RESERVOIR MONITORING MODEL STUDY

Reservoir monitoring by repeated seismic surveys is based on the assumption that changes in fluid properties cause detectable changes in the recorded seismic data. For an individual reservoir under study the validity of this assumption depends on many factors, such as lithology, fluid properties, production history, and seismic data quality. To analyze the influence that all these factors have on the potential success of seismic monitoring, we developed numerical tools for modeling all the relevant physical processes involved in reservoir monitoring and linked them in a unified modeling flow.

The starting point of our project was building a reservoir model, shown in Figure , whose lithology and structure resembles a North Sea reservoir. We then simulated the multiphase fluid flow and production history of the reservoir over a period of 3 years. We estimated the effective seismic properties of the reservoir combining the lithologic model and the fluid properties. Finally, we simulated multi-offset 3-D seismic surveys at different times during the production history. This process is shown as Steps A-D in Figure .

 

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Figure 1 Perspective view of the reservoir model, showing structure and faults. The reservoir lithology was fashioned after braided stream fluvial reservoirs common in many parts of the world including the North Sea. Production wells are indicated in white, and water injection wells are indicated in black. Arrows and ticks along the X and Y axes show extent of seismic imaging.

 

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Figure 2 Steps involved in monitoring reservoir performance.

While the macro-scale of our model reservoir is constant ($5\times 5$ km) the sampling of the reservoir physical parameters varies greatly between steps. The main scales that we used are described below:

Core Scale

is about $0.1 \times 0.1 \times 0.1$ m. Much of what is known about rock physics relations among rock, fluid, and seismic properties has come from measurements at the core scale. Description of the complete reservoir at this scale would yield 10¹³ (10,000 billion) voxels and is not practical. Nevertheless, in any reservoir, a subset of core data can yield seismic-to-reservoir properties relations that are critical to the seismic modeling.

Reservoir Geological Modeling Scale

is nominally $5.0 \times 5.0 \times 0.5$ m. Ideally, the geological model would be at the resolution of the best information, i.e., the core data. This is not practical and, instead, a scale is chosen small enough to capture the significant geological heterogeneities and large enough to yield a tractable number of cells or voxels. The scale chosen here yields 200 million voxels which is ambitious but practical for reservoir modeling. There are about 50,000 core scale voxels within a geological modeling cell.

Reservoir Flow Simulation Scale

The scale of the grid blocks in fluid flow simulation is a compromise between the desire to minimize artificial numerical artifacts, accounting for sufficient geological detail, and available computational resources. Full-field simulation, requiring order 10² grid blocks between wells can easily lead to systems with 10⁶-10⁸ grid blocks, which remains well beyond current computational possibilities. In this study, a discretization of the macro-scale into nx=132 $\times$ ny=140 $\times$ nz=12 resulted in grid blocks with an areal scale of $62.5 \times 62.5$ m in the coarse region and $31.25 \times 31.25$ m in the refined region. The vertical size of the grid blocks, on average, was 8.3 m.

Seismic Imaging Scale

is the scale of the resolution of the seismic images, that is linked with the wavelength of the seismic signal recorded at the surface ($\approx 50$ m in our model). We described the seismic properties of the reservoir on a $5\times 5\times 5$ meters scale. Description at this scale of the whole reservoir would yield about 50 million voxels. We have used a subset of the whole reservoir, for a total of about 1.5 million voxels.