PNNL

Abstract

We provide an evaluation of water by measuring its dissolved oxygen content. We quantify the effect of dissolved oxygen, a paramagnetic gas, on NMR signals at 2 MHz: small amounts (~7.4 mg/L) of O2 dissolved in water can reduce the NMR Transverse (T2) relaxation time by about 1000 ms at ambient atmospheric conditions. NMR T2 time reduces from about 3388 ms in deoxygenated water to about 2465 ms at ambient atmospheric conditions and down to about 36 ms at dissolved oxygen concentrations up to 2710 mg/L. Our empirical equation facilitates prediction of NMR T2 relaxation times for bulk water as a function of paramagnetic oxygen concentrations in solution. In water-saturated porous media, dissolved paramagnetic oxygen at concentrations of just 90 mg/L reduces the bulk water NMR T2 relaxation enough to mask NMR relaxation times attributed to water confined in pores. Given the sensitivity of NMR T2 relaxation coefficients to paramagnetic oxygen, all studies of low-field NMR response need to quantify control oxygenation levels, as changes in geographic location and barometric pressure alone can cause ~60 % variation in derived fluid properties with up to 36 % variation in pore derived properties.