Deep geothermal energy is currently being investigated in many Western European countries, including Belgium, as a promising renewable energy source for the future. On the 14^th of September 2015 the first drilling on the Balmatt site for the construction of the first deep geothermal energy plant in Flanders, Belgium took place. With this drilling, VITO wants to measure the available output of hot water at a depth of approx. 4 km below the surface. On the basis of these data, a geothermal energy plant will be built. The expectations are high because even in a minimal version the planned geothermal energy plant would be one of third largest in Western Europe, just after Iceland and Italy. While in these countries, as well as in the Netherlands and Germany similar geothermal systems are already in use, the project is pioneering in Belgium.

Geothermal energy plants rely on pumping up large amounts of deep hot groundwaters and transferring these groundwaters through above-ground facilities (such as heat exchangers, turbines or pumps) before reinjecting them back to the geological layer from which they were extracted (closed-loop configuration).

Previous and recent chemical and radiological analysis of these groundwaters have, however, shown that, in order to guarantee long service life without adversely affecting humans and the environment, such installations will need to be assessed both chemically and radiologically. Indeed, from a chemical point of view, very large quantities of dissolved salts, more specifically NaCl, have been found in many of these groundwaters (in the order of 120 to 150 g/L [1,2]). Radiologically, these groundwaters are also characterised by relatively elevated concentrations of ²²⁶Ra (up to 120 Bq/L) and ²²⁸Ra (up to 10 Bq/L) [1,3].

In European regulations, geothermal installations have been included in the so-called Basic Safety Standards for Radiation Protection (EU Directive 2013/59/Euratom) as a NORM practice of concern to the Member States. At Belgian level “geothermal energy production, including exploration and pumping activities in the development thereof” has recently been added as a NORM practice to national radiation protection legislation by the FANC-Decree of March 3, 2016, ensuring control of radiation protection by the national regulatory body (Federal Agency for Nuclear Control - FANC) in the installations and in the safe disposal or discharge of solid and liquid residues which exceed the exemption criteria.

The elevated activities of naturally occurring radionuclides (NORs) detected in deep geothermal groundwaters, as well as the high salt concentrations observed in these groundwaters, pose several challenges to the successful exploitation of deep geothermal energy. These challenges include:

1. To correctly assess possible radioactivity accumulation in the surface installations of a geothermal facility, both by (co-)precipitation of Ra-containing solid phases as well as by precipitation of its daughter products (²¹⁰Po and ²¹⁰Pb).

2. To correctly assess possible scaling formation in surface installations and clogging of injection wells due to precipitation of solid phases induced by temperature- and pressure-related gradients associated with operation of the geothermal facility.

3. To provide mitigation solutions to reduce the occurrence and effects of the phenomena described in the above points 1. and 2.

In order to address the challenges stated in the previous paragraph, the following research proposal outline is presented:

1. Literature study on the management and handling of process streams or equipment contaminated with NORs, based on common practices in similar geothermal installations or in the oil and gas industry.

2. Obtaining representative water samples for radiological and chemical analysis from existing geothermal installations. Such water (and gas) samples have already been obtained by VITO at the Balmatt site (Mol, Belgium), and more may be taken (from the Balmatt site and from other sites in Western Europe) if the research would call for a more elaborate data set.

3. Performing predictive thermochemical modelling to scope for the possibility of radioactivity accumulation in scalings, sludges and precipitates. The thermochemical models developed during this PhD will receive input from ongoing PhDs at VITO, looking at the detailed mineralogy of the formation. The challenge in thermochemical modelling lies within the brine-type nature of the groundwater, for which thermochemical data are relatively scarce.

4. Laboratory testing to validate and verify the modelling. Due to the scarcity of data, an experimental programme is needed in addition to model assessment. The experimental programme and forthcoming results will allow to adapt and fine-tune the modelling part, or to select the most appropriate thermochemical data set. The challenge in the experimental setup will be to mimick, as close as possible, the deep geothermal groundwaters and the changing physico-chemical conditions in the geothermal installation (mainly temperature and pressure changes). Post-mortem analysis will then allow to judge whether scaling formation has occurred, and whether NORs are associated with these scalings.

5. Development of a reactive transport model for the whole installation. If the conditions in the whole geothermal installation can be correctly mimicked through thermochemical models, these models can be elaborated by coupling them to a transport model. Such reactive transport model would then allow to assess if, and where scaling, sludge and/or precipitation might occur in the system. At this point, the PhD might benefit from the ongoing post-doc at KULeuven to simulate pore clogging in the system, using a simplified chemical model.

It is considered that the outcome of the research proposal outlined above will allow:

1. to gain more insight into the radiological and chemical aspects inherently associated with deep geothermal installations which tap into brine-type groundwater aquifers.

2. to help designing new installations (in terms of temperature and pressure gradients) in order to minimise contamination of installations and equipment with NORM-associated radionuclides.

3. to propose mitigating strategies in case such contamination would occur during testing and/or operation. At this moment, the testing of mitigating strategies is not taken up (yet) as part of the PhD. During the PhD it will be judged whether time and resources are available to perform such tests, or whether they should be performed within a separate experimental framework.

References

[1] Vandenberghe, N., Dusar, M., Boonen, P., Fan, L.S., Voets, R., Bouckaert, J. (2000) "The Merksplas-Beerse thermal well (17W265) and the Dinantian reservoir", Geologica Belgica, 3 (3-4), 349-367.

[2] Analytical results (unpublished) on groundwater samples from the first extraction well on the VITO Balmatt site (Mol, Belgium).

[3] Vasile, M., Bruggeman, M., Van Meensel, S., Bos,S., Laenen, B., (2016) "Characterization of the natural radioactivity of the first deep geothermal doublet in Flanders, Belgium", accepted for publication in Applied Radiation and Isotopes.