NMR Measurements of Tortuosity in Partially Saturated Porous Media

• Published in: Transport in porous media 2018-11, Vol.125 (2), p.271-288
• Main Authors: Zecca, Marco, Vogt, Sarah J., Connolly, Paul R. J., May, Eric F., Johns, Michael L.
• Format: Article
• Language: English
• Subjects: Civil Engineering; Classical and Continuum Physics; Diffusion coefficient; Earth and Environmental Science; Earth Sciences; Ethane; Geotechnical Engineering & Applied Earth Sciences; Hydrogeology; Hydrology/Water Resources; Industrial Chemistry/Chemical Engineering; Magnetic permeability; NMR; Nuclear magnetic resonance; Porous media; Sandstone; Systematic errors; Tortuosity
• ISSN: 0169-3913; 1573-1634
• DOI: 10.1007/s11242-018-1118-y

The tortuosity ( τ ), defined in the present context as the ratio of the free diffusion coefficient to the restricted diffusion coefficient of a contained fluid, is an important but difficult to measure characteristic of a porous medium, particularly when it is partially saturated with water. We develop and apply methodology, based on nuclear magnetic resonance (NMR) pulsed field gradient techniques, to measure τ for various sandstone rock cores as a function of residual water fraction. The NMR methodology requires the use of bipolar pulsed field gradient stimulated echo pulse sequences to avoid systematic errors due to magnetic susceptibility differences and D 2 O as a stationary immiscible water phase; this was selected as it provides no 1 H NMR signal. Tortuosity of the free pore space was successfully measured using liquid ethane as a probe fluid for three different sandstones over the full accessible range of residual water saturation. Generally, the tortuosity was observed to increase with residual water (D 2 O) content; however, significant variations were observed between the different sandstones.