Hydrothermal experiments at in-situ conditions to identify Li release reactions by water-mineral interactions in deep sedimentary basins of the North German Basin and Upper Rhine Graben

Lithium (Li) production from brines circulating in deep sedimentary basins as a byproduct of deep geothermal energy use could contribute to cover a significant proportion of the increasing global Li demand. Additionally, co-produced formation waters from the oil and gas industry represent another potential source of Li that could be utilized in a similar manner. Feasibility assessments for these operations require estimates for a possible Li replenishment by the recirculation of the Li-depleted brine after aboveground Li extraction. Currently, information about Li contents in reservoir rocks, the main Li carrier mineral phases of the respective reservoirs and possible Li release reactions are scarce. In this study, drill core material from two geothermal wells, Groß Buchholz Gt1 (Triassic Bunter sandstone) in the North German Basin (NGB) and Soultz-sous-Forêts EPS1 (granitic basement) in the Upper Rhine Graben (URG), as well as from the gas well Unterlüß T1 (Triassic Bunter sandstone) in the NGB, were investigated for both Li contents and Li release behavior. The bulk rock Li concentrations of the sample collection vary slightly in the range of 47 to 79 ppm. The main Li-bearing minerals were identified to be (i) chlorite (575 ppm) and illite (68 ppm) for Bunter sandstone samples and (ii) biotite for the Soultz sous forêts granite with maximum concentrations of up to 597 ppm. By grain-size fraction analysis phyllosilicates in the clay fraction were confirmed to exhibit the highest Li concentrations for the sandstones. To explore the Li release behavior, hydrothermal experiments were conducted at conditions of low enthalpy geothermal reservoirs (100-160 °C, 370 bar) for 14-60 days. The above described core samples were investigated by using ground rock powder and for one example the clay fraction. These solids were mixed with either bi-dist. H 2 O or synthetic brine in Au capsules or in flexible Au-Ti reaction cells and installed in high pressure reactors. Results of the experiments show that a nearly constant Li concentration was attained after 6 to 12 days in all experiments with the bi-dist. H 2 O. In these experiments, a total of 0.2 to 0.7 % of the Li contained in the granitic sample and 0.9 to 3.2 % of the Li contained in the Bunter sandstone samples were leached into the solution. It was found that different reaction mechanisms involving silicate minerals, as well as halogenide and carbonate minerals, contribute to the Li release. The experiment conducted with a clay fraction consisting mainly of illite and chlorite showed an enhanced leach of 6.9 % Li, indicating that Li is preferentially released by illite and chlorite in all experiments. Variations in temperature point only to a minor influence on the amount of Li released in the experiments. Experiments conducted with a synthetic brine instead of bi-dist. H 2 O stimulated the release of Li by a factor of up to 2 for the Bunter sandstone samples and by a factor of 5 for the granitic sample. The results of hydrothermal experiments implicate that replenishment of Li in a geothermal extraction operation is occurring and is only slightly dependent on the reservoir temperature in the range of 100 to 160 °C. • Mineral analysis by femtosecond laser ablation inductively coupled plasma mass spectrometry (fs-LA-ICP-MS) revealed chlorite (up to 575 ppm Li) and illite (68 ppm Li) for the Bunter sandstone sample set and biotite (up to 597 ppm Li) for the granite sample to be main Li bearing minerals. • In hydrothermal experiments with powdered samples and bi-distilled H 2 O a total of 0.2 to 3.2 wt% Li were released into solution. The highest amount of Li was leached in an experiment conducted with a < 2 μm clay fraction extracted from one of the Bunter sandstone samples (6.9 wt%). • Hydrothermal experiments conducted with a synthetic brine increased the amount of Li released into solution by a factor of up to 2 for experiments with Bunter sandstone and by a factor of 5 for experiments with a granite sample, hinting to enhanced Li release of phyllosilicates in hypersaline solutions.