Construction of a magnetic nano-tracer library with metal-doped encoding options and doping bias analysis insight from DFT calculations

This study presents a novel approach to overcoming the shortage of oilfield tracers through the development of an encodable magnetic nano-tracer library. We surveyed and screened 49 metal elements for doping to encode tracers, significantly increasing the number of available tracers. To validate the concept, six cost-effective and abundant elements (Mg, Mn, Co, Ni, Cu, Zn) were selected as dopants , and single-element doped magnetic nano-tracers were synthesized through co-precipitation. Transmission electron microscopy (TEM) revealed that the tracers exhibit a spherical morphology with an average size of approximately 10 nm. Vibrating sample magnetometer (VSM) measurements indicated saturation magnetizations ranging from 57.69 to 84.46 emu/g, confirming their excellent superparamagnetic properties. The tracers are compatible with fracturing fluids , increasing viscosity by 13.9–40.0 % at a concentration of 120 mg/L while maintaining post-breaking viscosity at approximately 5 mPa·s. Density functional theory (DFT) calculations showed that the minimum formation energies for Cu, Ni, Co, Zn, Mn, and Mg are −0.03 eV, −0.38 eV, −0.52 eV, −1.10 eV, −2.43 eV, and −3.95 eV, respectively. This indicates that the doping difficulty decreases in the order of Cu < Ni < Co < Zn < Mn < Mg, which is consistent with the doping concentrations measured by ICP-MS. This study provides valuable insights into the development of a nano-tracer library with element-doped encoding capabilities, enabling highly customizable tracers for oilfield applications. • 49 encodable Fe₃O₄ nanotracers via single-element doping exceed quantum dot limits (6). • Good fracturing fluid compatibility: 13.9–40 % viscosity boost, stable post-break (∼5 mPa·s). • DFT-guided doping trends (Cu < Ni < Co < Zn < Mn < Mg) align with ICP-MS quantification.