Using Low-Field NMR to Determine Wettability of, and Monitor Fluid Uptake in, Coated and Uncoated Sands

Abstract

Abstract Methods commonly used to measure the wettability of unconsolidated porous media are quick and easy to perform, but these tests do not provide reproducible results. This paper outlines the use of low-field nuclear magnetic resonance (NMR) as an alternative method of wettability assessment due to this tool's ability to discriminate bound fluid from bulk fluid. Water bound to the grain surfaces of water-wet samples relaxes quickly and produces signal amplitude peaks at low transverse relaxation (T2) values. Bulk water, on the other hand, relaxes much more slowly and signal amplitude peaks consequently appear at higher T2 values. It is expected that the contribution of surface-bound water in water-repellent samples is lower than in water-wet samples. NMR measurements performed on water-wet sand and on the same sand coated with organic matter clearly show significant differences in solid-fluid interactions between water and sands of different wettabilities. Numerous NMR measurements were obtained over time to monitor relaxation shifts because it was expected that the resulting spectra would provide insight into the rate of fluid uptake in sands of varying wettability. Water uptake appears to be spontaneous in water-wet samples and much slower in water-repellent samples, however all samples will eventually reach the same equilibrium endpoint regardless of wettability. NMR spectra also show that the results from water in uncoated and coated sands closely resemble that from water in wettable and water-repellent soils. Consequently, uncoated and coated sands can be used to analyze wettability mechanisms in unconsolidated porous media. The results also show that causative agents of soil water repellency include asphaltenes that are insoluble to n-pentane. Introduction Wettability is defined as "the tendency of one fluid to spread on or adhere to a solid surface in the presence of other immiscible fluids(1)." This parameter is not a fixed and constant property(2) due to factors such as saturation and climate history, as well as sample handling(3). Wetting and non-wetting fluids interact with a porous medium differently and these differences can be detected by low-field nuclear magnetic resonance (NMR)(4–6). One can infer the state of fluids within a porous medium from looking at NMR spectra based on type-specific transverse relaxation time (T2) cutoffs(7–9). Relaxation time is the time that a hydrogen-bearing molecule needs to revert to its original state, after it is excited by an oscillating magnetic field pulse sequence. Contributions to spectra at values of T2 that are lower than the cutoff are considered due to bound water while contributions at higher T2 values are considered due to water in the bulk phase(7, 10). Amplitude peaks at progressively lower T2 values indicate the presence of fluid in progressively smaller pores. The location of the amplitude peaks in NMR spectra can be used to infer sample wettability(9). While this has been done in porous media such as sandstones, carbonates, and chalk(11–14), no work has been done on soils. NMR can be a viable tool for assessing soils wettability because other wettability measurements such as the contact angle are impossible to make in unconsolidated porous media.