Redox Reactions During Sandstone Flow-through Experiments at Geothermal Conditions

Copper ions (Cu+ or Cu2+) in formation fluids can be reduced by carbon steel of the wells casing at geothermal conditions resulting in native copper (Cu(0)) precipitation, and oxidation and dissolution of iron (Fe). Indeed, high amounts of Cu(0) were observed at the bottom of a production well at the geothermal site Groß Schönebeck (Germany). It was expected that this reaction may also reduce the permeability of the reservoir rock close to the well casing. This study aimed to simulate this reaction at laboratorial conditions with special focus on the effect of reaction temperature and kinetics.A flow-through experiment was designed to simulate the conditions in a geothermal reservoir and well: a saline, and Cu-containing solution flowing through a sandstone media in contact with a steel capillary representing the well casing. Cu and Fe concentration as well as pH-, and redox values were monitored over time. After the experiment, pore spaces were characterized by X-ray computer tomography and by microprobe analysis using EDX on thin sections.Precipitation of Cu (without Fe) occurred within the sandstone near the wall and bottom of the capillary. The reaction-induced precipitation reduced the flow rate, but did not reach equilibrium after 49h. While the brine salinity had little impact on the reaction kinetics, an increasing temperature from 25 to 80°C strongly enhanced the overall reaction.