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Water–rock interactions in the Bruchsal geothermal system by U–Th series radionuclides
Geothermal Energy ( IF 4.2 ) Pub Date : 2020-09-08 , DOI: 10.1186/s40517-020-00179-4
Lena Kölbel , Thomas Kölbel , Ulrich Maier , Martin Sauter , Thorsten Schäfer , Bettina Wiegand

Uranium and thorium decay series disequilibria in deep geothermal brines are a result of water–rock interaction processes. The migratory behavior of radionuclides provides valuable site-specific information and can therefore be an important tool for reservoir characterization and sustainable management of geothermal sites. In this study, we present data from long-term monitoring of naturally occurring 238U, 232Th and 235U series radionuclides analyzed in brine samples collected from the Permo-Triassic sedimentary reservoir rock at the Bruchsal geothermal site (SW Germany). The results show that radionuclides of the elements radium (226Ra, 228Ra, 224Ra, 223Ra), radon (222Rn), and lead (210Pb, 212Pb) are rather soluble in brine, while isotopes of uranium (238U, 234U, 235U), thorium (232Th, 228Th, 230Th), polonium (210Po), and actinium (227Ac, 228Ac) have low solubilities and are mostly immobile. Activities of radium isotopes in the geothermal brine exceed those of their thorium progenitors (average 226Ra = 29.9 Bq kg−1, about 103 times that of its 230Th parent). Modelling the observed disequilibria allows the following conclusion on water–rock interaction processes: (1) supply from alpha-recoil depends on isotope half-life because it is limited by the rate of diffusion through microfractures causing isotopic fractionation. (2) Radium retardation due to adsorption is low (226Ra/222Rn = 1.3) resulting in adsorption–desorption rate constants in the order of 10−10 s−1 for k1 and 10−9 for k2. (3) Scavenging of 226Ra from brine can best be explained by co-precipitation with barite resulting in an observed 226Ra anomaly in the solids of the reservoir section. The precipitation rate constant amounts to ca. 3.4 × 10−8 s−1 corresponding to a mean removal time of radium from brine by mineral precipitation to approximately 1 year.

中文翻译:

U–Th系列放射性核素在布鲁赫萨尔地热系统中的水岩相互作用

深层地热盐水中铀和or的衰减系列失衡是水-岩相互作用过程的结果。放射性核素的迁移行为提供了特定地点的有价值信息,因此可以成为储层表征和地热场址可持续管理的重要工具。在这项研究中,我们从长期监测的天然存在的238U,232Th和235U系列放射性核素中获得数据,这些放射性核素是从Bruchsal地热站点(德国德国)的Permo-Triassic沉积储层岩石中收集的盐水样品中分析的。结果表明,镭(226Ra,228Ra,224Ra,223Ra),ra(222Rn)和铅(210Pb,212Pb)的放射性核素在盐水中溶解度较高,而铀(238U,234U,235U),th的同位素(232Th,228Th,230Th),po(210Po)和act(227Ac,228Ac)的溶解度低,并且大部分是不动的。地热盐水中镭同位素的活度超过了其pro祖的活度(平均226 Ra = 29.9 Bq kg-1,约为其230 Th母体的103倍)。对观察到的失衡模型进行建模,可以得出有关水-岩相互作用过程的以下结论:(1)α-反冲的供给取决于同位素的半衰期,因为它受到通过微裂隙引起同位素分馏的扩散速率的限制。(2)由于吸附引起的镭延迟低(226Ra / 222Rn = 1.3),导致k1的吸附-解吸速率常数约为10-10 s-1,k2约为10-9。(3)从盐水中清除226Ra可以最好地解释为与重晶石共沉淀导致在储层段固体中观察到226Ra异常。沉淀速率常数约为。3.4×10-8 s-1对应于通过矿物沉淀从盐水中去除镭的平均时间到大约1年。
更新日期:2020-09-09
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