C0₂ Neutral energy system utilising the subsurface. WP3 Evaluating chemical reactions upon seasonal heat storage in the hot deep Gassum Sandstone Formation, Aalborg, Denmark

The possibility of storing heated formation water in deep hot aquifers is investigated with laboratory experiments. In particular, geochemical reactions expected to take place upon injection of the heated formation water is tested with focus on potential heat promoted dissolution and precipitation processes in the Upper Triassic – Lower Jurassic Gassum Formation in the Aalborg area. The Gassum Formation is a roughly 200 meter thick geological formation rich in sandstones and with very good reservoir (aquifer) properties. It occurs in approximately 1500 meters depth below the Aalborg area.

A core flooding experiment at in situ pressure and the temperatures 23°C, 60°C, 80°C, 100°C, and 120°C has been conducted to simulate the injection of heated formation water. To represent one of the most abundant Danish geothermal reservoirs, core material from the Upper Triassic – Lower Jurassic Gassum Formation in the Farsø-1 well is used for the laboratory experiments. A synthetic CO2 -bearing Farsø brine is used as the flooding fluid. Geochemical analysis of the effluent from the flooding experiment is combined with petrographic analysis of the sandstone before and after the experiment and provide input to a geochemical numerical model that constrain and support the interpretation of the results from the laboratory experiments.

The experimental data, petrographic analysis and geochemical modelling in this study indicate that reinjection of heated formation water into the Gassum formation in the Aalborg area may induce or enhance albitisation of microcline, dissolution of quartz, ankerite and barite and cause precipitation of calcite.

The main changes in the aqueous composition induced by heating of the brine are increased concentrations of silicium, iron and barium caused by the dissolution processes. Re-precipitation of silicium, barium and iron upon cooling of the brine is a potential risk and appropriate removal of these elements e.g. by filtration in the surface facility is essential to prevent clogging of the injection well to ensure a continuous heat production from the heat storage plant.

The concentration of aqueous silicium increases stepwise with increasing temperature from c. 1 mg/L at 60°C to c. 8 mg/L at 120°C. The silicium concentration at each temperature is controlled by a quasi-steady-state between kinetically controlled dissolution and/or precipitation of microcline, kaolinite and quartz combined with equilibrium with albite at temperatures ≥ 80°C. Both the conversion of microcline to albite and the dissolution of quartz increases with increasing temperature. However, only an insignificant fraction of the microcline and the quartz is expected to dissolve during a 6 month storage period at both 100°C and 120°C and therefore dissolution of microcline and quartz is not expected to deteriorate the reservoir properties. Similarly, the formation of albite is not expected to affect the reservoir properties, since the albite precipitates inside the feldspar or as overgrowths on the microcline and thus has little effect on the reservoir permeability. Dissolution of microcline may, however, lead to the formation of illite, especially at higher temperatures. Since illite is very fiberous, the formation of illite may have a damaging effect on the reservoir permeability. The geochemical model indicates that the effluent is supersaturated with respect to illite and thus that illite may potentially precipitate during the experiment, although not identified by the petrographical analysis.

The concentration of barium also increases stepwise with increasing temperature. Thus, the concentration of aqueous barium increases from c. 1 mg/L at 60°C to c. 20 mg/L at 120°C and appears to be kinetically controlled by dissolution of barite. The concentration of aqueous iron increases from c. 10-15 mg/L at 60°C to c. 35 mg/L at 120°C and appears to be controlled by ankerite dissolution and calcite precipitation. Neither the dissolution of barite nor ankerite is expected to damage the formation, as these are present as cementing phases between framework grains in very small amounts in the reservoir.

In conclusion, the results of this study show that the dissolution/precipitation processes that may potentially be induced by storage of heated formation water with temperatures up to 120°C in the Gassum Formation in the Aalborg area are not likely to damage the reservoir and that heat storage in this reservoir may be possible provided operational precautions are taken.