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首页> 外文期刊>Geothermics >Surface water chemistry at Torfajokull, Iceland-Quantification of boiling, mixing, oxidation and water-rock interaction and reconstruction of reservoir fluid composition
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Surface water chemistry at Torfajokull, Iceland-Quantification of boiling, mixing, oxidation and water-rock interaction and reconstruction of reservoir fluid composition

机译：冰岛Torfajokull的地表水化学-沸腾，混合，氧化和水-岩相互作用的定量和储层流体组成的重建

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摘要

The surface geothermal water chemistry and alteration mineralogy associated with rhyolitic rocks at Torfajdkull central volcano, Iceland was studied. The geothermal waters ranged in pH and temperature from 2.33 to 9.77 and 6-98 degrees C, respectively, and was characterized by variable alteration products including amorphous silica, quartz, hematite, goethite, kaolinite, elemental sulfur, pyrite, anatase, montmorillonite. alunite, amorphous iron silicates, pyrite, goethite, hematite and illite. The chemical composition of these waters and the associated mineralogy is influenced by several processes occurring from the geothermal reservoir to the surface including boiling, mixing, degassing, oxidation and water-rock interaction. In order to quantify these processes and explain the observed geothermal surface water composition and mineralogy, a geochemical model was applied that involved three steps: (i) defining the composition of the end-member fluid types present in the system, (ii) applying a mixing model based on conservation of non-reactive elements and enthalpy (temperature) and (iii) quantifying the process of progressive fluid-rock interaction and secondary mineral formation in the surface zone. The model may be applied to any geothermal system. Geothermal waters at Torfajokull represent either a mixture of non-thermal water and condensed steam with insignificant fraction of boiled reservoir water or boiled reservoir water that has been mixed. Two types of steam-heated waters were observed, acid and carbonate rich, the difference thought to be related to the boiling process. Steam-heated carbonate waters are formed from <10% steam originated by boiling and phase segregation at >200 degrees C followed by mixing with non-thermal water at shallow depth whereas steam-heated acid waters are formed upon extensive boiling and steam condensation and mixing with non-thermal surface in the surface zone. The surface alteration mineralogy and associated elemental mobility is largely influenced by the formation mechanism and chemistry of the geothermal surface water in the surface zone. At acid pH and under oxidized conditions Na, K, Mg and Ca were observed to be mobile and leached out whereas Fe, Ti and to a less extent Si, were retained in the alteration product forming amarphous silica, kaolinite, anatase and pyrite as well as some smectites and sulfates. For steam-heated carbonate waters, Na and K were observed to be mobile whereas Fe and Si are retained in amorphous silica, ferrihydrites and iron rich silicates. Carbonates were not calculated or observed to form associated with carbonate springs. Magnesium, Ca and K were observed to be mobile at pH < 6 whereas they are quantitatively retained into smectites and eventually also zeolites and carbonates with increasing pH. As a consequence, the mobility of Mg and K and to a less extent Ca and Na are greatly reduced under alkaline conditions. Based on the above, the key factors controlling the fluid-rhyolite interaction under surface geothermal conditions (similar to 100 degrees C) are acid supply, oxidation state and extent of reaction. The surface geochemical exploration methods developed and applied here are suitable for any geothermal system in order to explore geochemical processes occurring in active geothermal systems including boiling and fluid mixing, fluid and elemental sources, reservoir fluid properties and when applied before and during geothermal exploitation and utilization how these may have changed in nature with time. (C) 2015 Elsevier Ltd. All rights reserved.
机译：研究了冰岛Torfajdkull中央火山的地热水化学及与流纹岩相关的蚀变矿物学。地热水的pH和温度范围分别为2.33至9.77和6-98摄氏度，其特征在于变化的蚀变产物包括无定形二氧化硅，石英，赤铁矿，针铁矿，高岭石，元素硫，黄铁矿，锐钛矿，蒙脱土。亚矾石，无定形硅酸铁，黄铁矿，针铁矿，赤铁矿和伊利石。这些水的化学成分和相关的矿物学受到地热储层到地表的几种过程的影响，这些过程包括沸腾，混合，脱气，氧化和水-岩相互作用。为了量化这些过程并解释观察到的地热地表水组成和矿物学，应用了一个地球化学模型，该模型涉及三个步骤：（i）定义系统中存在的末端流体类型的组成，（ii）应用基于非反应性元素守恒和焓（温度）的混合模型，以及（iii）量化表层流体与岩石相互作用和次生矿物形成的过程。该模型可以应用于任何地热系统。 Torfajokull的地热水代表非热水和冷凝蒸汽的混合物，而沸腾的水库水或已混合的沸腾的水库水的比例很小。观察到两种类型的蒸汽加热水，富含酸和碳酸盐，差异被认为与沸腾过程有关。蒸汽加热的碳酸水是由<10％的蒸汽形成的，该蒸汽来自> 200摄氏度的沸腾和相分离，然后与浅深度的非热水混合，而蒸汽加热的酸性水则是在大量沸腾和蒸汽冷凝并混合后形成的在表面区域具有非热表面。表面变化的矿物学和相关的元素迁移率在很大程度上受表层地热地表水的形成机理和化学作用的影响。在酸性pH值和氧化条件下，观察到Na，K，Mg和Ca可以移动并浸出，而Fe，Ti和少量Si保留在蚀变产物中，形成了高级二氧化硅，高岭石，锐钛矿和黄铁矿。如一些绿土和硫酸盐。对于蒸汽加热的碳酸盐水，观察到Na和K是可移动的，而Fe和Si被保留在无定形硅石，三水铁矿和富铁硅酸盐中。没有计算出碳酸盐或发现碳酸盐与碳酸泉相关。观察到镁，钙和钾在pH <6时可移动，但随着pH的增加，它们被定量保留在绿土中，最终也保留在沸石和碳酸盐中。结果，在碱性条件下，Mg和K的迁移率以及在较小程度上的Ca和Na的迁移率大大降低。基于以上所述，在地热条件下（类似于100摄氏度），控制流纹石相互作用的关键因素是酸的供给，氧化态和反应程度。此处开发和应用的地表地球化学勘探方法适用于任何地热系统，以探究活跃地热系统中发生的地球化学过程，包括沸腾和流体混合，流体和元素源，储层流体性质以及在地热开采和利用之前和期间应用的情况。随着时间的流逝，这些自然变化的方式。 （C）2015 Elsevier Ltd.保留所有权利。

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