Hydrogeological Characterisation of Sherwood Sandstone using BNMR and Geophysical Logs

The Sherwood Sandstone is an important aquifer for geothermal energy in Northern Ireland. The Northern Ireland Government has identified geothermal energy, including aquifer thermal energy storage (ATES) systems, as a viable green energy source. This study investigates the hydrogeologic characteristics of the Sherwood Sandstone, with a focus on its porosity, using borehole nuclear magnetic resonance (BNMR) and geophysical models (Archie and Waxman-Smits).

BNMR and geophysical logging were done inside three borehole installations drilled into the Sherwood Sandstone aquifer at a depth of about 100 m. The data (BNMR, resistivity log, EC log, temperature log, and natural gamma log) obtained from this exercise were analysed using WELLCAD.

The results of the processed data showed that the porosity calculated from the BNMR and the two petrophysical logs are similar, demonstrating the relationship between nuclear magnetic resonance and petrophysical-derived porosity. The average porosity of the Sherwood Sandstone at this location ranges between 19.9% and 14.5%, the minimum ranges between 2.1% and 9.9% and the maximum ranges between 42.5% and 30.2%.

This study confirmed the viability of Sherwood Sandstone for ATES systems, and further hydrogeologic characteristics such as hydraulic conductivity, transmissivity, and structure delineation are needed before its installation.