Abstract
It is commonly known that heat extraction and decompression can lead to mineral precipitation and reservoir clogging in geothermal systems. In the Upper Rhine Graben, the precipitating minerals are mainly barite and calcite. This study focuses on clogging processes due to mineral precipitation in porous reservoir rocks, i.e., sandstone. The goal is to develop, build, and put into operation the HydRA apparatus, a facility for performing experiments on forced precipitation of barite in the pore spaces of sandstone under geothermally relevant pressure and temperature conditions. Barite precipitation during the flow-through is provoked by using barite-supersaturated solutions with a saturation index (SI) of 1.75. Scanning electron microscopy (SEM) investigations are used to detect barite crystal agglomerations and clogging of the pore spaces by overgrowths on these agglomerates. Following this, different crystal shapes are observed. The results are confirmed by permeability analyses before and after the flow-through experiments. Comparison of the major and trace element compositions of the original and reacted sandstones indicates element mobility due to water–rock interaction, even during the short-time experimental runs (max. 24 h).
Highlights
When heat is extracted from geothermal fluids, the chemical equilibrium in the geothermal system is changed and some mineral phases become supersaturated and precipitate
Experimental apparatus and procedure All experiments were performed in HydRA, in which rock samples are percolated at a fixed flow rate under geothermally relevant temperature and pressure conditions by a barite-supersaturated fluid to induce barite precipitation
Using the HydRA facility it was possible to fulfill the second aim of this work, the experimental investigation of the effects of barite precipitation in reservoir rocks under similar temperature and pressure conditions like those observed in the Upper Rhine Graben
Summary
When heat is extracted from geothermal fluids, the chemical equilibrium in the geothermal system is changed and some mineral phases become supersaturated and precipitate. Dissolution of minerals may alter the rock structure, whereas precipitation of mineral phases from the fluid may have an effect on the permeability. Flow-through experiments with sandstones were performed in numerous studies with regard to geothermal energy use for heat storage, energy extraction, or C O2-storage. In all these studies, a fluid is forced to flow through reservoir rocks, like limestone, Orywall et al Geotherm Energy (2017) 5:20 crystalline rocks, or sandstones. Another study shows that the decrease in permeability of kaolinite containing sandstones is related to the dissolution and re-precipitation of the kaolinite (Rosenbrand et al 2014)
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