Image-D5.05 Database: Potential Supercritical Conditions

• Main Author: Manzella, Adele
• Format: Text
• Language: English
• Published: Zenodo 2017
• Subjects: deep geothermal; exploration; geophysics; high-temperature systems; indicators; drilling; GIS; favourability; Curie Point; Nesjavellir; Iceland
• Online Access: https://dx.doi.org/10.5281/zenodo.1288704; https://zenodo.org/record/1288704

Very high-temperature reservoirs are a possible target for future geothermal exploration either through the direct exploitation of super-critical fluids or as a potential high-temperature reservoir for Enhanced Geothermal Systems. By exploiting subsurface fluids at super-critical conditions, i.e. high temperature (>375 °C) and high pressure (>22 MPa), the energy output per well will increase by a factor of ~10. This will reduce development costs by decreasing the number of wells needed (IEA Technology Roadmap 2011). In order to contribute to the EU strategic energy and climate targets for 2020 and 2050 by fostering increased growth in the geothermal energy market through enhanced awareness of the potential of geothermal energy production, a database of potential supercritical resources has been launched by the IMAGE project. Superhot and supercritical resources are expected in the surrounding of still hot magmatic intrusions in the crust. A large part of the IMAGE activity focused on understanding what are the favorable conditions at a few km depth for shallow magmatic emplacement, beside improving exploration and investigation techniques for their detection and the related hot water circulation. In a typical crust with an average thermal gradient of the order of 30-35 °C/km the critical temperature of a brine (temperature above 450 °C) is reached at depth greater than 12-15 km. However, in many sites around the world (e.g. Larderello and Phlegraean Fields in Italy, Nesjavellir in Iceland, The Geysers in California) where exploratory boreholes were drilled in high-temperaturegeothermal system (T > 370 °C), reservoir pressures above supercritical conditions (>22.1 MPa) were encountered. These evidences confirm that geothermal reservoirs in supercritical conditions, both in temperature and pressure, exist in the vicinity of cooling magmatic intrusions. Volcanic rifts, extensional basins and/or subduction zones with related shallow crustal magma emplacements, are the more promising environments in which supercritical conditions may be found in the upper to middle crust levels, The compilation of a European database of the favourable indicators of the presence of supercritical geothermal resources has been a main task in the IMAGE project. The objective is to define areas in Europe where supercritical fluids occur at a drillable depth with a manageable chemistry composition can be found. Where do we find fluids at 4-5 km depth with a temperature exceeding 400 °C? In various regions in Europe, including Iceland, Italy, Azores, Montserrat, Canary Island. What characterizes these areas? Can we find other areas with similar features at greater depth? We focus on Iceland, where these resources have been searched and studied in the last decade. We used the experience of research in Larderello, described in detail in other IMAGE deliverables (e.g. D5.01), to look for indicators applicable over broad areas, in search of potential supercritical resources in continental Europe. Since temperature is the key parameter controlling the presence of supercritical reservoirs at (relatively) shallow depth, mapping of supercritical resources was mainly driven by thermal models derived from crustal and lithospheric constraints and data interpolation from available deep wells. Other information providing indirect indication of crustal thinning and shallow magmatic emplacement have been searched and analyzed. In particular, we mapped the following indicators: the depth of 400 °C isotherm; the MOHO depth and crustal thickness; the earthquake density combined with the estimated depth of the Brittle-Ductile Transition in Europe. Other interesting indicators, e.g. He3/He4 ratio values from which fluids of crustal origin may be inferred, or Curie Point depth that refers to deep temperature regime, are available only for local areas, and are too restricted to be of use at regional and European scale. The location of recent (Pleistocene-Holocene) volcanism, also dispersed, was mapped since it provides useful information, but was not used in the computation of final maps. After defining the indicators, their spatial correlation was established by Geographic Information System (GIS) models, and a database was organized. By prioritizing favourable conditions using GIS spatial analysis methods, the “favourability” map of geothermal resources at supercritical condition was then obtained. It provides a clear overview of the distribution of potential resources in Europe, based on analytical data. : FP7