The risk of scaling and corrosion predicted from formation water composition for Danish geothermal plants. GEOTHERM - Geothermal energy from sedimentary reservoirs - Removing obstacles for large scale utilization. Innovation Fund Denmark: project 6154-00011B (Final report in WP3)

Precipitation of solids, i.e. scaling, from geothermal water is a commonly experienced problem. Formation water may contain soluble species in concentrations up to 300 g/L. During production of geothermal water, the change in temperature and pressure may lead to precipitation of solids (scaling) and/or corrosion in the plant facilities. Scaling and corrosion may jeopardise geothermal energy production by reducing the injectivity from the injection well to the reservoir or destruction of the well casing, pipes and plant equipment. This study addresses the risk of scaling in Danish geothermal plants (Margretheholm, Sønderborg and Thisted) based on geochemical modelling of formation water from these plants. The present report is one of the final reports of WP3 under the project GEOTHERM – Geothermal energy from sedimentary reservoirs – Removing obstacles for large scale utilization supported by the Innovation Fund Denmark: project 6154-00011B.

Formation water was sampled during operation of the Margretheholm, Sønderborg and Thisted geothermal plants from both the production well and close to the injection well, and the chemical composition of the brine was measured. The potential risk of scaling was modelled by MARTHE-PHREEQC, which considers the potential for mineral precipitation in the reservoir in a radial flow from the injection well. The Pitzer database was applied in order to address the very saline formation water; however, the drawback with this database is that silicate precipitation cannot be considered. Therefore, additional reactive transport modelling was performed using PHREEQC and its phreeqc database to include a larger range of minerals in the evaluation of potential geochemical reactions upon injection of cooled formation water. Input parameters were besides mineralogy and porosity of the reservoir, formation water composition and production data such as depth of well head, formation water temperature, flow rates etc. Furthermore, the risk of barite nucleation in the injection wells was modelled. The modelling only addresses the risk of scaling and thus several processes such as the formation of biofilm, corrosion processes and transport of particles have not been taken into account because of lack of data in the model databases and current software capabilities (e.g. transport of eroded particles).