Anisotropic Variation Law of Rock Permeability with the Burial Depth of Limestone

Abstract  Permeability tensors of both macrofracture and microfracture systems were measured progressively along the depth of limestone formations at severed sites. It was found that the principal permeability values Kx, Ky and Kz in these permeability tensors all decrease simultaneously and logarithmically with depth. However, the limestone aquifers are composed of an upper region where the larger permeability ellipsoid is upright or prolate and characterized by KzKx and Kz>Ky, a transitional zone, and a lower zone whose smaller permeability ellipsoid is horizontal or oblate and characterized by Kz<Kx and Kz<Ky. The inversion of the permeability ellipsoids in direction indicates that the anisotropy of rock permeability with the depth of limestone formations has evidently changed. The anisotropic variation law of permeability tensors in a macrofracture system displays a similar pattern with that in a microfracture system. It is next to impossible to examine the rock permeability tensor of the aquifer just by measuring the hydraulic parameters of macrofracture system directly, unless the limestone aquifer is exposed on or near the earth's surface. Therefore, the permeability tensors of a macrofracture system at any depth may be indirectly and roughly determined from the gaugeable permeability tensors of the microfracture system by conversion. This anisotropic variation law of rock permeability with depth is of great significance in the study of three‐dimensional fracture water flow the huge carbonate formations and in the research on the conditions of karst development and karst distribution.