Reinjection of excess water from the power production process in the Nesjavellir geothermal field has increased the temperature of shallow groundwaters, posing a risk to cold water wells used for the power plant as well as the ecosystem in Lake Thingvellir. Here, we present a numerical model of fluid flow and heat transport in the shallow reinjection zone to elucidate the flow path of reinjected liquid and the impact of reinjection on the temperature of groundwaters. The permeability structure of the model is based on a 3D geological model of the area. The numerical simulation is calibrated against underground water temperature data measured between 1998 and 2018 and data from a tracer test performed in 2018–2019. The model reproduces the overall temperature field and shows how a high-permeability lava flow together with rift-parallel normal faults act as permeable channels controlling fluid transport. If injection continues, the temperature along the lava flow increases considerably and spreads vertically to much deeper levels, generating a narrow warm zone along the main fault. If shallow injection ceases, temperature drops rapidly at the surface, but decreases slowly around the reinjection zone over 20 years. The numerical model in this study allowed a better characterization of the fracture–matrix interface and the porosity of post-glacial lava flows, contributing to sustainable management of the geothermal resource and the surrounding environment.

中文翻译：

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冰岛 Nesjavellir 地热系统浅层含水层回注和示踪剂输送的数值模拟

Nesjavellir 地热田发电过程中多余的水回注导致浅层地下水的温度升高，对用于发电厂的冷水井以及辛格维利尔湖的生态系统构成威胁。在这里，我们提出了浅层回注区流体流动和热传递的数值模型，以阐明回注液体的流动路径以及回注对地下水温度的影响。该模型的渗透率结构基于该地区的 3D 地质模型。数值模拟根据 1998 年至 2018 年间测量的地下水温度数据和 2018 年至 2019 年进行的示踪剂测试数据进行了校准。该模型再现了整体温度场，并显示了高渗透性熔岩流与裂谷平行正断层如何作为控制流体输送的渗透通道。如果继续注入，沿熔岩流的温度会显着升高，并垂直扩散到更深的水平，从而沿主断层形成一个狭窄的暖区。如果浅层注入停止，地表温度会迅速下降，但在 20 年内回注区周围温度会缓慢下降。本研究中的数值模型可以更好地表征裂缝-基质界面和冰后熔岩流的孔隙度，有助于地热资源和周围环境的可持续管理。沿熔岩流的温度显着升高并垂直扩散到更深的水平，沿主断层形成了一个狭窄的暖区。如果浅层注入停止，地表温度会迅速下降，但在 20 年内回注区周围温度会缓慢下降。本研究中的数值模型可以更好地表征裂缝-基质界面和冰后熔岩流的孔隙度，有助于地热资源和周围环境的可持续管理。沿熔岩流的温度显着升高并垂直扩散到更深的水平，沿主断层形成了一个狭窄的暖区。如果浅层注入停止，地表温度会迅速下降，但在 20 年内回注区周围温度会缓慢下降。本研究中的数值模型可以更好地表征裂缝-基质界面和冰后熔岩流的孔隙度，有助于地热资源和周围环境的可持续管理。