Can sodium NMR provide more than a tracer for brine in petrophysics?

Abstract

Sodium has been suggested as a tracer for brine in reservoir formations, where sodium ions are found exclusively in the aqueous phase, and can be detected by time-domain nuclear magnetic resonance (NMR). To date, petrophysical applications of sodium-23 NMR have focused on concentrated NaCl electrolyte solutions where the nuclear spin relaxation time is related to ion concentration. Therefore, a measure of brine volume is achieved directly from the sodium signal amplitude and relaxation time, available in a single measurement. However, real reservoir formation or injection brines contain many different ionic moieties. Sodium-23 relaxation times and diffusion coefficients are measured using time-domain NMR and pulsed field gradient (PFG) NMR techniques, respectively, and shown to depend strongly on the ions present in solution. Correlations between sodium-23 relaxation times and sodium ion concentration are found to differ depending on the cations and other anions present in the brine. However, consistent correlations are obtained for sodium-23 relaxation times and diffusion coefficient, and brine viscosity, regardless of the ionic content of the brine. In general, sodium-23 NMR remains a qualitative technique for monitoring changes in sodium concentration or brine volume (at constant salinity) in reservoir formations. However, if knowledge of the brine chemistry is available, then sodium-23 NMR offers a non-invasive and quantitative method of robustly measuring brine volume and viscosity in petrophysical applications.