Toward Better Understanding of the Ferrofluid Impregnation Process and Potential Artefacts—A Prerequisite for Reliable Interpretation of Magnetic Pore Fabrics

Abstract

AbstractMagnetic pore fabrics (MPFs) serve as efficient proxy for pore alignment and preferential flow directions. Anisotropy of magnetic susceptibility of impregnated samples reflects the shape and arrangement of ferrofluid‐filled pores. Reliable interpretation requires that the entire connected pore space is reached by ferrofluid. This study investigates impregnation efficiencies of various methods (standard vacuum, pressure injection, magnetic flowthrough, and resin impregnation) and ferrofluids (water‐based versus oil‐based) on sandstone and calcarenite with different wettabilities. Mass‐based and susceptibility‐based impregnation efficiencies further evaluate the influence of impregnation time and flow rate. Because directional impregnation may introduce artificial fabrics, mutually perpendicular cores were measured, and subsamples served to capture spatial variability. Impregnation efficiencies vary between <1% and >100%, with resin impregnation and pressure injection using oil‐based ferrofluids being least efficient. Oil‐based ferrofluid generally leads to lower susceptibility‐based compared to mass‐based impregnation efficiencies, suggesting particle filtering. Water‐based ferrofluids show larger ranges of susceptibility‐based compared to mass‐based impregnation efficiencies. In particular, impregnation efficiencies >100% exclusively occur for susceptibility‐based impregnation efficiencies of water‐based ferrofluid, using either standard vacuum or pressure injection methods. MPFs after magnetic flowthrough impregnation reflect the impregnation direction rather than pore fabric. A major challenge is to apply enough force to reach sufficiently high impregnation efficiencies without creating MPFs that do not represent pore fabrics. While we cannot define a best practice for ferrofluid impregnation yet, this study improves our understanding of the impregnation process, and provides experimental procedures to identify artifacts.