The Ristrup area, NW of Aarhus, Denmark
The nuclear magnetic resonance (NMR) technique has become popular in groundwater studies because it responds directly to the presence and mobility of water in a porous medium. There is a need to conduct laboratory experiments to aid in the development of NMR hydraulic conductivity models, as is typically done in the petroleum industry. However, the challenge has been obtaining high-quality laboratory samples from unconsolidated aquifers. Working with these materials offers unique challenges including borehole conditions and laboratory sample conditions and size.
At our study site in Denmark, we employed sonic drilling, which minimizes the disturbance of the surrounding material and provides both good borehole conditions for logging, as well as core samples for laboratory studies. We acquired logging data consisting of NMR logging, electromagnetic (EM) induction, gamma, and magnetic susceptibility. In addition, we extracted twelve 7.6 cm diameter samples for laboratory measurements. Our samples were used for laboratory NMR measurements in their initial condition and size. We also prepared two sets of smaller subsamples to investigate the importance of sample size and condition on the NMR laboratory measurement.
This study addressed the main challenges inherent in conducting laboratory and logging NMR in unconsolidated near-surface aquifers: the borehole conditions and the sample conditions and size. A detailed comparison of laboratory and logging NMR data from the study site suggests sonic drilling to be a suitable drilling method as it is known to produce essentially no disturbed zone around the PVC casing and provide continuous, undisturbed samples of the unconsolidated materials. With this and with our careful sample preparation, the results of this study showed good agreement between the logging and laboratory NMR estimated water contents and relaxation times. We investigated the influence of sample size and condition by acquiring NMR data on two sets of smaller undisturbed and disturbed subsamples. When the T2 distributions of the small-disturbed and small-undisturbed samples were compared to the T2 distributions of the large samples, we observed different distributions both in shape and time. This study recommends use of larger samples for laboratory NMR experiments and concludes that repacking of samples has a large impact on the NMR measurements.
Figure: Results of laboratory and logging NMR measurements at the study site