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With the economic establishment of the shale gas exploitation, horizontal drilling and hydraulic fracturing have become nowadays common procedures, but not without any controversy. In parallel, the emergent case of deep geothermal energy systems is claimed to not have much to do with the fracking process. Through an intensive review of the available literature and data, we aim to lift the veil on the differences and similarities between shale gas and deep geothermal energy regarding the chemical substances used during the stimulation phase, as far as possible. Such a comparison appears finally not so obvious. In a general way, the effective used quantity of each chemical should not be neglected, even if advertised as being an extremely small percentage of the total stimulation fluids composition. Although some of these substances are considered purely environment/human health friendly, the diversity of …