Towards a quantitative interpretation of magnetic pore fabrics

Abstract:

Magnetic fabrics are widely used as proxies for mineral alignment, and when measured on rocks impregnated with strongly magnetic fluid, they provide information on the average pore fabric. So far, magnetic pore fabrics were interpreted based on empirical relationships: (1) the maximum susceptibility axis indicates the direction of pore elongation and the maximum permeability or preferred flow direction, (2) the higher the ratio of maximum to minimum susceptibility, the more strongly anisotropic the pore shapes and flow properties, and (3) the shape of the magnetic susceptibility ellipsoid reflects the average pore shape. Compared to other pore fabric characterization methods, the magnetic method has the advantage that the full anisotropy tensor can be measured on a single core, and a wide range of pore sizes, supposedly down to 10 nm, is captured in a representative sample volume. A major challenge, however, is the large variability of reported empirical relationships between magnetic anisotropy and either pore shapes or permeability anisotropy. Here, we use measurements on synthetic samples with known simple pore space, in combination with numerical modelling of shape and distribution anisotropies to reach a quantitative understanding of magnetic pore fabric data. Results show a strong influence of the fluid susceptibility, where the anisotropy degree of a given non-equidimensional pore increases non-linearly with the susceptibility of the fluid. This may explain a major part of the variability in previous studies. Additional scatter may be related to the frequency-dependence of the magnetic properties of the fluid. These observations and their mathematical description will eventually enable a full quantitative interpretation of magnetic pore fabric measurements in rocks, making them more useful in fluid flow applications.