NMR-Relaxometry – a new, reliable and non-destructive method to estimate the fluid content in rock salt

In the context of the site selection procedure for a high-level radioactive waste repository in Germany, the fluid content in salt rocks is an important parameter to evaluate their barrier properties (i.e. gas formation potential and fluid pathways). Moreover, it can affect hydraulic conductivity, heat resistance, rock mechanical properties and it can be used for the spatial characterisation of the host rock. Fluids in rock salt (halite) are typically present in small, unconnected inclusions located within crystals or arranged along crystal boundaries and fissures, and can consist of brine, minor amounts of gases and hydrocarbons. When estimating the fluid content of rock salt, the main challenges are the low fluid content, the low permeability of the bulk rock preventing fluid extraction and the spatial heterogeneous fluid distribution. In summary, the measurements are time intensive and the results are highly dependent on the sample selection and preparation.

Nuclear magnetic relaxation (NMR) relaxometry is an established petrophysical method for a non-invasive characterisation of hydrocarbon host rocks and its fluids but it has not been used for rock salt so far. We present first results obtained from steeply inclined Zechstein rock salt (z2HS2) and compare them with a water content estimated by an extensive grinding of the salt samples in dried acetone and a subsequent analysis of the extracted fluids using infrared-spectroscopy. First results show, that NMR has promising features including a low limit of detection, non-destructive and quick measurements, no need for extensive sample preparations, and averaging over a large sample volume.