Hydrogeochemical modelling of corrosion effects and barite scaling in deep geothermal wells of the North German Basin using PHREEQC and PHAST

Highly mineralised formation water can cause compositional and structural alteration in well casing materials. Furthermore, precipitating minerals can lead to the clogging of wells and damage to pumping equipment. Hydrogeochemical modelling is a helpful tool for evaluating and predicting such natural hydrogeochemical and technically induced processes (e.g., scaling, corrosion, outgassing), during drilling and operation of a geothermal well. The software PHREEQC and its databases are used to model such processes at chemical equilibrium conditions. For a better understanding of the spatial distribution of chemical species and precipitating phases within an aquifer the software PHAST, which combines three-dimensional water flow and chemical equilibrium modelling is also applied. ion waters from the North German Basin are taken as an example of the capabilities and limitations of such hydrogeochemical models. The groundwater in the deep aquifers in this area is dominated by Na+ and Cl− or by Na+, Ca2+ and Cl−. The amount of total dissolved solids ranges from 100 g L−1 up to 400 g L−1 and increases with depth and temperature. During the operation of geothermal wells in the North German Basin, various scalings (e.g., Ba and Sr sulphates, Fe sulphides and Pb hydroxides) are observed. Several model applications elucidating the hydrogeochemical behaviour of deep geothermal systems are presented. The first model is a simple example of iron corrosion at different environmental conditions, and is followed by modelling of the hydrogeochemical processes during the operation of one geothermal well in the North German Basin, at a depth of 4000 m, a temperature of 150 °C and with a Total Dissolved Solids (TDS) content of about 250 g L−1. To evaluate the effects of barium sulphate scaling in a deep aquifer, induced by injected surface water containing Ca2+ and SO42−, a one- and a three-dimensional model scenario are presented. Furthermore, one scenario is calculated in which EDTA is added, to prevent the precipitation of barite by the formation of aqueous complexes.