Verma Mahendra P, van Geldern Robert, Barth Johannes A C, Monvoisin Gael, Rogers Karyne, Grassa Fausto, Carrizo Daniel, Huertas Antonio Delgado, Kretzschmar Thomas, Villanueva-Estrada Ruth Esther, Godoy José Marcus, Mostapa Roslanzairi, Cortés Hugo Alberto Durán
Geotermia, Instituto Nacional de Electricidad y Energías Limpias, Reforma 113, Col. Palmira, Cuernavaca, Mor., C.P. 62490, México.
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Department Geographie und Geowissenschaften, GeoZentrum Nordbayern, Schlossgarten 5, 91054, Erlangen, Germany.
Rapid Commun Mass Spectrom. 2018 Oct 30;32(20):1799-1810. doi: 10.1002/rcm.8233.
Knowledge of the accuracy and precision for oxygen (δ O values) and hydrogen (δ H values) stable isotope analyses of geothermal fluid samples is important to understand geothermal reservoir processes, such as partial boiling-condensation and encroachment of cold and reinjected waters. The challenging aspects of the analytical techniques for this specific matrix include memory effects and higher scatter of delta values with increasing total dissolved solids (TDS) concentrations, deterioration of Pt-catalysts by dissolved/gaseous H S for hydrogen isotope equilibration measurements and isotope salt effects that offset isotope ratios determined by gas equilibration techniques.
An inter-laboratory comparison exercise for the determination of the δ O and δ H values of nine geothermal fluid samples was conducted among eleven laboratories from eight countries (CeMIEGeo2017). The delta values were measured by dual inlet isotope ratio mass spectrometry (DI-IRMS), continuous flow IRMS (CF-IRMS) and/or laser absorption spectroscopy (LAS). Moreover, five of these laboratories analyzed an additional sample set at least one month after the analysis period of the first set. Statistical evaluation of all the results was performed to obtain the expected isotope ratios of each sample, which were then subsequently used in deep reservoir fluid composition calculations.
The overall analytical precisions of the measurements were ± 0.2‰ for δ O values and ± 2.0‰ for δ H values within the 95% confidence interval.
The measured and calculated δ O and δ H values of water sampled at the weir box, separator and wellhead of geothermal wells suggest the existence of hydrogen and oxygen isotope-exchange equilibrium between the liquid and vapor phases at all sampling points in the well. Thus, both procedures for calculating the isotopic compositions of the deep geothermal reservoir fluid - using either the analytical data of the liquid phase at the weir box together with those of vapor at the separator or the analytical data of liquid and vapor phases at the separator -are equally valid.
了解地热流体样品中氧（δO值）和氢（δH值）稳定同位素分析的准确性和精密度，对于理解地热储层过程（如部分沸腾 - 冷凝以及冷水和回注水的侵入）非常重要。针对这种特定基质的分析技术具有挑战性的方面包括记忆效应、随着总溶解固体（TDS）浓度增加δ值的更高离散度、用于氢同位素平衡测量的Pt催化剂因溶解/气态H₂S而劣化以及同位素盐效应，这些效应会抵消通过气体平衡技术测定的同位素比率。
来自八个国家的11个实验室开展了一项实验室间比对活动，用于测定9个地热流体样品的δO和δH值（CeMIEGeo2017）。通过双进样同位素比率质谱法（DI - IRMS）、连续流IRMS（CF - IRMS）和/或激光吸收光谱法（LAS）测量δ值。此外，其中5个实验室在第一组样品分析期至少一个月后分析了另一组样品。对所有结果进行统计评估以获得每个样品的预期同位素比率，随后将其用于深层储层流体成分计算。
在95%置信区间内，测量的总体分析精密度为δO值±0.2‰，δH值±2.0‰。
在地热井的堰箱、分离器和井口采集的水样中测量和计算得到的δO和δH值表明，在井中的所有采样点，液相和气相之间存在氢和氧同位素交换平衡。因此，计算深层地热储层流体同位素组成的两种方法——使用堰箱处液相和分离器处气相的分析数据，或使用分离器处液相和气相的分析数据——同样有效。
