Magnetic pore fabrics: The role of shape and distribution anisotropy in defining the magnetic anisotropy of ferrofluid‐impregnated samples

Biedermann, Andrea R. (2019). Magnetic pore fabrics: The role of shape and distribution anisotropy in defining the magnetic anisotropy of ferrofluid‐impregnated samples. Geochemistry, geophysics, geosystems, 20(12), pp. 5650-5666. American Geophysical Union AGU 10.1029/2019GC008563

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Pore fabrics define physical properties of a rock, such as permeability and elasticity, both of which are important to many geological, hydrological and environmental applications. Minerals and hence pores are often preferentially aligned, leading to anisotropy of physical properties and preferred flow directions. Preferred flow paths are defined by the shape and arrangement of pores, and a characterization of this pore fabric forms the basis for prediction of fluid flow directions. Magnetic pore fabrics (MPF), i.e., magnetic anisotropy measurements on ferrofluid‐impregnated samples, are a promising and fast way to characterize the pore fabric of connected pores in 3D, while analysing a large number of pores with sizes down to 10 nm, without the need for any a priori knowledge about fabric orientation. Empirical relationships suggest that MPF is related to the pore shape and orientation and approximates permeability anisotropy. This study uses models including shape and distribution anisotropy to better understand and quantify MPF, using simple pore shapes and pore assemblies measured in previous studies. The results obtained in this study show that (1) shape anisotropy reliably predicts the MPF of single pores, (2) both shape and distribution anisotropy are needed to predict MPF of pore assemblies, and (3) the anisotropy parameters P, L, and F are affected by the intrinsic susceptibility of the ferrofluid in addition to pore geometry. These findings can help explain some of the variability in empirical relationships, and are an important step towards a quantitative understanding and application of MPF in geological and environmental studies.

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