Quantification of dissolved O(2) in bulk aqueous solutions and porous media using NMR relaxometry

Effects of dissolved paramagnetic oxygen (O(2)) in water on (1)H nuclear magnetic resonance (NMR) Carr-Purcell-Meiboom-Gill (CPMG) experiments is evaluated at a (1)H Larmor frequency of 2 MHz. Dissolution of O(2) into water significantly reduces the (1)H 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 (1)H NMR CPMG experiments of confined water in a model system composed of Berea sandstone. For this system, 90 mg/L O(2) in H(2)O 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.