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

High temperature aquifer thermal storage (HT-ATES) in hot deep aquifers is considered to optimize the usage of commonly available energy sources. This study investigates potential near-well mineralogical changes induced by HT-ATES in two different Danish geothermal reservoirs: the mineralogically mature and calcite-free Gassum Formation, and the calcite-containing Bunter Sandstone Formation.

Core flooding experiments at reservoir conditions and temperatures up to 150°C were performed with a core specimen from each of the two geothermal reservoirs and with synthetic formation water as the flooding fluid. Effluent brines for chemical analysis were collected during the experiments and mineralogical changes of the core material identified by mineralogical characterisation of the core material before and after the experiment. A 1D reactive transport model was constructed to simulate the laboratory experiment and the model was calibrated against the experimental data. This calibrated model was then extended to simulate the continuous injection for six months of heated formation water into the two sandstone reservoirs to evaluate potential near-well mineralogical and porosity changes. For the Bunter Sandstone Formation, the formation water was modified to a saturation index for calcite of -0.1 prior to injection to replicate a situation where precautions are made to avoid loss of injectivity due to calcium carbonate scaling at elevated temperatures.

For the Gassum Formation, injecting formation water up to 100°C showed no significant changes in porosity. At temperatures ≥120°C, albite and siderite contents decreased, but the impact on reservoir porosity was minimal. However, calcite precipitation near the injection well at temperatures ≥120°C may reduce injectivity. In the Bunter Sandstone Formation, the use of slightly modified formation water resulted in considerable calcite dissolution in the reservoir at all investigated temperatures and dolomitization at 150°C. Our findings indicate a potential risk of near-well clogging caused by the precipitation of dolomite at this temperature. At lower temperatures, the calcite dissolution led to increased porosity, posing a potential risk to the rock's integrity.

The study suggests that HT-ATES in reservoirs with high calcium carbonate content, like the Bunter Sandstone Formation, is challenging. Precautions to avoid calcium carbonate scaling may lead to dissolution, while neglecting precautions may cause scaling and clogging issues. In contrast, mineralogically mature and calcite-free sandstones, such as the Gassum Formation, may be suitable for excess heat storage, particularly at temperatures <100°C.