Numerical investigation on the implications of spring temperature and discharge rate with respect to the geothermal background in a fault zone

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Abstract

Geothermal springs are some of the most obvious indicators of the existence of high-temperature geothermal resources in the subsurface. However, geothermal springs can also occur in areas of low average subsurface temperatures, which makes it difficult to assess exploitable zones. To address this problem, this study quantitatively analyzes the conditions associated with the formation of geothermal springs in fault zones, and numerically investigates the implications that outflow temperature and discharge rate from geothermal springs have on the geothermal background in the subsurface. It is concluded that the temperature of geothermal springs in fault zones is mainly controlled by the recharge rate from the country rock and the hydraulic conductivity in the fault damage zone. Importantly, the topography of the fault trace on the land surface plays an important role in determining the thermal temperature. In fault zones with a permeability higher than 1 mD and a lateral recharge rate from the country rock higher than 1 m3/day, convection plays a dominant role in the heat transport rather than thermal conduction. The geothermal springs do not necessarily occur in the place having an abnormal geothermal background (with the temperature at certain depth exceeding the temperature inferred by the global average continental geothermal gradient of 30 °C/km). Assuming a constant temperature (90 °C here, to represent a normal geothermal background in the subsurface at a depth of 3,000 m), the conditions required for the occurrence of geothermal springs were quantitatively determined.

Keywords

Geothermal spring Fault hydrogeology Numerical modeling Groundwater flow 

Etude numérique sur les interrelations entre la température et le débit d’une source avec le fonds géothermique dans une zone de faille

Résumé

Les sources géothermales sont parmi les indicateurs les plus évidents de l’existence de ressources géothermales à haute température dans le sous-sol. Cependant, des sources géothermales peuvent également se manifester dans des zones où les températures moyennes du sous-sol sont faibles, ce qui rend difficile l’évaluation de zones exploitables. Pour résoudre ce problème, cette étude analyse quantitativement les conditions associées à la formation des sources géothermales dans les zones de failles, et étudie du point de vue numérique les interrelations de la température et du débit des sources géothermiques avec le fond géothermique du sous-sol. On en conclut que la température des sources géothermales dans les zones de failles est principalement contrôlée par le taux de recharge au sein de la roche encaissante et la conductivité hydraulique dans la zone endommagée par les failles. Fait important, la topographie de la trace de la faille sur la surface joue un rôle important dans la détermination de la température thermale. Dans les zones de failles avec une perméabilité supérieure à 1 mD et un taux de recharge latérale de la roche encaissante supérieur à 1 m3/jour, la convection joue un rôle dominant dans le transport de chaleur plutôt que dans la conduction thermique. Les sources géothermales ne sont pas nécessairement localisées dans les secteurs ayant un fond géothermal anormal (avec une température à une certaine profondeur dépassant la température inférée par le gradient géothermique continental moyen de 30 °C /km). En supposant une température constante (ici 90 °C, pour représenter un fond géothermal normal dans le sous-sol à une profondeur de 3,000 m), les conditions requises pour l’apparition de sources géothermales ont été déterminées quantitativement.

Investigación numérica sobre las implicancias de la temperatura y el caudal de descarga de los manantiales con respecto al fondo geotérmico en una zona de falla

Resumen

Los manantiales geotérmicos son algunos de los indicadores más obvios de la existencia de recursos geotérmicos de alta temperatura en el subsuelo. Sin embargo, los manantiales geotérmicos también pueden ocurrir en áreas de bajas temperaturas promedio del subsuelo, lo que dificulta la evaluación de zonas explotables. Para abordar este problema, este estudio analiza cuantitativamente las condiciones asociadas con la formación de manantiales geotérmicos en zonas de fallas e investiga numéricamente las implicancias que la temperatura de salida y el caudal de descarga de los manantiales geotérmicos tienen sobre el fondo geotérmico en el subsuelo. Se concluye que la temperatura de los manantiales geotérmicos en zonas de fallas se controla principalmente por el caudal de recarga de la roca de campo y la conductividad hidráulica en la zona dañada por la falla. Es importante destacar que la topografía de la traza de falla en la superficie del terreno juega un papel importante en la determinación de la temperatura térmica. En las zonas de falla con una permeabilidad superior a 1 mD y un caudal de recarga lateral en la roca de campo superior a 1 m3/día, la convección desempeña un papel dominante en el transporte de calor en lugar de la conducción térmica. Los manantiales geotérmicos no necesariamente ocurren en el lugar que tiene un fondo geotérmico anormal (con la temperatura a cierta profundidad que excede la temperatura inferida por el gradiente geotérmico continental promedio global de 30 °C/km). Suponiendo una temperatura constante (en este caso 90 °C, para representar un fondo geotérmico normal en el subsuelo a una profundidad de 3,000 m), las condiciones requeridas para la ocurrencia de manantiales geotérmicos se determinaron cuantitativamente.

断层带地热背景下泉温度和排泄量的数值研究

摘要

地热泉是地表以下高温地热资源存在的最明显的指示。然而,地热泉也可出现在地表以下平均温度较低的地区,这就使评价可开采带变得十分困难。为了解决这个问题,本研究定量分析了与断层带地热泉形成相关的条件,数值上研究了地热泉流出的温度和排泄量对地表以下地热背景的影响。结论是,断层带地热泉温度主要受到来自围岩的补给量及断层损坏带水力传导率的控制。重要的是,地表上断层迹的地形在确定高温地区的温度上发挥着重要作用。在渗透率高于1 mD以及来自围岩的侧向补给量大于1 m3/day的断层带,对流而不是热传导在热传输中发挥着主导作用。地热泉不一定就出现在地热背景异常(在一定深度温度超过全球平均大陆地热梯度30 °C /km推断出的温度)的地方。假定一个恒定温度(这里为90 °C,代表着地表以下深度3,000 m 的正常地热背景),对地热泉存在所需的条件进行了定量确定。

Investigação numérica nas implicações da temperatura e da taxa de descarga de fonte em relação à resposta geotermal em uma zona de falha

Resumo

Fontes geotermais são alguns dos indicadores mais óbvios da existência de recursos geotérmicos de alta temperatura na subsuperfície. No entanto, as fontes geotermais também podem ocorrer em áreas de baixa temperatura subterrânea média, o que dificulta o acesso de zonas exploráveis. Para abordar este problema, este estudo analisa quantitativamente as condições associadas à formação de fontes geotermais em zonas de falhas e investiga numericamente as implicações que a temperatura de saída e a taxa de descarga de fontes geotermais têm como resposta geotermal na subsuperfície. Conclui-se que a temperatura das fontes geotermais nas zonas de falha é, principalmente, controlada pela taxa de recarga da rocha fonte e pela condutividade hidráulica na zona de falha afetada. Importantemente, a topografia do traço de falha na superfície da terra desempenha um papel importante na determinação da temperatura termal. Nas zonas de falha com uma permeabilidade superior a 1 mD e uma taxa de recarga lateral da rocha fonte superior a 1 m3/dia, a convecção desempenha um papel dominante no transporte de calor e não na condução térmica. As fontes geotermais não ocorrem, necessariamente, em locais que apresentam uma resposta geotérmica anormal (com a temperatura, em certa profundidade, excedendo a temperatura inferida pelo gradiente geotérmico continental médio global de 30 °C/km). Assumindo uma temperatura constante (90 °C aqui, para representar uma resposta geotérmica normal na subsuperfície a uma profundidade de 3,000 m), as condições necessárias para a ocorrência de fontes geotermais foram quantitativamente determinadas.

Notes

Acknowledgements

We thank Christoph Schrank and Daniel Owen from Queensland University of Technology, Australia, and three reviewers for their constructive suggestions, which helped improving this article.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Key Laboratory of Groundwater Resources and Environment, Ministry of Education, College of Environment and ResourcesJilin UniversityChangchunChina
  2. 2.Institute of Earth Surface DynamicsUniversity of LausanneLausanneSwitzerland

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