Reactive transport modelling of potential near-well mineralogical changes during seasonal heat storage (HT-ATES) in Danish geothermal reservoirs

Geochemical Reactions upon Injection of Heated Formation Water in a Danish Geothermal Reservoir

“…These results are comparable to those found for a calcite-free and siderite-cemented sample of the Gassum Formation in the eastern part of Denmark . Holmslykke and Kjøller extended geochemical models simulating laboratory experiments to near-well environment models for a calcite-free and a partly calcite-cemented reservoir and found that in neither sandstone reservoir types was the dissolution/precipitation of silicates considered problematic for the stability of the reservoir. Based on this, we find it highly unlikely that the injection of heated formation water into the Gassum Formation at Farsø will induce reactions with the silicate minerals that may deteriorate the reservoir.…”

“…Calcite is, however, not observed in the Gassum Formation in the western part of Denmark at the depth of the tested sample. Holmslykke and Kjøller showed that for a calcite-free siderite-cemented sandstone reservoir, the equilibrium state of calcite controlled, whether siderite dissolved or precipitated in the reservoir. Thus, at small temperature increases above the reservoir temperature, where the injected water was subsaturated with calcite, siderite precipitated, whereas siderite dissolved at higher temperatures when the injected water became supersaturated with respect to calcite.…”

“…Such precaution may involve the reduction of the Ca concentration. Holmslykke et al , showed that injection of Ca-depleted formation water into a calcite containing reservoir may have serious consequences for the reservoir due to calcite dissolution. Because the amount of ferroan dolomite/ankerite is very low, injection of Ca-depleted formation water into the Gassum Formation at Farsø may not have a detrimental impact on the reservoir properties.…”