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Comparing Lab and Borehole NMR in Unconsolidated Aquifers

The Problem

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. Borehole NMR can also be used to infer the hydrologic properties of unconsolidated aquifers. However, it is challenging to obtain reliable estimates of the NMR parameters from borehole data collected in unconsolidated aquifers. Specifically, the formation can be altered due to severe washout during drilling and there can be a large disturbed zone around the casing. Laboratory-NMR data could aid in the development of NMR-K models, as is typically done in the petroleum industry. However, the challenge has been obtaining high quality laboratory samples from unconsolidated aquifers.

The Approach

In this project, we present a detailed comparison of laboratory and borehole NMR data under most promising conditions at a study site in Denmark. To address the challenges facing both laboratory and borehole measurements in unconsolidated aquifers, we drilled using a sonic rig that minimally disturbs the formation and produces essentially no disturbed zone around the PVC casing. Moreover, continuous undisturbed samples, ~ 7.6 cm (3 inch) in diameter, were obtained for the entire 32 m depth of the borehole. With this drilling method, we obtained high quality borehole NMR measurements, other logging data, and lithologic descriptions. Laboratory-NMR data showed general agreement with the borehole NMR estimated parameters. We also investigated the impact of sample size and condition on laboratory NMR measurements.