Causes of abundant calcite scaling in geothermal wells in the Bavarian Molasse Basin, Southern Germany

The carbonate-dominated Malm aquifer in the Bavarian Molasse Basin in Southern Germany is being widely exploited and explored for geothermal energy. Despite favorable reservoir conditions, the use of geothermal wells for heat and power production is highly challenging. The main difficulty, especially in boreholes >3000 m deep with temperatures >120 °C, is that substantial amounts of calcite scales are hindering the proper operation of the pumps within the wells and of the heat exchangers at the surface. To elucidate the causes of scaling we present an extensive geochemical dataset from the geothermal plant in Kirchstockach. Based on chemical analyses of wellhead water samples, chemical and mineralogical analyses of scales collected along the uppermost 800 m of the production well, and chemical analyses of gas inclusions trapped in calcite-scale crystals, four processes are evaluated that could promote calcite scaling. These are (i) decompression of the produced fluid between the reservoir and the wellhead, (ii) corrosion of the casing that drives pH increase and subsequent calcite solubility decrease, (iii) gas influx from the geothermal reservoir and subsequent stripping of CO2 from the aqueous fluid, and (iv) boiling within the geothermal well. The effectiveness of the four scenarios was assessed by performing geochemical speciation calculations using the codes TOUGHREACT and CHILLER, which explicitly simulate boiling of aqueous fluids (CHILLER) and take into account the pressure dependence of calcite solubility (TOUGHREACT). The results show that process i causes notable calcite supersaturation but cannot act as the sole driver for scaling, whereas ii and iii are negligible in the present case. In contrast, process iv is consistent with all the available observations. That is, scaling is controlled by the exsolution of CO2 upon boiling at the markedly sub-hydrostatic pressure of 4–6 bar within the production well. This process is confirmed by the visible presence of gas inclusions in the calcite scales above the downhole pump, where the production fluid should nominally have been in the homogeneous liquid state. Whereas minor calcite scaling may have been triggered by fluid decompression within the production well, we conclude that the abundant scaling along the pump casing is due to cavitation induced by operating the pump at high production rates

Effects of progressive burial on matrix porosity and permeability of dolostones in the foreland basin of the Alpine Orogen, Switzerland

The changes in rock-matrix porosity and permeability that carbonate reservoirs undergo with increasing burial depth are poorly understood. This lack of understanding raises the risks involved in exploring and engineering deep reservoirs for geo-energy applications. To provide more insight into compaction processes, the present study examines the e ects of progressive burial on two dolomitized mudstone units belonging to the Middle Triassic Muschelkalk within the Swiss Molasse Basin, situated in the foreland of the Alpine Orogen. Based on investigations of wireline logs and drill cores retrieved from up to 5000 m depth, we report the burial mod- i cation of crystal textures, pore sizes, pore geometries and their impact on matrix porosity and permeability. Within the rst 1500 m below surface, porosity is found to drop from 40 ± 2 to 18 ± 1 vol% and permeability drops from 105 ± 15 to ∼1 mD. At depths > 3000 m, porosity and permeability maintain nearly constant values of 6 ± 2 vol% and 1900 m, mechanical compaction is inactive and pressure solution at crystal contacts and along stylolites (both di- agenetic and tectonic), without any associated cementation, accounts for porosity loss. At depths > 3000 m, collapse of pores by pressure solution is compounded by pore-clogging by hydrothermal dolomite introduced by external uids. Throughout the entire depth range, stylolitization incrementally thins the formations, however, the dissolved material is not locally reprecipitated

Identifying deep hydrothermal fluids that leach metals from the oceanic crust and generate seafloor VMS deposits

Metasomatic alteration of the deep oceanic crust by heated seawater produces a variety of hydrothermal–metamorphic mineral assemblages including spilites and epidosites. Epidosites have been proposed as markers of deep upflow in hydrothermal convection cells and as source rocks for the metals in seafloor VMS deposits. Whereas spilitization is attributed to fluid of seawater salinity, the salinity of epidotizing fluids is debated. In addition, the metal contents of the fluids have been constrained by experiments and observations of vent fluids, but no direct analyses are available. We have analyzed primary fluid inclusions in spilites and epidosites in the Semail ophiolite (Oman). The results confirm that both fluids have salinity close to that of seawater (2.6–4.1 wt.% dissolved solids). The fluid inclusion data reveal that the alteration occurred over a range of P-T conditions: 170–430 °C for spilitization and 235– 400 °C for epidotization, all at hydrostatic pressures of 27–50 MPa. Laser-ablation-ICP-MS analyses show that the spilitizing fluid is enriched in Fe, Cu and Zn, whereas the epidotizing fluid is metal- depleted. Therefore, we view the spilitizing fluid rather than the epidotizing fluid as the major carrier of metals to black-smoker sulfide deposits on the seafloor