NMR-Responsive Paramagnetic [M-EDTA] (M = Fe³⁺, Mn²⁺, Cu²⁺) Complexes to Differentiate T₂-Distribution Signals of Crude Oil and Brine

Nuclear magnetic resonance (NMR) well logging tools are being commercially utilized for the characterization of various fluids confined within reservoir rocks. Conventional NMR T₂-distribution measurements are unable to quantify the actual fractions of crude oil and brine simultaneously present in the reservoir rocks. Herein, we reported NMR-responsive paramagnetic [M-EDTA] (M = Fe³⁺, Mn²⁺, Cu²⁺) complexes having the capability to differentiate NMR T₂-distribution signals coming from crude oil and brine by accelerating the water (¹H) relaxation. The formation of these [Fe-EDTA]^-, [Mn-EDTA]^2-, and [Cu-EDTA]^2- complexes was explored by UV-visible spectroscopy, and their chemical stability in brine solution was monitored by measuring the change in percent transmittance and backscattering intensities with time. Spin-spin (T₂) relaxation signals and T₂-distribution spectra of various carbonate and sand packs were recorded. The separation of NMR T₂-distribution signals was achieved using the optimized concentration of synthesized paramagnetic complexes. The separation of NMR T₂-distribution signals with the optimum concentration of complexes and their long-term chemical stability in a brine solution suggest them to be commercially reliable contrast agents for the characterization of various rock fluids confined in oil reservoirs.