Geothermal energy will play an important role in achieving CO2 emission reduction targets by 2030. Carboniferous carbonate rock reservoirs in Ireland provide both shallow and deep geothermal targets. The porosity-permeability system in these carbonates is dominated by secondary porosity such as faults, fractures and karst. Despite their importance, the fracture and mechanical stratigraphy is not well understood.
Therefore, this study will analyse fault and fracture stratigraphy of potential onshore carbonate reservoir rocks. It will provide upscaling by integrating regional-, outcrop- and bed-scale observations, comparing surface with subsurface observations.
Field reservoir analogue studies will be conducted in quarries and key outcrops. This includes mapping of lithofacies, bed-scale parameters including thickness and variability, and fracture intensity and patterns. Field mapping will be upscaled through digital outcrop mapping using automated feature extraction on structure-from-motion drone photogrammetry. Present-day mechanical strength and elastic properties will be determined on representative lithologies using rock deformation experiments, to constrain mechanical stratigraphy.
Fracture, lithological and alteration parameters will be extracted from diamond drillcores. They will be compared with on-core petrophysical analysis and existing downhole geophysics, especially sonic, density and image data. Downhole survey, core data and analogue studies will be compared, to investigate the implications of stress-release jointing, bed-parallel slip and depth-related changes in karst development, on associated transmissivity and fluid flow.
Cenozoic N-NW-trending strike-slip faults, key conductive structures, and loci for karst conduits, will be mapped from Tellus airborne EM and magnetic surveys. This will extend previous lineament studies and will be integrated with existing constraints on these structures from mines and quarries. An integrated assessment of the impact on deep geothermal potential will then be performed, in particular of fracture permeability and how it relates to reservoir bulk permeability, and possible implications for critically stressed faults. These results will feed directly into ongoing deep geothermal projects.
Lead Applicant: Koen Torremans
Scholar: Simon Vokes
Host: University College Dublin