Abstract
Hydrogeochemical modelling has become an important tool for the exploration and optimisation of deep hydrogeothermal energy resources. However, well-established software and model concepts are often run outside of its temperature, pressure, and salinity ranges. The core of the model codes are thermodynamic databases which contain a more or less complete subset of mineral phases, dissolved species, and gases occurring in geothermal systems. Although very carefully compiled, they should not be taken for granted at extreme conditions but checked prior to application. This study compares the performance of four commonly used geochemical databases, phreeqc.dat, llnl.dat, pitzer.dat, and slop16.dat. The parameter files phreeqc.dat, pitzer.dat, and llnl.dat are distributed with PHREEQC and implement extended Debye–Hückel and Pitzer equations. Thermodynamic data in slop16.dat represent an implementation of the revised Helgeson–Kirkham–Flowers formalism and were converted to be used with the Gibbs Energy Minimizer ChemApp for this work. Test calculations focussed on the solubility for some of the most common scale-forming mineral phases (barite, celestite, calcite, siderite, and dolomite). The databases provided with PHREEQC performed comparatively well, as long as the range of validity was respected for which the corresponding database was developed. While it is obvious to use pitzer.dat at high salinities instead of phreeqc.dat, the range of validity for high temperature is usually not given in the database and has to be checked manually. The results also revealed outdated equilibrium constants for celestite in slop16.dat and llnl.dat and the lack of adequate temperature functions for the solubility constants of siderite and dolomite in all databases. For those two minerals, new thermodynamic data are available. There is, however, a lack of experimental thermodynamic data for scale-forming minerals in low-enthalpy geothermal settings, which has to be addressed for successful modelling.
Highlights
Geothermal energy has acquired a key position in the mix of renewable energies due to its very effective heat supply and the ability to provide base load electrical energy
In order to assess the reason for this discrepancy, log10 K values for the reaction SrSO4 ⇋ Sr2+ + SO42− were calculated from CHNOSZ for standard conditions (298.15 K and 1 bar) and compared to phreeqc.dat, pitzer.dat, and literature data
This implies that the equilibrium constant in slop16. dat and llnl.dat favours the solid phase in the reaction more than phreeqc.dat and pitzer.dat do, resulting in higher solubilities
Summary
Geothermal energy has acquired a key position in the mix of renewable energies due to its very effective heat supply and the ability to provide base load electrical energy. Ambitious facilities running at high volumetric flow rates and temperatures revealed that even small changes of the hydrochemical equilibria can lead to serious problems with precipitations in subsurface and surface level facilities Such precipitations tend to clog casings, heat exchangers, and submersible pumping systems of the facility and may even lead to porosity reduction in the reservoir. Any of these risks can lead to economically adverse situations for the operator, due to the necessity of maintenance and plant downtime. This has led to an increasing awareness for hydrochemical processes in the engineering community
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