Reservoir simulation grids: opportunities and problems
Grid is one of the most difficult and time consuming tasks in the simulation of geologically complex.
A reservoir simulator predicts reservoir performance by solving flow equations on a discrete grid that is chosen by the simulation engineer to represent the reservoir. The grid is normally selected with one or more of the following considerations in mind:
- Geology and size of the reservoir and the available data used for reservoir description;
- Type of fluid displacement or depletion process to be modelled;
- Past and anticipated field development (location and type of wells);
- Numerical accuracy desired;
- Available software options;
- Objectives of the simulation study;
- Competence of the simulation engineer or team;
- Available computer resources, time constrain or project budget.
In the early days of reservoir simulation it was often the last of the above considerations that determined the number of grid blocks. Research in reservoir simulation and hardware developments, especially over the past ten years, has greatly extended the gridding options available to the user. Modern commercial simulators typically offer one or more of the following techniques:
- Local grid refinement;
- Hybrid grid;
- Curvilinear (steam-tube) grid;
- PEBI grid (generalization of point- distributed grid);
- Corner point geometry;
- Dynamic grid;
- Automatic grid generation.
Simulation Grids
Global Orthogonal Grids – most commonly used grids are constructed by aligning the grid blocks along orthogonal coordinate directions and then distorting the grid where necessary, to fit major reservoir features. The block-centred grid is advantageous for the calculation of accumulation terms, while the point-distributed grid is more accurate for the calculation of flow between blocks.
Local Grid Refinement – for large reservoir simulation problems, fine grid is only needed in parts of the reservoir where saturations or pressure are changing rapidly. Here the problem is the accurate calculation of flow between blocks at the intersection pf coarse and fine grids. Hybrid grids are however useful for accurate calculation of WOR and GOR.
The simulation engineer has the opportunity to use many kinds of flexible grids. However, from a practical standpoint, the most important problem is : “What grid should one select for a specific problem?”
Here are some comments and guidelines:
a) Local grid refinement improves the prediction of WOR and GOR when sharp saturation gradients exist near wells The refined region should be large enough to include the extent of the reservoir with sharp gradients in saturation;
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b) Unless there are compelling reasons, grid should be orthogonal (at least locally), and as uniform as possible. Large blocks next to small blocks should be avoided. Generalization of point-distributed grid usually more reliable than block-centred grid;
c) So far dynamic grids have proven to be of limited value in field applications.
| Grid | сетка |
| Simulation | моделирование |
| Reservoir performance | Поведение резервуара |
| Flow (equation) | Уравнение потока |
| Discrete (grid) | дискретный |
| Reservoir description | Описание резервуара |
| Fluid displacement | Вытеснение жидкости |
| Depletion process | Процесс истощения запасов |
| field development (anticipated) | разработка месторождения |
| time constraints | ограничение на шаг по времени |
| local grid refinement | локально-измельченная сетка |
| hybrid grid | гибридная сетка |
| curvilinear grid | криволинейная сетка |
| pebi grid (_generalization of point-distributed) | соединено перпендикулярная сетка |
| corner point geometry | геометрия угловой точки |
| dynamic grid | динамическая сетка |
| globally orthogonal grid | глобальная ортогональная сетка |
| coordinate direction | направление осей координат |
| distort the grid | искажать сетку |
| block-centred grid | сетка, в которой узел сетки находится в центре блока |
| point-distributed grid | сетка, в которой узел сетки смещен относительно центра блока |
| intersection | пересечение |
| coarse grid | грубая крупная сетка |
| fine grid | мелкая сетка |
| WOR(water oil ratio) | водонефтяной фактор |
| GOR (gas oil ratio) | газовой фактор |
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