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Numerical simulations to assess thermal potential at Tauranga low-temperature geothermal system, New Zealand

Simulations numériques en vue d’évaluer le potentiel du système geothermal basse-température de Turanga, Nouvelle Zélande

Simulaciones numéricas para evaluar el potencial termal en el sistema geotermal de baja temperatura de Tauranga, Nueva Zelanda

数值模拟评价新西兰Tauranga地温地热系统的地热潜力

Simulações numéricas para avaliação do potencial térmico do sistema geotérmico de baixa temperatura de Tauranga, Nova Zelândia

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Abstract

Tauranga low-temperature geothermal system (New Zealand) has been used for the last 40 years for direct uses including space heating, bathing and greenhouses. Warm-water springs in the area are between 22 and 39 °C, with well temperatures up to 67 °C at 750 m depth. A heat and fluid flow model of the system is used to determine reservoir properties and assess thermal potential. The model covers 130 km by 70 km to 2 km depth, and was calibrated against temperatures measured in 17 wells. Modelling shows that to maintain the observed primarily conductive heat flow regime, bulk permeability is ≤2.5 × 10−14 m2 in sedimentary cover and ≤1 × 10−16 m2 in the underlying volcanic rocks. The preferred model (R 2 = 0.9) corresponds to thermal conductivities of 1.25 and 1.8 W/m2 for sedimentary and volcanic rocks, respectively, and maximum heat flux of 350 mW/m2. The total surface heat flow is 258 MW over 2,200 km2. Heat flux is highest under Tauranga City, which may be related to inferred geology. Model simulations give insights into rock properties and the dynamics of heat flow in this low-temperature geothermal system, and provide a basis to estimate the effects of extracting hot fluid.

Résumé

Le système géothermal basse-température de Turanga, Nouvelle Zélande, a été exploité durant les 40 dernières années pour des usages directs incluant le chauffage de l’air, des bains et des serres. Les sources chaudes de cette zone sont entre 22 et 39 °C, avec des températures au puits allant jusqu’à 67 °C à 750 m de profondeur. Un modèle du flux de chaleur et d’écoulement du fluide dans ce système est utilisé pour déterminer les propriétés du réservoir et évaluer le potentiel thermique. Le modèle couvre 130 km par 70 km sur 2 km de profondeur et a été calé par rapport aux températures mesurées dans 17 puits. La modélisation montre que pour maintenir le flux de chaleur conductive initialement observé la perméabilité est en général de ≤2.5 × 10−14 m2 dans la couverture sédimentaire et ≤1 × 10−16 m2 dans les roches volcaniques sous-jacentes. Le modèle retenu (R 2 = 0.9) correspond à des conductivités thermiques de respectivement 1.25 et 1.8 W/m2 pour les roches sédimentaires et les roches volcaniques et un flux de chaleur maximum de 350 mW/m2. Le flux de chaleur total en surface est de 258 MW à l’intérieur d’une zone de 2,200 km2. C’est sous la Ville de Tauranga que le flux de chaleur est le plus élevé, ce qui peut être imputé à la géologie telle qu’on la suppose. Le modèle de simulation permet une bonne compréhension des propriétés de la formation et de la dynamique du flux de chaleur dans ce système geothermal basse-température et fournit une base pour estimer les effets d’une exploitation du fluide thermal.

Resumen

El sistema geotermal de baja temperatura de Tauranga (Nueva Zelanda) ha sido usado durante los últimos 40 años para usos directos, incluyendo calefacción, baños e invernaderos. Los manantiales de agua cálida en el área están entre 22 y 39 °C, con temperaturas de pozo de hasta 67 °C a 750 m de profundidad. Se utiliza un modelo de flujo de fluido y calor del sistema para determinar las propiedades del reservorio y evaluar el potencial termal. El modelo cubre 130 km por 70 km y hasta una profundidad de 2 km, y fue calibrado contra temperaturas medidas en 17 pozos. El modelado muestra que para mantener el régimen primario observado del flujo de conductividad de calor, la permeabilidad general es ≤2.5 × 10−14 m2 en la cubierta sedimentaria y ≤1 × 10−16 m2 en las rocas volcánicas subyacentes. El mejor modelo (R 2 = 0.9) corresponde a conductividades termales de 1.25 y 1.8 W/m2 para las rocas sedimentarias y volcánicas respectivamente, y un máximo flujo de calor de 350 mW/m2. El flujo de superficie total de calor es 258 MW sobre 2,200 km2. El flujo de calor es mayor bajo la ciudad de Tauranga, lo cual puede estar relacionado con la geología inferida. Las simulaciones de modelos dan conocimientos de las propiedades de las rocas y de la dinámica del flujo de calor en este sistema geotermal de baja temperatura, y proporciona una base para estimar los efectos de extracción del fluido caliente.

摘要

(新西兰)Tauranga地温地热系统已经利用40年了, 直接用于空间加热、洗澡和温室。本地区的温水泉温度为22到39度, 750米深的井水温度达67度。系统的热量和液体流模型用于确定热储的特性, 评价热能潜力。模型覆盖的面积130公里x70公里, 深度2米, 根据17个井测量的温度进行了校准。模拟显示要维持观测到的导热流状态, 整个透水率在沉积层中应该为≤2.5 × 10−14 m2, 在下伏的火山岩中应该 为≤1 × 10−16 m2。首先的模型(R 2 = 0.9)对应沉积岩和火山岩的热传导率分别为1.25 和 1.8 W/m2, 最大热通量为350 mW/m2。总的地表热流2200平方公里为258 MW。热通量在Tauranga城之下最高, 可能与推断的地质条件有关。模型模拟使人们可以了解这个地温地热系统中岩石特性和热流动力学, 为评估抽取热流的影响提供了依据。

Resumo

O sistema geotérmico de baixa temperatura de Tauranga (Nova Zelândia) foi usado nos últimos 40 anos para usos diretos incluindo o aquecimento de espaços, banhos e estufas. As nascentes de água quente da zona têm entre 22 e 39 °C, com temperaturas em furos até 67 °C à profundidade de 750 m. Foi usado um modelo de fluxo de fluidos e de calor do sistema para determinar as propriedades do reservatório e avaliar o seu potencial térmico. O modelo abrange 130 km por 70 km e 2 km de profundidade e foi calibrado com as temperaturas medidas em 17 furos. A modelação mostra que para a manutenção do regime de fluxo de calor condutivo primário que se observa, a permeabilidade bruta seria ≤2.5 × 10−14 m2 na cobertura sedimentar e ≤1 × 10−16 m2 nas rochas vulcânicas subjacentes. O modelo preferido (R 2 = 0.9) corresponde a condutividades térmicas de 1.25 e 1.8 W/m2 para as rochas sedimentares e vulcânicas, respetivamente, com um fluxo de calor máximo de 350 mW/m2. O fluxo de calor superficial total é 258 MW para 2,200 km2. O fluxo de calor é máximo sob a cidade de Tauranga, o que pode se relacionar com a geologia inferida. As simulações do modelo contribuem para o conhecimento das propriedades da rocha e a dinâmica do fluxo de calor neste sistema geotérmico de baixa temperatura e proporcionam uma base para se estimarem os efeitos da extração de fluidos quentes.

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Acknowledgements

This work benefited from the advice of Mike O’Sullivan at the University of Auckland. Reviews from Paul White, Nico Fournier and two anonymous reviewers improved the manuscript. The work was supported in part by Bay of Plenty Regional Council, and by GNS Science Geothermal Core Research Program.

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Correspondence to S. C. P. Pearson.

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Published in the theme issue “Hydrogeology of Shallow Thermal Systems”

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Pearson, S.C.P., Alcaraz, S.A. & Barber, J. Numerical simulations to assess thermal potential at Tauranga low-temperature geothermal system, New Zealand. Hydrogeol J 22, 163–174 (2014) doi:10.1007/s10040-013-1076-y

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Keywords

  • Numerical modelling
  • Thermal conditions
  • New Zealand
  • Heat flow
  • TOUGH2