Temperature dependence on the mass susceptibility and mass magnetization of superparamagnetic Mn-Zn-ferrite nanoparticles as contrast agents for magnetic imaging of oil and gas reservoirs.

The mass susceptibility (χ_mass) and mass magnetization (M_mass) were determined for a series of ternary manganese and zinc ferrite nanoparticles (Mn-Zn ferrite NPs, Mn_xZn_1−xFe₂O₄) with different Mn:Zn ratios (0.08 ≤ x ≤ 4.67), prepared by the thermal decomposition reaction of the appropriate metal acetylacetonate complexes, and for the binary homologs (M_xFe_3−xO₄, where M = Mn or Zn). Alteration of the Mn:Zn ratio in Mn-Zn ferrite NPs does not significantly affect the particle size. At room temperature and low applied field strength the mass susceptibility increases sharply as the Mn:Zn ratio increases, but above a ratio of 0.4 further increase in the amount of manganese results in the mass susceptibility decreasing slightly, reaching a plateau above Mn:Zn ≈ 2. The compositional dependence of the mass magnetization shows less of a variation at room temperature and high applied fields. The temperature dependence of the mass magnetization of Mn-Zn ferrite NPs is significantly less for Mn-rich compositions making them more suitable for downhole imaging at higher temperatures (>100 °C). For non-shale reservoirs, replacement of nMag by Mn-rich Mn-Zn ferrites will allow for significant signal-to-noise enhancement of 6.5× over NP magnetite. [ABSTRACT FROM AUTHOR]