Quantitative measure of a drainage area 

r_e = \sqrt{\frac{A_e}{\pi}}

It makes a physical sense of a geometrical average distance from well to the drainage area boundary.

For a circular-shape drainage area with centred well it makes a direct sense of a distance between a well and drainage area boundary

The radius of a  around a producing or injecting well where reservoir fluid is forced moving by the well operation.

In other words, the area, where reservoir fluid is moving along the streamlines which either stem from injecting well or end up at producing well.

See also

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Physics /  Fluid Dynamics / Percolation

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