Geochemical Constraints on the Operations of High Temperature Aquifer Energy Storage (HT-ATES) in Abandoned Oil Reservoirs

Heat for industrial processes and housing is an important part of the energy consumption. In Germany it accounts for more than 50 % of the total demand. The need for a sustainable heat production and/or the use of existing waste heat is key for the energy transition. Many industrial applications produce large amounts of heat, which cannot be used in the summer months. Heat storage is of increasing importance and technical solutions are widely discussed. In this context, the reuse of abandoned oil reservoirs as aquifer thermal energy storage systems is receiving even greater attention. Their widespread distribution and the very often already existing data and pre-knowledge of the future reservoir is very advantageous. Numerous aquifer thermal storage systems in the low-temperature range (25 °C < T < 40 °C) have been realized worldwide and are currently in operation. High temperature systems (T > 100 °C) with the advantage of higher efficiency and a better match to the requirements of process heat are completely lacking. Especially large temperature differences between storage-, formation- and injection temperatures add complexity to the operations, particularly by increasing significantly the perturbation of the geochemical system, leading to an increased scaling potential in the surface installations as well as in the reservoir. ... mehrTo address this gap of knowledge the Karlsruhe Institute of Technology plans establishing the demonstrator DeepStor in an abandoned oil reservoir below its campus with a foreseen half-yearly cyclic high-temperature energy storage and reproduction.
Herein, the assessment of a reservoir and the hosted geothermal fluid in the marine sandstones of the Meletta beds, part of the Cenozoic graben filling, in around 1250 m to 1350 m depth is demonstrated. Main focus is on the expected fluid-solid interactions with the associated scaling potential. In a first step, the unperturbed formation temperature, key parameter controlling the chemical system and therefore the basis for further analyses, is calculated using the MulT_predict multicomponent geothermometer. Then, the old and rather low-quality wellhead fluid data is corrected applying an equilibrium approach. Finally, the scaling formation is modelled, both for the testing phase (single-borehole application), where formation water needs to be stored in an open pond for later re-injection, as well as during the cyclic storage and production phase with two boreholes (cold and warm side of the reservoir). It is shown that the major scaling phases are iron hydroxides, calcite, celestite and barite. During the testing phase the change of the chemical system to ambient p/T-conditions and the exposition to atmospheric oxygen are the driving forces for the precipitation within the pond. During regular two-well operations a cyclically recurring process of dissolution processes in the reservoir and precipitation in the surface installation is evolving and remains stable in the long term.