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

Abstract

One possible solution to overcome the mismatch between the supply and use of available renewable energy resources is to store excess heat in low-enthalpy geothermal reservoirs. In this study, we evaluate the possible risk of damaging the reservoir through chemical dissolution and precipitation processes in the reservoir induced by the increased reservoir temperature upon injection of heated formation water. Evaluation of the reservoir performance is done by a combination of core flooding experiments, petrographic analysis, and geochemical modeling. A sample from the Upper Triassic–Lower Jurassic Gassum Formation in the northern part of Jutland, Denmark, representing one of the most important potential geothermal reservoirs in Denmark, is used in a core flooding experiment at reservoir conditions and elevated temperatures up to 120 °C. During the experiment, chemical changes in the effluent were monitored. Petrographic analysis of the core sample prior to and after the experiment is used as input in a geochemical model simulating the conditions of the core flooding experiment. The results show that the increased temperature induces the albitization of microcline; dissolution of quartz, ferroan dolomite/ankerite, and barite; and precipitation of kaolinite and calcite. The results indicate that the risk of reduced injectivity due to calcium carbonate scaling at elevated temperatures should be considered as part of any thermal energy storage project even in a calcium carbonate poor reservoir.