Anisotropic Permeability

In reality, most reservoirs are not homogeneous in terms of permeability (isotropic permeability). Usually permeability varies significantly between the vertical and horizontal planes within a formation. This variation in permeability in different planes or directions is known as anisotropic permeability.

Anisotropic permeability is especially important when dealing with horizontal or partially penetrated wells since flow occurs in both the vertical and horizontal planes.

In general, it is not uncommon to assume that the permeability in the x-direction (kx) is close to that in the y-direction (ky). However, the permeability in the z-direction (kz) is typically significantly different, usually less, than the horizontal permeabilities (kx and ky).

Thus, vertical radial flow is controlled by ky and kz (where x is the direction of the horizontal wellbore) and horizontal radial flow is controlled by kx and ky.