Physical and Chemical Stability of Nanoparticles in Ferrofluid Before and After Impregnation: Implications for Magnetic Pore Fabric Studies

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2023-11, Vol.24 (11), p.n/a
• Main Authors: Biedermann, Andrea R., Mazurek, Martin, Schröder, Johanna, Arenz, Matthias
• Format: Article
• Language: English
• Subjects: Aggregation; Anisotropy; colloidal stability; Epoxy compounds; ferrofluid; Groundwater; Hydrocarbons; magnetic fabrics; magnetic pore fabrics; Magnetic properties; Mobility; Nanoparticles; Particle motion; Particle settling; Permeability; Polymerization; pore space characterization; Rock; Rocks; Shape
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2023GC011125

Magnetic pore fabrics (MPFs) are a promising approach to explore the 3D pore space of rocks. There exist empirical relationships between the orientation, degree, and shape of MPF with pore space geometry and permeability anisotropy. Nevertheless, the precise nature of these relationships remains elusive. A common assumption in MPF studies is that ferrofluid uniformly fills the pore space, establishing a constant nanoparticle density and magnetic properties in space and time. However, this assumption is challenged by observations of particle sedimentation and time‐dependent magnetic properties, which render the quantitative interpretation of MPFs challenging. This study explores the physical and chemical stability of ferrofluid, mobility of particles after impregnation, and the magnetic properties of impregnated rocks over time. Water‐based ferrofluids are physically more stable than oil‐based ferrofluids, though with higher magnetic variability. In impregnated rock, magnetic nanoparticles display a certain degree of mobility, resulting in changes in MPF degree and shape. When ferrofluid is mixed with epoxy, particles are less mobile, though some changes were observed both during polymerization and aging. The susceptibility of ferrofluid‐epoxy mixtures is lower than for ferrofluid and carrier liquid at the same concentration. Interestingly, the susceptibility of impregnated rock increases over time, regardless of the ferrofluid used for the impregnation. Understanding and controlling these processes will enhance the reliability of MPF interpretations and increase the applicability of the method. Plain Language Summary To predict how groundwater, contaminants, or hydrocarbons flow through rock, it is important to understand the pore space, in particular, whether pores have a preferred shape and orientation, in which case flow is faster along certain directions. One way to describe the pore space is to fill the pores with a strongly magnetic liquid, the so‐called ferrofluid, and measure the magnetic properties to estimate the pore space characteristics. To interpret these reliably, certain assumptions have to be valid, for example, that the ferrofluid properties are constant throughout the pore space and over time. These assumptions were tested for different types of ferrofluids. Results show that physical processes, including particle aggregation or particle motion in the pore space, and chemical processes, for example, alteration or surface changes, may occ