Modeling the effects of silica deposition and fault rupture on natural geothermal systems
Modeling the effects of silica deposition and fault rupture on natural geothermal systems
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DOI：
10.1029/2012jb009218
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发表时间：
2012-05
影响因子：
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通讯作者：
D. Dempsey;J. Rowland;G. Zyvoloski;R. Archer
中科院分区：
文献类型：
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作者：
D. Dempsey;J. Rowland;G. Zyvoloski;R. Archer
[1]新西兰北岛中部的陶波火山区（TVZ）内有大量的天然地热对流。在许多地方，覆盖地貌的高度多孔的喷发产物已经被热液的吞吐量和随之而来的二氧化硅沉积所改变。我们详细介绍了一个数值模型，认为在二氧化硅沉积/溶解的存在下，控制不断变化的渗透率分布的地热羽的演变。二氧化硅的沉淀根据梯度反应方案发生，其中溶解的二氧化硅浓度由温度依赖性二氧化硅溶解度控制。在120 kyr的时期内，持续的地热循环导致在主要地热上升流区上方形成了约200 m厚的低渗透性帽带。帽带鼓励上升流体的横向流动，增加了地热表达的观察区域。它还对盖层以下的流体具有绝缘作用，导致温度、焓和油气田的储集层潜力增加。第二个模型被构造来考虑通过不渗透帽带的断层破裂的具体情况。同震时渗透率沿着断层面增加，产生了通过地热田中心的强有力的、更新的流动，暂时减少了侧向流动。然而，近地表渗透率的重新封闭是迅速的，并且在1.1万年内发生了侧向流动的恢复和地热储层的恢复。这些影响进行了讨论的背景下，两个TVZ地热田：灭绝Ohakuri领域，和Te Kopia，这是位于一个主要的活动正断层。
[1] Natural geothermal convection abounds within the Taupo Volcanic Zone (TVZ) of New Zealand's Central North Island. In many locations the highly porous eruptive products that blanket the landscape have been altered by the throughput of hydrothermal fluids and the consequent deposition of silica. We detail a numerical model that considers the evolution of a geothermal plume in the presence of silica deposition/dissolution that controls an evolving permeability distribution. Precipitation of silica occurs according to a gradient reaction regime, in which the dissolved silica concentration is controlled by the temperature dependent silica solubility. Over a period of 120 kyr, continuous geothermal circulation leads to the development of a low permeability cap-zone, approximately 200 m thick, above the main geothermal upflow zone. The cap-zone encourages lateral flow of rising fluids, increasing the area across which geothermal expression is observed. It also has an insulating effect on fluids below the cap, causing increases in temperature, enthalpy, and the reservoir potential of the field. A second model is constructed to consider the specific scenario of fault rupture through the impermeable cap-zone. Coseismic increases in permeability along the fault plane produce vigorous, renewed flow through the center of the geothermal field, temporarily reducing lateral flows. However, resealing of near surface permeability is rapid, and the restoration of lateral flows and recovery of the geothermal reservoir occurs within ∼10 kyr. These effects are discussed in the context of two TVZ geothermal fields: the extinct Ohakuri field, and Te Kopia, which is situated on a major active normal fault.