Quantification of dissolved O₂ in bulk aqueous solutions and porous media using NMR relaxometry

Effects of dissolved paramagnetic oxygen (O₂) in water on ¹H nuclear magnetic resonance (NMR) Carr-Purcell-Meiboom-Gill (CPMG) experiments is evaluated at a ¹H Larmor frequency of 2 MHz. Dissolution of O₂ into water significantly reduces the ¹H transverse relaxation coefficient (T₂). For deoxygenated water, T₂ is 3388 ms, water at ambient atmospheric conditions (7.4 mg/L O₂) exhibits a T₂ of 2465 ms, and dissolution of 2710 mg/L O₂ further reduces T₂ to 36 ms. The results were fit with an empirical model to facilitate prediction of T₂ times for bulk water as a function of paramagnetic oxygen concentrations in solution. Dissolved O₂ also greatly influences ¹H NMR CPMG experiments of confined water in a model system composed of Berea sandstone. For this system, 90 mg/L O₂ in H₂O enhances T₂ relaxation of bulk water such that the relaxation time is comparable to physically confined water in the sandstone pores. Given the sensitivity of NMR T₂ coefficients to paramagnetic oxygen, low-field NMR-based characterization of fluid and porous media structure requires control of dissolved oxygen, as geospatial variation in the partial pressure of O₂ alone is expected to perturb fluid and pore relaxation times by up to 60 and 36%, respectively.