Abstract
Decreasing production rates and massive precipitation of native copper (Cu(0)) were observed in the production well at the geothermal research facility Groß Schönebeck (Germany). The Cu precipitates filling up the well are a product of an electrochemical corrosion reaction between dissolved copper (Cu2+, Cu+) in the brine and iron (Fe(0)) of the carbon steel liner. It was hypothesized that this reaction occurs not only within the borehole, but also on the outside of the casing at contact between casing and reservoir rock as well as in the pores of the reservoir rock. To verify the assumption of potential clogging of the rock pores as well as to quantify the reaction and to determine reaction kinetics, a flow-through experiment was designed mimicking the reaction at depth of the well between sandstone samples (24 cm3 Fontainebleau), steel (carbon steel or stainless steel), and artificial formation water containing 1 mM Cu2+ at oxic or anoxic (O2 < 0.2 mg/L) conditions in dependence of temperature and salinity. Obtained experimental data served as input for a numerical reaction model to deepen the process understanding and that ultimately should be used to predict processes in the geothermal reservoir. Results showed that (1) with increasing temperature, the reaction rate of the electrochemical reaction increased. (2) High amounts of sodium and calcium chloride (NaCl + CaCl2) in the solution decreased the overall reaction inasmuch more Fe and less Cu was measured in the salt-poor solutions over time. (3) Strongest oxidation was observed in oxic experiments when not only native copper but also iron hydroxides were identified after the experiments in the pore space of the rock samples. (4) No reaction products were observed when stainless steel was used instead of carbon steel to react with the Cu2+ solution. A numerical flow-through reactor model was developed for PHREEQC based on the assumption that Fe(0) corrosion is kinetically controlled and subsequent Cu(0) precipitation occurs in thermodynamic equilibrium within the investigated experimental set-up. Calculated coefficients of determination comparing measured and simulated reaction rates for Fe and Cu underline the validity of the approach.
Highlights
Mineral precipitation in the pores of geothermal reservoir rocks reduces the permeability and productivity and injectivity of geothermal wells (Putnis and Mauthe 2001; Milsch et al 2009; Civan 2015)
The performed experiments mimic field observations from the geothermal research site Groß Schönebeck, where fluid–casing–rock interactions were assumed to be responsible for clogging the pores of the reservoir rock near the production well casing
The type of steel was found to have the largest effect on the electrochemical corrosion at geothermal conditions
Summary
Mineral precipitation in the pores of geothermal reservoir rocks reduces the permeability and productivity and injectivity of geothermal wells (Putnis and Mauthe 2001; Milsch et al 2009; Civan 2015). To prevent the threat of reservoir formation damage, the geochemical reactions within the pore space of the reservoir rock need to be understood and predictive models need to be developed. Precipitation is caused by shifts of the chemical equilibrium due to changes of the temperature, pressure, or interactions between geothermal fluid, reservoir rock, and the material of the well casing. This approach requires an experimental set-up at elevated fluid temperatures, pressures, and salinity. The obtained experimental results represent a dataset that secondly can be used as basis for geochemical models to give prognoses about processes in the wells
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
