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

Abstract

Potential near-well mineralogical changes caused by seasonal high temperature aquifer thermal storage were investigated by numerical reactive transport modelling for two mineralogically different Danish geothermal reservoirs; the mineralogical mature and calcite free Gassum Fm. and the calcite containing Bunter Sandstone Fm. Modelling scenarios simulated injection of heated formation water with temperatures up to 150 °C into the reservoirs for a period of 6 months. For the Bunter Sandstone Fm., 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. Based on mineral dissolution and precipitation calculated by the numerical model, the changes in porosity as a result of heat storage were estimated. For the Gassum Fm., no significant changes in porosity were calculated when injecting formation water with temperatures up to 100 °C. At temperatures ≥120 °C, the content of albite and siderite was significantly reduced due to dissolution, but these mineralogical changes were not expected to significantly affect the reservoir porosity due to the low content of these minerals in the reservoir. However, significant precipitation of calcite near the injection well at temperatures ≥120 °C may reduce injectivity at these temperatures. For the calcite containing Bunter Sandstone Fm., injection of the slightly modified formation water led to significant calcite dissolution in the reservoir at all investigated temperatures and to dolomitisation at 150 °C. In the case of dolomitisation, our results suggest a risk for clogging of the near well environment by freshly precipitated dolomite. For the remaining temperatures, the calcite dissolution causes an increased porosity and a potential risk for compromising the rock integrity which should be further investigated. For both reservoirs, only very small changes were predicted in the aqueous composition of the formation water. The mineralogical changes in the reservoir are therefore not easily detected by monitoring changes in the aqueous composition of the formation water.Our results imply that high temperature aquifer thermal energy storage in reservoirs with a high calcium carbonate content such as the Bunter Sandstone Fm. is challenging. If precautions are taken to avoid calcium carbonate scaling by reducing the calcium content of the formation water, injection of the Ca-depleted formation water into the reservoir will cause dissolution of calcium carbonates. On the other hand, if precautions are not taken, it is well-known that precipitation of calcium carbonates will cause scaling and clogging problems. In contrast, our results suggest that storage of excess heat in mineralogical mature and calcite free sandstones such as the Gassum Fm. may be feasible. This is especially the case at temperatures <100 °C.