Glasgow UKGEOS different lithology samples: unravelling their internal structure and fluid migration patterns within them

The Glasgow UK GeoEnegy Observatory is a shallow geothermal energy research facility. Its underpinning concepts are to explore the role of shallow mine heat energy on energy supply decarbonisation, the risks involved, and the environmental management regulation needed for this net-zero policy. The research field site has currently a network of twelve boreholes (Phase 1) for characterisation and monitoring purposes targeting former, flooded coal mines. In this work, we use samples from the GGC01 borehole (BGS, Keyworth) aiming to further characterise the subsurface material and understand the fluid migration within the different lithologies (e.g., interlayered sandstones and mudstones). We have used X-ray and Neutron tomography for this purpose. We show that these nondestructive full-field methods, with different resistivity and resolutions, can be powerful tools for both rock mechanics and physics research on the UKGEOS samples. X-rays are sensitive to density variations, which characterise the different lithologies along the length of the borehole. Neutrons are sensitive to hydrogen and its isotopes and, thus, ideal for monitoring hydrogen-rich fluid displacements within this heterogeneous material. Fluid flow experiments were performed for this purpose; where water displaced heavy water to acquire proper contrast within the neutron images. Both methods provide information about the spatio-temporal evolution of properties and textural characteristics of the tested material. Moreover, the combined use of such methods is vital when the material collection is limited (e.g., borehole samples). We have collected samples of 38 mm in diameter and 76 mm in length from different borehole depths. X-ray tomography (3D analysis) images have revealed the internal structure (different lithologies, textural spatial differences due to deposition and natural or lab-induced deformation fractures) of the tested material. Individual seams or network of seams, all characterised by lower density material, are visualised in some of the samples. Neutron tomography (3D analysis) images have indicated that these seams absorbed more water after flow experiments in some of the samples. The same observation stands for local patches of low-density material within the samples. We have carried out High Speed Neutron tomography to capture water migration within samples containing lab-induced fractures during flow experiments (4D analysis). Initial results have demonstrated that within those samples the matrix permeability was less important than the fracture permeability.