Proppants are hard spherules that are commonly used for the stimulation of wells in the oil and gas industry, but they are not widely applied in geothermal reservoirs. High-strength corundum proppants were used at the Gross Schonebeck geothermal site. The long-term reaction of these corundum (Al2O3)-based compounds under geothermal reservoir conditions (high temperature and high salinity) could result in their dissolution and the eventual re-precipitation of secondary Al-Si minerals, thereby clogging the pores of the reservoir and severely reducing its permeability. We studied the chemical stabilities of two types of proppants (resin-coated and uncoated) under different reservoir-like conditions over time. In a batch experiment, proppants were placed into Teflon autoclaves containing either natural brines from Gross Schonebeck or synthetic brines. They were kept at similar to 2 bar and at either 150 degrees C or 25 degrees C for 12, 28,42 or 80 days in NaCl and CaCl2 brines. The resulting precipitates are mainly composed of Na and Ca, with traces of Si and Fe. Up to 153 mg/L of Si were measured in the solutions, with occasional Al (normally <1 mg/L) and no Fe. The metal release was not affected by the resin coating. The high silica release during the experiment indicates some chemical instability of the proppants, which depends strongly on the temperature and salinity of the reservoir fluid. (c) 2013 Elsevier Ltd. All rights reserved.