Permeability anisotropy and its relations with porous medium structure

The complete permeability tensor of 18 porous rock cores was determined by means of X-ray tomography monitoring during the displacement of a salty tracer. To study the effect of the pore space geometry on the anisotropy of permeability, we compared the three-dimensional shape of the invasion front with the X-ray porosity maps obtained before injection. The samples (clean and shale-bearing sandstones, limestones, and volcanic rocks) belong to a broad variety of granulometry and pore space geometry. Their porosity ranges from 12 to 57%, and their permeability ranges from 1.5 x 10(-14) to 4 x 10(-12) m(2). For sandstones the permeability anisotropy is well correlated with the presence of bedding. For volcanic rocks it is clearly related to the orientation of vesicles or cracks. However, for limestones, no evident link between the geometry of the porous network and the permeability anisotropy appears, probably because of the influence of the nonconnected porosity that does not contribute to the hydraulic transport. This systematic work evidences the ability and the limitations of the tracer method to characterize the anisotropy of permeability in the laboratory in a simple and rapid way.