Abstract：
The occurrence of enhanced concentration of the radium triplet 226Ra, 228Ra and 224Ra is a frequently observed property of highly saline anoxic deep water as used e.g. in geothermal plants. In the present study we develop a model to explain the observed activity levels in the brines. The model considers processes at the rock-fluid interface of the aquifer like alpha recoil, sorption and surface precipitation and is implemented by means of a Monte Carlo simulation. The outcomes of the simulations indicate the dominating role of fine-grained constituents of the reservoir rock, e.g. claystone with enhanced specific activities of the natural decay chains. Mass fractions of such material in the order of a few percent are sufficient to result in radium fluid concentrations >1 Bq l-1. Also a generally valid relation between the Th/U ratio in the aquifer rock and the 228Ra/226Ra activity ratio in the fluid was found. This link improves the agreement between radium fluid data and the mean Th/U ratio of the Earth\'s crust. The 224Ra/228Ra fluid ratios reflect the transport time from the location of last radium release to the sampling point. The model findings were applied to a well investigated aquifer used in a geothermal plant in the North German Basin. An eight component system of the aquifer rock was established as the basis for the simulation of the radium concentrations in the deep fluid. The comparison between simulation and fluid analyses revealed a degree of radium sorption of about 50 %, which is necessary to match the model\'s results with the measurements. On the other hand, the 228Ra/226Ra fluid ratio of the brine was well reproduced by the simulation, showing the suitability of the model even in complex heterogeneous reservoirs. From the 224Ra/228Ra fluid ratios a transition from pore-to fracture-guided transport < 10 m distance from the production well is suggested. Precipitates from such deep fluids occurring after changes of the thermodynamic conditions are able to accumulate radium isotopes in Ba/Sr-sulphate phases. The time dependence of the radioactive disequilibrium between 226Ra, 228Ra and its child 228Th in such scales is described by a mathematical model and is applied to two different uptake models. Based on this approach, age determinations on precipitates found in different components of a geothermal plant are conducted. They reveal the triggering of scale formation due to modifications in the plant. The results are suitable for drawing conclusions about the operation of the system, which result in a reduction in the amount of scale and a reduction in downtimes.
摘要：
镭三重态226Ra的浓度增加，228Ra和224Ra是例如在地热装置中使用的高盐水缺氧深水的经常观察到的性质。在本研究中，我们开发了一个模型来解释盐水中观察到的活性水平。该模型考虑了含水层岩石-流体界面的过程，如α反冲，吸附和表面沉淀，并通过蒙特卡罗模拟实现。模拟结果表明，储层岩石的细粒成分具有主导作用，例如，具有增强的自然腐烂链的特定活动的粘土石。这种材料的质量分数在几个百分比的量级足以导致镭流体浓度>1Bql-1。还发现了含水层岩石中的Th/U比与流体中的228Ra/226Ra活度比之间的普遍有效关系。这种联系改善了镭流体数据与地壳平均Th/U比之间的一致性。224Ra/228Ra流体比率反映了从最后一次镭释放的位置到采样点的传输时间。模型结果已应用于北德盆地地热工厂中使用的经过充分调查的含水层。建立了含水层岩石的八组分系统，作为模拟深层流体中镭浓度的基础。模拟和流体分析之间的比较显示，镭的吸附程度约为50％，这是使模型的结果与测量结果匹配所必需的。另一方面,模拟结果很好地再现了盐水的228Ra/226Ra流体比，表明该模型即使在复杂非均质储层中也具有适用性。根据224Ra/228Ra流体比，建议从孔隙到裂缝引导传输的过渡距离生产井<10m。在热力学条件改变后发生的这种深层流体中的沉淀物能够在Ba/Sr-硫酸盐相中积聚镭同位素。226Ra之间的放射性不平衡的时间依赖性，228Ra及其子228Th在这种尺度上由数学模型描述，并应用于两种不同的摄取模型。基于这种方法，对地热工厂不同组件中发现的沉淀物进行了年龄测定。它们揭示了由于植物中的修饰而引起的水垢形成的触发。结果适合得出有关系统运行的结论，这导致规模的减少和停机时间的减少。
