Large-scale Reservoir Simulation Figure 1: Areal View of Geological and Upscaled Simulation Models Figure 3: Mega-cell Simulation – Saturations
Flow Channels No Flow Channels Water
25x25m grid
OIl 250x250m grid
Figure 2: Full-field Ghawar Model
250m
Figure 4: Giga-cell Simulation – Saturations
Water
Oil
42m
approach is important in capturing advancing water and gas fingers and similar fluid movements inside the reservoir.
Benefits of More Accurate Numerical Solutions (Fine Grid)
With the upscaled models, large grid blocks smear the advancing fluid fronts, often predicting premature fluid breakthrough at the wells4
due
to a well-known phenomenon called numerical dispersion. To match the water and gas arrivals at the wells, reservoir engineers often introduce pseudorelative permeability functions – permeability modifications – which could be unrealistic. For example, for a 1km well spacing, only one grid block can be placed between the wells if a 0.25km-sized grid block is used. The pressure field calculated by this approach will be averaged over the three cells. Hence, sharp pressure profiles at the wells known from the analytical solutions will not be calculated by the simulator. This omission will result in inaccurately simulated fluid velocities and predicted breakthrough times at wells. If finer and finer grids are used, as GigaPOWERS now allows, more cells can be placed between the wells and therefore true pressure profiles between the wells can be captured closer to the reality. As an example of a giant reservoir that we have simulated, we used an areal grid size of 42m. This size allowed us to
104
place nearly 40 grid blocks between the wells – increased from three grid blocks for mega-cell models and one grid block for kilo-cell models. This change provided higher numerical accuracy for the pressure and saturation solutions, and therefore the well pressures and water and gas breakthroughs could be calculated more accurately.
One Billion Cell Example
The largest onshore oil reservoir of the world, Ghawar, was simulated as a full-field model by using 1.032 billion active cells (see Figure 2). The reservoir has thousands of wells and 60 years of production/injection history. The model was run in a three-phase black oil mode. By using 4,000 cores of a cluster computer, the GigaPOWERS simulator completed the run in 21 hours.
Some Benefits of Billion-cell Models Figures 3 and 4 compare the predicted water and oil saturations for a region of Ghawar at a given time. Figure 3 represents the mega- cell simulation model with 0.25km areal grid. Figure 4 illustrates the results of the giga-cell simulator with 0.042km grid for the same reservoir at the same time. The red colour represents oil and the blue colour represents water.
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 1
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