Local effects of the injection of undersaturated waters in geothermal applications

The Northern Alpine Foreland Basin (NAFB) in southwest Germany is home to more deep geothermal plants than any other region in the country. The upper Jurassic, its main aquifer, consists of permeable carbonates which bear waters with temperatures of up to 150 °C on the southern border, and total dissolved salts values of up to 2 g/L. Geothermal applications in the NAFB include medical spas, geothermal district heating and power generation.

In the case of heating and power generation, cooled-off waters are reinjected underground where rock-fluid interactions can lead to changes in the rock matrix and flow pathways. These interactions are thus an important factor to consider in the maintenance of geothermal plants. In carbonate systems, a decrease of temperature after heat extraction leads to an undersaturation of the previously equilibrated waters with regard to the host formation. The injected water dissolves the rock matrix along the borehole and the flow paths into the reservoir. This can increase the risk of a thermal breakthrough between injection and production well and possibly affect the borehole integrity. While the overall amount of dissolution has been monitored and modelled previously, the microscopic changes to the flow paths are still under investigation.

We used rock samples coated with a 2-component paint and produced one microfracture in the coating to overcome experimental restrictions due to a limited fluid volume of the autoclave. The experiments were run at typical injection temperatures between 40°C and 75°C. The CO₂ partial pressure wasadjusted to the measured values in the injection borehole. Microscopic and Raman images were taken before and after the exposition of the rock to the undersaturated waters and complemented by hydrochemical analyses.

The dissolution of the limestones picked up microstructures and led to a heterogeneous development of the flow path. In an early stage of the injection, an underprediction of the increase of the hydraulic conductivity is thus expected.

These assessments allow insights into the kinetics taking place at the artificial disturbance, which will make it possible to characterize and quantify the 3-dimensional pattern of calcite dissolution at a localized scale which is important for the development of the hydraulic properties, and affects further dissolution.