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 $$\mu$$ mol/( $$\hbox {m}^2\,\cdot$$ s). Scale rates below the pump reach up to 1.5 $$\mu$$ mol/( $$\hbox {m}^2\,\cdot$$ 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 ( $$\Delta$$ log (p $$\hbox {CO}_2$$ ), $$\Delta$$ total pressure, $$\Delta$$ pH, and $$\hbox {SI}_{{calcite}}$$ ). 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 ( $$\hbox {CO}_2$$ -stripping and magnesium inhibition).

中文翻译：

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巴伐利亚Molasse盆地地热设施中的水化学和运行参数驱动碳酸盐垢动力学

在巴伐利亚莫拉西盆地探索马尔姆含水层的地热设施中观察到的大部分垢是碳酸盐。它们的形成是由于在生产过程中由于压力变化和脱气而导致的二氧化碳分压降低而导致了石灰-碳酸平衡的破坏。这些水垢存在于泵，生产管，过滤器，热交换器中，偶尔也存在于注入管中。在这项研究中，采用了地热设施所有部分的规模。该数据库包含来自13个地热泵，6,000 m生产管（采样间隔为10-12 m），11个换热器版本，2个注入管和大量过滤元件的规模样品。通过SEM-EDX，XRD，拉曼光谱和酸消解法分析样品，以评估其化学和矿物组成。根据泵更换期间六个设备的直接仪表测量，确定了沿生产管道的水垢率。通过间接测量（多指卡尺测量），可以得出泵下方区域的比例速率。从13个站点进行了井口的水化学分析，以提供水文地球化学模型。从泵到井口，生产管道中的方解石水垢率增加，达到1.5-4.1 $$ / mu $$ mol /（$$ \ hbox {m} ^ 2 \，\ cdot $ s）。泵下方的水垢率高达1.5 $$ \ mu $$ mol /（$$ \ hbox {m} ^ 2 \，\ cdot $$ s）。鉴于通过生产管道上升过程中水化学的细微变化，其中<4％的溶解钙离子会随着水垢沉淀而沉淀，因此无法从井口的水样中得出水垢率，而需要直接进行量规测量。少量的降水，加上充分的湍流条件，表明所有测得的速率均由遵循Appelo和Postma（2004）命名的方解石结晶的表面反应控制。有两种方法可用于缩放比例的建模。第一种方法是基于PHREEQC的水文地球化学模型。通过这种方法计算出的缩放比例比所测量的缩放比例高一个数量级。第二种方法基于在六个设施处的井口处测得的水垢速率与已确定的热力学水垢驱动力之间的相关性（$$ \ Delta $$ log（p $$ \ hbox {CO} _2 $$），$$ \ Delta $总压力，$$ \ Delta $$ pH和$$ \ hbox {SI} _ {{calcite}} $$）。相关性允许进行线性回归，用于预测井口的比例率，沿着整个生产管道，以及以下。建模结果表明，基于新的依赖于热力学尺度驱动器的回归的尺度预测比现有的水文地球化学模型更有效，而没有实施动力学参数（$ \ hbox {CO} _2 $$-剥离和镁抑制）。