Induced rare earth element fractionation in brines by hydraulic fracturing of their aquifer rocks

Stimulation of rocks deteriorates the equilibrium distribution of rare earth elements and yttrium (REY) between solids and brines. By stimulation of the Permian, Lower Rotliegend formations in the well Groß Schönebeck 3/90 the volcanites are less affected by stimulation than the overlying conglomerate and sandstones. Brines from different lithological sections differ in REY patterns and gain anomalies of Eu, Gd, and Y during processes of recrystallization. The initially small anomalies increase during stimulation but decrease with both ongoing pumping and recrystallization. The REY patterns and their Eu, Gd, Y anomalies of brines from the conglomerate and volcanites distinctively differ from those of the sandstones. The Gd and Y anomalies are strongly correlated and both increase in sandstones with distance from the underlying conglomerate and volcanite. The correlation plot of Gd and Y anomalies does not describe a mixing line between brines from the volcanite and conglomerate but it reveals the extent of recrystallization of rapid precipitations and growth of minerals immediately after the stimulation and the slow re-equilibration of REY between solids and brines in each section of rocks. Extreme recrystallization occurred within the conglomerates, the least ones in the volcanite. REY patterns with their characteristic Eu, Gd, and Y anomalies are reliable indicators of the source of brines after stimulation. The time-dependence of changes of Eu, Gd and Y anomalies reflect the process of recrystallization.