# 5 Calculating Properties

Once the three-dimensional framework is defined and gridded, the next step is to populate the grid with the hydraulic property values needed for the simulation

# 5.1 Gross Thickness

The simplest property is the thickness of the reservoir or aquifer, which is the vertical distance between the bounding surfaces (Figure 8).

For dipping or folded strata, it may be necessary to apply a trigonometric dip correction (Figure 9). In the Test Site example, structural deformation is relatively minor, so no dip correction is applied.

# 5.2 Net Thickness

Clays and related rocks such as shale have volumetrically large but disconnected porosity and represent a volume of the aquifer that is not involved in active flow, which is important in evaluating groundwater contamination sites (Payne et al., 2008). The “flowing” portion of the reservoir or aquifer is identified as “net section”. To determine this, a cutoff-value of coarseness is selected, and the geologic logs are “blocked” into net (coarse sediments) and non-net (fine sediments) zones. The thickness of the net section is tallied for each log, and then used to create contours of net thickness as shown in Figure 10. In this case, the map is an isopach where isocontours represent lines of constant thickness rather than constant elevation.

The thickness contours are then gridded using the same process as the horizons in the previous steps (Figure 11).

# 5.3 Net to Gross Ratio

Once data for net and gross thickness have been mapped and gridded, it is straightforward to take the ratio of the two. Net thickness divided by total thickness is the net to gross thickness ratio, a value between 0 and 1 that is usually just referred to as net to gross (Figure 12). Regardless of sedimentary facies, high net to gross layers tend to prove both permeable and hydraulically well connected. While low net to gross layers can be permeable at individual wells, they are much more likely to be broken into disconnected compartments.

# 5.4 Properties

In oil and gas reservoir appraisal, the net to gross thickness ratio in clastic reservoirs has been long known to correlate reliably with several bulk reservoir properties. Porosity and permeability are often mapped directly from the net to gross value using interpolation functions unique to a particular oil field. At the Test Site, porosity and hydraulic conductivity are estimated based on the correlation between net to gross ratio and the porosity and hydraulic conductivity measurements made in the permanent monitoring wells as shown in Figure 13.

The equations for the lines in Figure 13 are (Equations 1 and 2):

 (1)
 (2)

Once these aquifer specific relationships have been established, they can be calculated for each grid location given its net to gross value to create aquifer properties for each cell of the 20×20 grid (Figure 14).