Hydrochemical and operational parameters driving carbonate scale kinetics at geothermal facilities in the Bavarian Molasse Basin

Abstract

The majority of scales observed at geothermal facilities exploring the Malm Aquifer in the Bavarian Molasse Basin are carbonates. They form due to a disruption of the lime–carbonic acid equilibrium during production caused by a reduction of the partial pressure of carbon dioxide due to pressure change and degassing. These scales are found at the pumps, production pipes, filters, heat exchangers, and occasionally in the injection pipes. In this study, scales of all sections of geothermal facilities were taken. The database consists of scale samples from 13 geothermal pumps, 6,000 m production pipe (sample interval 10 - 12 m), 11 heat exchanger revisions, 2 injection pipes, and numerous filter elements. The samples were analyzed by SEM-EDX, XRD, Raman spectroscopy, and acid digestion to assess their chemical and mineralogical composition. From direct gauge measurements at six facilities during pump changes, scale rates were determined along the production pipes. From indirect measurements (multifinger caliper measurements) scale rates are derived for the region below the pump. Hydrochemical analyses from the wellhead were taken from 13 sites to feed the hydrogeochemical models. The calcite scale rates in the production pipes increase from the pump to the wellhead, where they reach 1.5 - 4.1 $$\mu$$ μ mol/($$\hbox {m}^2\,\cdot$$ m 2 · s). Scale rates below the pump reach up to 1.5 $$\mu$$ μ mol/($$\hbox {m}^2\,\cdot$$ m 2 · s). Given the slight change of hydrochemistry on the rise through the production pipe, where < 4 % of dissolved calcium ions precipitate as scale, scale rates cannot be derived from water samples at the wellhead, but require direct gauge measurements. The small amount of precipitation, together with fully turbulent conditions suggests that all measured rates are controlled by the surface-reaction of calcite crystallization following the nomenclature of Appelo and Postma (2004). Two approaches are used for the modeling of the scale rates. The first approach is based on hydrogeochemical modeling with PHREEQC. Scale rates calculated by this method are one order of magnitude higher than the measured ones. The second approach is based on correlations between the measured scale rates at the wellhead at six facilities and identified thermodynamic scale drivers ($$\Delta$$ Δ log (p$$\hbox {CO}_2$$ CO 2 ), $$\Delta$$ Δ total pressure, $$\Delta$$ Δ pH, and $$\hbox {SI}_{{calcite}}$$ SI calcite ). The correlations allow linear regressions which are used for the prediction of the scale rate at the wellhead, along the whole production pipe, and below. The modeling results show that scale prediction based on the new regressions that rely on thermodynamic scale drivers works better than existing hydrogeochemical models, already without implementation of kinetic parameters ($$\hbox {CO}_2$$ CO 2 -stripping and magnesium inhibition).

Ultrasonic and Impact-Echo Testing for the Detection of Scaling in Geothermal Pipelines

“…Tables 1 and 2 show the evolution of the calcium and magnesium ion concentrations over time. Since the concentration of magnesium ions was negligible, the assumption of pure calcite scaled on the test rig is valid and in accordance with the findings of Köhl et al [2]. Before the first experiment, a scaling layer of (6.62 ± 0.24) mm was measured by a sliding caliper.…”

“…A number of methods are used to monitor fouling (a broader term for all types of organic and inorganic deposition) in other industries, such as the oil, gas and food industries. These include direct and indirect, offline, in-situ and online, as well as inline methods, each of which have pros and cons [2,3]. An effective monitoring solution specific to the high temperature conditions in geothermal plants, which optimises maintenance and cleaning measures in dependency of scaling, has yet to be developed.…”

“…The predominant type of scaling in the Bavarian Molasse basin, where the Sauerlach facilities are located, is calcite [2]. The research described in the following therefore focuses on this locally most common pipe-scaling combination of steelcalcite.…”

Transformational Geothermal Fluids in the Upper Jurassic Carbonate Reservoir of the South German Molasse Basin

“…The minor variation of the 87 Sr/ 86 Sr values of these samples neither correlate with the strontium content nor the Mg/Ca ratio (Figure 7d & Figure 8c). These groundwaters show the highest gas contents (CO2 and CH4) of Upper Jurassic groundwater in the SGMB [4,97]. Thus, the missing correlations may be related to degradation of hydrocarbons, resulting in CH4 and CO2 generation, which is also recognisable in elevated levels of alkalinity (Table 3).…”

Scaling and corrosion formation mechanism in geothermal piping system based on Pourbaix-Lindal diagram-Ryznar index and fluid velocity profile

“…The function of this piping system is to flow geothermal fluid from the reservoir to the turbine to drive a generator that will produce electrical energy. Geothermal fluid consists of steam containing non-volatile anions such as Cl -, HCO3, SO 4-, NH4 -, F -, I -, Br-and cations such as Na + , K + , Mg 2+ , Mn 2+ , Fe 2+ , Al 3+ , and non-condensable gas (NCG), CO2, H2S, H2 and the inert gas He and Ar and brine [3,4,5,6]. Some of these anion and cation species are also influenced by the character of production wells which have a composition of wet vapor, impurity gases, non-condensed gases, and a scale that has the possibility of solid particles.…”