Porous media typical values

Diw is the molecular diffusion coefficient of the chemical in water, x is tortuosity, and aL is the (longitudinal) dispersivity (dimension L). The first term describes molecular diffusion in a porous medium (Eq. 18-57), the second the effect of dispersion (Eq. 22-52). Typical values of the dispersivity aL for field systems with flow distances of up to about 100 m lie between 1 and 100 m. Since aL depends strongly on the scale... 

The relationship between i/j and the water content of a porous medium is also determined empirically Fig. 3-23 shows typical ijs — 0 curves. When is zero, the porous medium is at its saturated moisture content, which is equal to porosity (assuming that there is negligible trapped gas). The pressure head can become somewhat negative before air enters (and water leaves) some soils the value ijja at which air begins to enter is called the air entry value. Water leaves the matrix as i/j (defined as pressure head) becomes more negative than ifsa as water leaves the soil, the hydraulic conductivity also decreases. 

Diffusion, convection, and dispersion all contribute to the spread of a front. Let us see how much each mechanism contributes to the spread. First, let us see when the diffusion transport is important as compared to the convective transport. We use v2Dot to calculate the spreading distance from a point source 68% of the injected source is within this distance. Table 2.2 shows the results for different time periods compared with the traveled distances during the same time periods by a convective flow of 1 m/day. A typical flow rate in petroleum reservoirs is 1 m/day (interstitial velocity). A typical value of diffusion coefficient of 4 X 10 mVs in a porous medium is used. In the first 5 seconds, the diffusive transport is more important than the convective transport. Soon after, the convective flow becomes the dominant mechanism. 

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