching parameters was conducted on the nonorthogonalgrid model.
The Thamama reservoir comprises three zones, Zones A, B, and C.Characterized by high faulting and low permeability, this carbonate reservoircontains about 3.6 billion STB of oil in place (OIP). Zones B and C, whichcontain about 80% of the total OIP, have been the focus of major developmentsince the field discovery in 1967. Fig. 1 shows the general structuralconfig-uration of the reservoir, which consists of a large dome with faultsoriented north-west/southeast.
Faulting in the field, which partitions the reservoir into numerous smallfault blocks, has caused some concern about implementation of the full-fielddevelopment plan. Although lateral and vertical communication between upthrownand downthrown faulted blocks along major faults was suspected, it was neverfully identified. Throughout the field's producing life, the extent of lateraland vertical communication within the reservoir, particularly between the oilzone and the underlying aquifer, has been questioned. This question became acritical operating consideration when a peripheral water injection plan wasproposed in the late 1970's.
Although an extensive reservoir study was undertaken to predict the field'sfuture producing performance under different waterflood operations, no waterinjection program was implemented. We decided that further detailed reservoirstudies should be conducted to understand the communication and crossflowbetween fault blocks fully before a full-scale injection program could bedesigned.
A preliminary 3D, coarse, full-field simulation study of the Thamamareservoir with an in-house simulator having a rectangular (orthogonal) gridsystem showed the significant effect of faulting on reservoir performance.Unfortunately, the crossflow and communication levels between the fault blockscould not be determined accurately because the model could not represent thefaults properly owing to simulator limitations. Therefore, we decided to use asimulator capable of handling distorted (nonorthogonal) grids, or corner-pointgeometry, to fit the faults and reservoir boundaries better.
Two approaches were considered for constructing the model grid. The firstapproach uses a rectangular (orthogonal) gridding system that represents faultsat block boundaries. As a result, reservoir geology and geometry cannot berepresented properly. However, finite-difference techniques can be used in astraightforward manner with good accuracy. The second approach uses anonrectangular (nonorthogonal) gridding system that accommodates accuratedescription of the fault pattern by placing block boundaries along faulttraces. However, the nonrectangular gridding systems could lead to seriouscalculation errors.1
We conducted full-field simulation studies using both orthogonal andnonorthogonal grid models. The results of the history match of both models withthe same input data were compared to determine the magnitude of the calculationerror and differences in model performance.
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