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

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