IRETHERM: Developing a Strategic and Holistic Understanding of Ireland's Geothermal Energy Potential Through Integrated Modelling of New and Existing Geophysical, Geochemical and Geological Data

Abstract

The academia-government-industry collaborative IRETHERM project (www.iretherm.ie), initiated in 2011 and funded by Science Foundation Ireland, is developing a strategic understanding of Ireland's (all-island, south and north) deep geothermal energy potential through integrated modelling of new and existing geophysical, geochemical and geological data. Potential applications include both low enthalpy district space heating of large urban centres and electricity generation from intermediate-temperature waters. IRETHERM is unique in the breadth of the targets studied and the manner in which disparate data are being acquired, collated and combined in a unified, holistic manner. High-resolution geophysical modelling tools are being constructed for imaging aquifers, fault zones and granitic bodies in the depth range 0–5 km. The new tools are being tested on seven “type” geothermal targets with a comprehensive program of electromagnetic field-surveys to identify those geological settings and localities that present the greatest potential for significant geothermal energy provision. New borehole temperature and heat-flow measurements and analyses of radiogenic element compositions of an island-wide suite of multi-depth crustal samples are being used to derive the first 3-D model of Irish crustal heat-production. Thermal variations are being modelled using new crustal thermal conductivity measurements and heat-production constraints and existing controls on lithospheric structure to determine the origin of the regional variation in heat-flow and identify high-temperature anomalies at upper-crustal levels for immediate and future targeting. IRETHERM comprises three broad geothermal target types, 1) Assessment of the geothermal energy potential of Ireland’s radiogenic granites (EGS), 2) Assessment of the geothermal energy potential of Ireland’s deep sedimentary basins (HSA), and 3) Assessment of the geothermal energy potential of warm springs. Linking these three together is subsurface imaging using both controlled-source (CSEM) and natural-source (magnetotellurics, MT) electromagnetic methods. Electrical conductivity, being a transport property, is a proxy for permeability, and appropriate porosity-permeability relations are being developed. Modelling of the data will include constraints from other geoscientific data, both in a formal (e.g., joint inversion) manner and a less formal (i.e. co-operative) manner. These results will be complemented by (hydro)geothermal modelling of the processes relevant to the target-specific geothermal energy potential. To date, MT measurements have been made at 466 sites over sedimentary basins (190 sites), granites (156 sites) and warm springs (120 sites), with CSEM across one warm spring. An ongoing continuous geochemical (temperature and electrical conductivity every 15 mins) and time-lapse seasonal hydrochemical sampling programmes are in progress at six warm spring sites. A database on heat production in Irish rocks has been compiled, of more than 3,300 geochemical sample measurements, with 3,000 retrieved from various archives and over 300 new analyses. Geochemistry, geochronology and isotopic analyses have been conducted on subsurface granites and exposed analogues astride the Iapetus Suture Zone in order to understand the underlying reasons for their radiogenic heat production. Finally, thermal conductivity measurements have been made on borehole samples from representative lithologies. This paper will review all of the IRETHERM work within Ireland’s low enthalpy setting.