Using NMR Decay-time Measurements to Monitor and Characterize DNAPL and Moisture in Subsurface Porous Media

Abstract

Knowing how environmental properties affect dense nonaqueous phase liquid (DNAPL) solvent flow in the subsurface is essential for developing models of flow and transport in the vadose zone necessary for designing remediation and long-term stewardship strategies. For example, one must know if solvents are flowing in water-wetted or solvent-wetted environments, the pore-size distribution of the region containing DNAPLs, and the impact of contaminated plumes and their transport mechanisms in porous media. Our research investigates the capability and limitations of low-field proton nuclear magnetic resonance (NMR) relaxation decay-rate measurements for determining environmental properties affecting DNAPL solvent flow in the subsurface. The measurements that can be performed with the laboratory low-field system can also be performed in situ in the field with the current generation of commercial borehole logging tools. The oil and gas industry uses NMR measurements in deep subsurface, consolidated formations to determine porosity and hydrocarbon content and to estimate formation permeability. These determinations rely on the ability of NMR to distinguish between water and hydrocarbons in the pore space and to obtain the distribution of pore sizes from relaxation decay-rate distributions. In this paper, we will show how NMR measurement techniques can be used to characterize, monitor, and evaluate the dynamics of mixed fluids (water-DNAPL) in unconsolidated near-surface porous environments, and describe the use of proton NMR [Formula: see text] (spin-spin relaxation time) measurements in unconsolidated sandy-soil samples to identify and characterize the presence of DNAPLs in these environments. The potential of NMR decay-rate distributions for characterizing DNAPL fluids in the subsurface and understanding their flow mechanisms has not previously been exploited; however, near-surface unsaturated vadose zone environments do provide unique challenges for using NMR measurements. These challenges are addressed through systematic laboratory experiments and a program of research to extend and adapt current field NMR measurements to near-surface environmental problems.

Estimation of water retention parameters from nuclear magnetic resonance relaxation time distributions

“…[3] The NMR relaxation behavior encodes relevant information about the water filled pore space, even at partial saturation [ Bird et al ., 2005; Hertzog et al ., 2007]. On the laboratory scale, as well as on the field scale, NMR techniques are expected to have great potential for characterizing the vadose zone [ Roy and Lubczynski , 2005; Costabel and Yaramanci , 2011a].…”

Relative hydraulic conductivity and effective saturation from Earth’s field nuclear magnetic resonance – a method for assessing the vadose zone

“…So far, reliable strategies to estimate the hydraulic parameters under unsaturated conditions from NMR data are not available. Although, the NMR relaxation behaviour encodes relevant information about the water filled pore space, even at partial saturation (Bird et al ; Hertzog et al ). Chen et al () proposed a formula to estimate the ratio from NMR relaxation times.…”

Practical limitations and applications of short dead time surface NMR

“…Water in the unsaturated zone tends to be held in the smallest available pore spaces. Hence, we generally expect water in an unsaturated formation to exhibit faster transverse relaxation (shorter and ) than water in the same formation when saturated (Hertzog et al ; Costabel and Yaramanci ). Results presented here indicate that both bound and mobile water in unsaturated/unconsolidated sediments can exhibit relaxation times of less than 30 ms and can be detected using short dead time surface NMR instrumentation.…”