A measure of ability of a porous formation to allow a certain fluid to pass through it.

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body	k_a

absolute permeability to air which is defined by the reservoir pore structure only, also denoted as

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body	k_{abs}

or

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body	k_{air}

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body	k_r

relative permeability to a given fluid which is defined by the interaction between fluid and reservoir matrix

In general case, permeability is anisotropic both in vertical and lateral directions and quantified by symmetric tensor value:

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anchor	ktensor
alignment	left

k=\begin{bmatrix}
    k_{11}  & k_{12} & k_{13}   \\
    k_{12}   & k_{22} & k_{23}   \\
    k_{13}    & k_{23} & k_{33} 
\end{bmatrix}

which can be diagonalized for a proper selection of coordinate axis

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body	({\bf e_1}, {\bf e_2}, {\bf e_3}) \rightarrow ({\bf e_x}, {\bf e_y}, {\bf e_z})

:

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anchor	XPHJL
alignment	left

k=\begin{bmatrix}
    k_x}  & 0 & 0}   \\
    0   & k_y & 0}   \\
    0    & 0 & k_z 
\end{bmatrix}

and characterized by 3 principal tensor components

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body	k = (k_x, \, k_y, \ k_z)

If not mentioned otherwise the permeability usually means horizontal permeability:

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body	k = k_h = \sqrt{k_x^2+k_y^2}

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