Injection of Ca-depleted formation water in the Lower Triassic Bunter Sandstone Formation for seasonal heat storage in geothermal sandstone reservoirs: Effects on reservoir quality

Optimisation of the use of commonly available energy sources through seasonal storage of excess heat in hot deep aquifers is considered. The chemical effects of heating the Bunter Sandstone Formation to up to 150 °C are investigated by laboratory core flooding experiments at reservoir conditions, petrographic analysis and geochemical modelling.

Experiments are performed with a Ca-depleted synthetic formation water in order to avoid loss of injectivity by calcium carbonate scaling at elevated temperatures. The synthetic formation water is injected into a Bunter Sandstone Formation sample at 25 °C, 75 °C (reservoir temperature), 100 °C and 150 °C with a velocity of 0.05 PV/h.

Results show a significant increase in the aqueous concentration of calcium, silicium and barium upon heating, while the concentration of magnesium decreases. The main chemical processes taking place upon heating of the reservoir to up to 150 °C is dolomitisation, the replacement of plagioclase with albite, the dissolution of quartz and barite and the precipitation of mica.

A significant portion of the cementing calcite dissolved during the experiment, and consequently the tested Bunter Sandstone sample disintegrated after the experiment, indicating that injection of heated calcium depleted Bunter brine into the Bunter Sandstone Formation may damage the reservoir. The results highlight the importance of investigating the effects that removal of selected ions may have on the reservoir properties, since the injection of modified formation water may cause new problems in the reservoir.