Novel Simulation Techniques Used in a Gas Reservoir With a Thin Oil Zone: Troll Field
- Adolfo Henriquez (Statoil) | Odd J. Apeland (Statoil) | Oystein Lie (Statoil) | Ian Cheshire (ECL)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1992
- Document Type
- Journal Paper
- 414 - 418
- 1992. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 4.3.4 Scale, 5.1.5 Geologic Modeling, 5.4.2 Gas Injection Methods, 5.6.4 Drillstem/Well Testing, 1.6 Drilling Operations, 4.1.2 Separation and Treating, 5.1.1 Exploration, Development, Structural Geology, 2 Well Completion, 2.2.2 Perforating, 5.5 Reservoir Simulation
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Choice of production strategy in modern reservoir management relies heavilyon numerical simulation. Large fields may require prohibitively large computertimes. New techniques were developed to save computer and engineering time:local grid refinement with small timesteps and flux boundary conditions forsimulating regions of special interest. The combined use of these techniquesallowed flexible, non-time-consuming, user-friendly reservoir simulation of avariety of reservoir management scenarios for Troll field.
Simulation of large fields may require large amounts of CPU and engineeringtime. When three phases are present and detailed spatial and temporaldevelopment of the fluid flow has to be described, these needs are increased. Asustained effort to reduce these requirements led to the specification anddevelopment of novel simulation techniques.
The first technique is a user-friendly way to refine the grid in regions ofthe reservoir where needed. Compared with conventional techniques, computertime is dramatically reduced by use of small timesteps for the locally refinedgrid and larger timesteps for the global grid.
The second technique allows simulation of regions of the reservoir whiletaking into account the full-field behavior. The inter-flows between the partand the whole calculated in a full-field reference run are used as boundaryconditions for simulating the region of interest.
In this paper, these techniques are described summarily and theirapplication to the calculation of oil production profiles from a large gasfield with an oil rim is presented.
The Troll field, located about 60 lun offshore Norway, is a large gas fieldwith thin oil zones. Discovered by Norske Shell in 1979. the field coversroughly 750 km2. The hydrocarbons in place, used only for planning purposes,are approximately 615 x 10-6 std m3 and 1671 x 10-9 std m3 for oil and gas,respectively. Three main easterly tilted fault blocks contain the hydrocarbonreserves (see Fig. 1). Troll East contains about two-thirds of the gas inplace. Troll West Gas Province has a gas cap of up to 200 m underlain by an oilcolumn roughly 12 m thick; and the Troll Oil Province contains an oil columnbetween 22 and 28 m thick with a 10- to 50-m-thick gas cap. Refs. 1 and 2provide detailed geological and geophysical properties. properties. The Trollfield has four operators: Norske Shell A/S, Norsk Hydro Production A/S, SagaPetroleum A.S., and Statoil.
Need for Novel Simulation Techniques
An ongoing long-term horizontal well test of the potential for oilproduction from the Troll Oil Province confirmed the economic production fromthe Troll Oil Province confirmed the economic viability of recovering this oil.A 500-m-long horizontal well was drilled and produced by Norsk Hydro on behalfof the Troll license partners. Refs. 3 and 4 give the production history andother partners. Refs. 3 and 4 give the production history and other details ofthe well.
The possibility of economical exploitation of the thin oil zones byhorizontal wells and the impact of gas production on oil recovery called forthe study of an integrated field development, which required the investigationof different scenarios. Ref 5 gives an overview of some unconventionaltechniques used in a previous integrated reservoir simulation model for Trollfield. These unconventional techniques were improved and new ones developed inthe work described here.
The main frame of reference for further development is that between 16.5 x10-9 and 23.7 x 10-9 std m3/a gas will be produced from Troll East to satisfythe contracted gas sales for the first quarter of the next century. The exactlevel of off-take rate will be decided later by the Norwegian authorities.
Within this constraint and the development of the gas market, the level andtiming of gas production from the Troll West Gas Province and the optimizationof oil production from the thin oil Province and the optimization of oilproduction from the thin oil zones have to be considered.
The main features to be studied are the gas flux from the Troll West GasProvince to Troll East, the behavior of the underlying aquifer, and theinfluence of these phenomena on oil production from both the Troll West GasProvince and the Troll Oil Province. A detailed study of gas and water coningtoward the horizontal wells is required to determine the oil productionpotential within the framework of gas production. Coning phenomena have to beresolved within small simulation gridblocks and with small timesteps, gasproduction may be simulated with coarse gridblocks and large timesteps. Thecontrasting needs for spatial and temporal resolution in a large field are themain issues that determine the need for new simulation techniques. Fluid flowbetween reservoir provinces on a scale of kilometers and years has to be takeninto provinces on a scale of kilometers and years has to be taken into accountwhile calculating fluid flow on a scale of meters and days in the vicinity ofthe horizontal wells.
To meet these needs, the simulator features detailed in the next threesections were developed, tested, and used.
Local Grid Refinement With Small Timesteps
The need to refine the simulation grid in small reservoir regions oftenarises when large saturation changes appear in a relatively short time periodcompared with the rest of the reservoir. This may be caused by pressuregradients in the neighborhood of the wells, coning, condensate dropout orgeology-related problems, like fluid flow through small communication channels,or the need to represent reservoir heterogeneity in detail at a smallscale.
Historically, the first answer to this problem was to increase the number ofblocks in the regions of interest. However, the refinement extended to allblocks in the x, y, and eventually z directions, with corresponding increasesin computer time. Gradually, simulators, incorporated the possibility ofrefining just the areas of interest, usually through non-neighboring
connections and explicit input of transmissibilities between the coarse andrefined blocks, which kept the Cartesian geometry of the coarse grid. Ref. 5describes the calculation of production profiles with such techniques. Pedrosaand Aziz provided a profiles with such techniques. Pedrosa and Aziz provided aconsistent way to calculate these transmissibilities for the case of radialrefinements, which honors the geometry of flow into the wells better.
These solutions were hampered because the small timesteps needed to solvethe linear equations of the flow problem were propagated to the rest of thesimulation grid and computer time propagated to the rest of the simulation gridand computer time consequently increased.
Small timesteps were used for local grid refinements in the simulator usedat Statoil.
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