Fabien Magri

Numerical investigations of fault-induced seawater circulation in the Seferihisar-Balçova Geothermal system, western Turkey

The Seferihisar-Balçova Geothermal system (SBG), Turkey, is characterized by temperature and hydrochemical anomalies along the faults: thermal waters in northern Balçova are heated meteoric freshwater, whereas the hot springs of the southern Seferihisar region have a strong seawater contribution. Previous numerical simulations of fluid flow and heat transport indicated that focused upsurge of hot water in faults induces a convective-like flow motion in surrounding units. Salt transport is fully coupled to thermally driven flow to study whether fault-induced convection cells could be responsible for seawater encroachment in the SBG. Isotope data are presented to support the numerical findings. The results show that fault-induced convection cells generate seawater plumes that extend from the seafloor toward the faults. At fault intersections, seawater mixes with rising hot thermal waters. The resulting saline fluids ascend to the surface along the fault, driven by buoyant forces. In Balçova, thick alluvium, minor faults and regional flow prevent ascending salty water from spreading at the surface, whereas the weak recharge flow in the thin alluvium of the southern SBG is not sufficient to flush the ascending hot salty waters. These mechanisms could develop in any faulted geothermal system, with implications for minerals and energy migration in sedimentary basins.ResumeLe système géothermal de Seferihisar-Balçova (SGSB), en Turquie, est caracterisé par des anomalies thermiques et hydrochimiques le long de failles : les eaux thermales dans la Balçova du Nord sont des eaux douces météoriques réchauffées, tandis que les sources chaudes de la région de Seferihisar ont une contribution forte de l’eau de mer. Des simulations numériques antérieures des écoulements de fluide et du transport de chaleur indiquent que des venues d’eau chaude dans les failles provoquent un mouvement de l’eau de type convectif dans les terrains alentours. Le transport du sel est complètement couplé avec le calcul des écoulements induits par la chaleur pour étudier si les cellules de convection crées par les failles pouvaient être responsables de l’invasion marine dans le SGBS. Les données isotopiques sont présentées pour appuyer les conclusions des calculs numériques. Les résultats montrent que la convection induite par les failles crée un panache d’eau salée qui s’étend du fond de la mer vers les failles. A l’intersection avec les failles, l’eau de mer se mélange avec l’eau des sources chaudes. Les fluides salins qui en résultent montent et atteignent la surface le long des failles, sous l’effet de la poussée d’Archimèdes. A Balçova, des alluvions épaisses, des failles mineures et l’écoulement régional empêchent l’eau salée qui remonte de s’étaler en surface, alors que la faible recharge des alluvions peu épaisses du sud du SGBS est insuffisante pour repousser l’eau chaude salée en mouvement ascendant. Ces mécanismes pourraient se développer dans n’importe quel système faillé géothermal, avec des implications sur la migration des minéraux et de l’énergie dans les bassins sédimentaires.ResumenEl sistema geotermal de Seferihisar-Balçova (SBG), Turquía, está caracterizado por anomalías de la temperaturas e hidroquímicas a lo largo de las fallas: las aguas termales en el norte de Balçova son agua dulces meteóricas calientes, mientras que los manantiales calientes de la región del sur de Seferihisar tienen una fuerte contribución de agua de mar. Las simulaciones numéricas previas del flujo del fluido y el transporte de calor indicaban que la surgencia focalizada de agua caliente en las fallas inducen un movimiento de flujo convectivo en las unidades circundantes. El transporte de sales se acopla totalmente al flujo térmicamente forzado para estudiar si las celdas de convección inducidas por las fallas podrían ser responsables de la invasión de agua de mar en el SBG. Se presentan los datos de isótopos para respaldar los hallazgos numéricos. Los resultados muestran que las celdas de convección inducidas por las fallas generan plumas de agua de mar que se extienden desde el fondo del mar hacia las fallas. En las intersecciones de las fallas, el agua de mar se mezcla con las aguas termales calientes surgentes. Los flujos salinos resultantes ascienden a la superficie a lo largo de la fallas, forzados por las fuerzas de flotación. En Balçova el grueso aluvio, las fallas menores y el flujo regional impiden que el agua salada ascendente se propague en la superficie, mientras que el flujo débil de la recarga en el aluvio delgado del sur del SBG no es suficiente para barrer el flujo ascendente de las aguas saladas. Estos mecanismos podrían desarrollarse en cualquier sistema geotermal fallado, con implicancias para la migración de minerales y de energía en las cuencas sedimentarias.摘要土耳其Seferihisar-Balçova(SBG)地热系统表现为沿着断裂的温度和水化学异常:北部Balçova地区的热水来源于大气降水,南部Seferihisar地区热水海水来源较多。前期水流场和热传导的数值模拟表明断裂里热水的集中涌出导致邻近单元里出现对流。盐分运移模拟耦合了热导致的水流,用于研究SBG地热系统中断层导致的对流是否产生了海水入侵。同位素数据用于为模拟结果提供依据。结果表明断裂导致的对流产生了海水晕,且其从海底直接延伸至断裂处。在断裂交汇处,海水与上升的热水发生混合。在浮力作用下,形成的咸水沿着断裂上升至地表。在Balçova地区较厚的冲积层处,基本上没有断裂和区域水流阻碍上升的咸水扩展到地表,而南部SBG地热系统中薄层冲积层里的补给较差的水流不足以冲散上升的咸水。这些过程在任何有断裂的地热系统中都能产生,也能应用于沉积盆地中矿物和能源的运移研究。RiassuntoLe falde acquifere del sistema geotermico del Seferihisar-Balçova (SBG), Turchia, sono caratterizzate da anomalie termiche e chimiche lungo le faglie: le acque termali a Nord di Balcova sono acque meteoriche riscaldate mentre le sorgenti di acqua calda nella regione meridionale di Seferihisar hanno salinità molto elevate. Simulazioni numeriche di flussi idrici e di calore hanno indicato che la fuoriuscita di acque termali focalizzata nelle faglie induce una circolazione convettiva negli acquiferi circostanti. Il trasporto di sale viene ora accoppiato ai flussi idrici indotti termicamente per studiare se questi flussi convettivi possano essere responsabili delle intrusioni marine nel SBG. Dati isotopici vengono presentati a sostegno dei risultati numerici. Questi ultimi mostrano che le cellule convettive indotte dalle faglie generano piume di acqua marina che si estendono dal fondo marino verso le faglie stesse. All’intersezione delle faglie, l’acqua marina si mescola con le acque termali ascendenti. I fluidi salini risultanti da questa miscela raggiungono la superficie lungo le faglie grazie alle forze di galleggiamento. In Balcova, lo spesso strato di sedimenti alluvionali, le faglie minori e il flusso idrico regionale impediscono alle acque termali ascendenti di diffondersi mentre le deboli ricariche idriche nel sottile deposito alluvionale nel SBG meridionale non sono sufficienti a diluire le acque termali ascendenti. Questi meccanismi possono svilupparsi in qualsiasi sistema geotermico con faglie, il che ha implicazioni per il trasporto di minerali ed energia in bacini sedimentari.ResumoO sistema Geotérmico de Seferihisar-Balçova (SBG), na Turquia, é caracterizado por anomalias hidroquímicas e de temperatura ao longo das falhas: as águas termais no norte de Balçova são águas quentes de origem meteórica, ao passo que as fontes termais da região sul de Seferihisar têm uma forte contribuição de água do mar. Anteriores simulações numéricas de fluxo de fluidos e transporte de calor indicaram que a ascenção localizada de água quente nas falhas induz um movimento de fluxo de tipo convectivo nas unidades vizinhas. O transporte do sal foi totalmente associado ao fluxo termo-induzido para estudar de que modo as células de convecção induzidas pelas falhas poderiam ser responsáveis pela invasão da água do mar no SBG. São apresentados dados isotópicos para apoiar as constatações numéricas. Os resultados mostram que as células de convecção induzidas pelas falhas geraram plumas de água salgada que se estendem desde o fundo do mar em direção às falhas. Nos cruzamentos de falhas, a água do mar mistura-se com as águas termais quentes ascendentes. Os fluidos salinos resultantes ascendem até à superfície ao longo das falhas, actuados por forças de impulsão. Em Balçova, as aluviões espessas, as falhas menores e o fluxo regional evitam que a água salgada ascendente se espalhe até à superfície, enquanto que o fraco fluxo de recarga na fina camada de aluvião no sul do SBG não é suficiente para repelir as águas salgadas quentes ascendentes. Estes mecanismos podem desenvolver-se em qualquer sistema de falhas geotérmicas, com implicações na migração de minerais e energia em bacias sedimentares.ÖZTürkiye’nin batısındaki Seferihisar-Balçova Jeotermal Sistemi (SBJS), faylar boyunca oluşan sıcaklık ve hidrokimyasal anomalilerle tanımlanır. Seferihisar bölgesinde sıcak su kaynaklarında güçlü bir deniz suyu katkısı varken Balçova’da sıcak sular, ısınmış meteorik tatlı sulardan meydana gelmektedir. Önceki akışkan akımı ve ısı taşınım sayısal modellemeleri, faylardaki sıcak sulardaki yükselmenin çevre birimlerde konvektif akım benzeri harekete neden olduğunu belirtmektedir. SBJS’de faylarla kontrol edilen konveksiyon hücrelerinin, deniz suyu girişiminden sorumlu olup olmadığını araştırmak için tuz taşınımı tamamen sıcak su kontrollü akımla birleştirilmiştir. İzotop verileri de, sayısal sonuçları desteklemek için sunulmuştur. Sonuçlar, fay kontrollü konveksiyon hücrelerinin, deniztabanından faylara doğru uzanan denizsuyu girişimini yarattığını göstermiştir. Fay arayüzeylerinde deniz suları, yükselen sıcak sular ile karışmaktadır. Sonuçta oluşan tuzlu akışkanlar, sıcaklık farkı ile oluşan kuvvetlerle faylar boyunca yükselmektedir. Seferihisar’da ince alüvyondaki zayıf akımlar yükselen sıcak tuzlu sular üzerinde etkisiz kalırken, Balçova’da kalın alüvyon, faylar ve bölgesel akım tuzlu suyun yükselerek yüzeyden çıkmasına engel olmaktadır. Bu mekanizmalar sedimanter havzalardaki mineral ve enerji göçünün oluştuğu herhangi bir faylı jeotermal sistemde gelişebilir.

Thermal convection of temperature‐dependent viscous fluids within three‐dimensional faulted geothermal systems: Estimation from linear and numerical analyses

Linear stability analysis and numerical simulations of density-driven flow are presented in order to estimate the effects of temperature-dependent fluid viscosity variation on the onset of free thermal convection within a three-dimensional fault embedded into impermeable rocks. The strongly coupled equations of density-driven flow are linearized. The solution was obtained through expansion into Fourier series. Simple polynomial expressions fitting the neutral stability curves are given for a range of fault aspect ratios, fluid viscosity properties, and thermal conductivity heterogeneity, providing a new tool for the estimation of critical Rayleigh numbers in faulted systems. The results are validated against the limiting case of temperature-invariant viscosity (i.e., constant). 3-D numerical simulations of free convection within a fault are run using the finite element technique in order to verify the theoretical results. It turned out that at average geothermal temperature conditions, thermal convection can develop within faults which permeability is up to 4 times lower than the case of a fluid with constant viscosity, in agreement with the developed linear theory. The polynomial expressions of this study can be applied to any numerical model for testing the feasibility of fault convection in 3-D geothermal basin.

Joint Research Project Brine: Carbon Dioxide Storage in Eastern Brandenburg: Implications for Synergetic Geothermal Heat Recovery and Conceptualization of an Early Warning System Against Freshwater Salinization

Brine was a scientific joint-project implemented to accompany a prospective CO2 storage site in Eastern Brandenburg, Germany. In this context, we investigated if pore pressure elevation in a CO2 storage reservoir can result in shallow freshwater salinization involving the conceptual design of a geophysical early warning system. Furthermore, assessments of a potential synergetic geothermal heat recovery from the CO2 storage reservoir and hydro-mechanical integrity were carried out. The project results demonstrate that potential freshwater salinization is strongly depending on the presence and characteristics of geological weakness zones. The integrated geophysical early warning system allows for reliable monitoring of these potential leakage pathways at different spatial and time scales.

Modeling Highly Buoyant Flows in the Castel Giorgio: Torre Alfina Deep Geothermal Reservoir

The Castel Giorgio-Torre Alfina (CG-TA, central Italy) is a geothermal reservoir whose fluids are hosted in a carbonate formation at temperatures ranging between 120°C and 210°C. Data from deep wells suggest the existence of convective flow. We present the 3D numerical model of the CG-TA to simulate the undisturbed natural geothermal field and investigate the impacts of the exploitation process. The open source finite-element code OpenGeoSys is applied to solve the coupled systems of partial differential equations. The commercial software FEFLOW® is also used as additional numerical constraint. Calculated pressure and temperature have been calibrated against data from geothermal wells. The flow field displays multicellular convective patterns that cover the entire geothermal reservoir. The resulting thermal plumes protrude vertically over 3 km at Darcy velocity of about  m/s. The analysis of the exploitation process demonstrated the sustainability of a geothermal doublet for the development of a 5 MW pilot plant. The buoyant circulation within the geothermal system allows the reservoir to sustain a 50-year production at a flow rate of 1050 t/h. The distance of 2 km, between the production and reinjection wells, is sufficient to prevent any thermal breakthrough within the estimated operational lifetime. OGS and FELFOW results are qualitatively very similar with differences in peak velocities and temperatures. The case study provides valuable guidelines for future exploitation of the CG-TA deep geothermal reservoir.

Assessing anthropogenic impacts on limited water resources under semi-arid conditions: three-dimensional transient regional modelling in Jordan

Both increasing aridity and population growth strongly stress freshwater resources in semi-arid areas such as Jordan. The country’s second largest governorate, Irbid, with over 1 million inhabitants, is already suffering from an annual water deficit of 25 million cubic meters (MCM). The population is expected to double within the next 20 years. Even without the large number of refugees from Syria, the deficit will likely increase to more then 50 MCM per year by 2035 The Governorate’s exclusive resource is groundwater, abstracted by the extensive Al Arab and Kufr Asad well fields. This study presents the first three-dimensional transient regional groundwater flow model of the entire Wadi al Arab to answer important questions regarding the dynamic quality and availability of water within the catchment. Emphasis is given to the calculation and validation of the dynamic groundwater recharge, derived from a multi-proxy approach, including (1) a hydrological model covering a 30-years dataset, (2) groundwater level measurements and (3) information about springs. The model enables evaluation of the impact of abstraction on the flow regime and the groundwater budget of the resource. Sensitivity analyses of controlling parameters indicate that intense abstraction in the southern part of the Wadi al Arab system can result in critical water-level drops of 10 m at a distance of 16 km from the production wells. Moreover, modelling results suggest that observed head fluctuations are strongly controlled by anthropogenic abstraction rather than variable recharge rates due to climate changes.RésuméTant l’augmentation de l’aridité que la croissance de la population posent un problème important concernant les ressources en eau douce dans des zones semi-arides, telle que la Jordanie. Le deuxième plus grand gouvernorat du pays, Irbid, avec plus d’un million d’habitants, souffre déjà d’un déficit hydrique annuel de 25 millions de mètres cubes (MCM). La population devrait doubler dans les 20 prochaines années. Même sans le grand nombre de réfugiés de Syrie, le déficit augmentera vraisemblablement à plus de 50 MCM par année d’ici 2035. La ressource exclusive du Gouvernorat est l’eau souterraine, exploitée à partir de vastes champs captants d’Al Arab et de Kufr Asad. Cette étude présente le premier modèle d’écoulement régional tridimensionnel en régime transitoire de l’ensemble du Wadi al Arab pour répondre aux importantes questions concernant la qualité dynamique et la disponibilité de l’eau au sein du bassin. L’accent est mis sur le calcul et la validation de la recharge dynamique des eaux souterraines, dérivés d’une approche multi-proxy, comprenant (1) un modèle hydrologique concernant un ensemble de données de 30 ans, (2) des mesures de niveaux d’eau souterraine et (3) des informations sur les sources. Le modèle permet d’évaluer l’impact des prélèvements sur le régime d’écoulement et sur le bilan des ressources en eau souterraine. Les analyses de sensibilité des paramètres de contrôle indiquent que des prélèvements intenses dans la partie sud du système du Wadi al Arab peut entraîner des diminutions sévères du niveau d’eau de 10 m à une distance de 16km des puits de production. En outre, les résultats du modèle suggèrent que les fluctuations de la charge hydraulique sont fortement contrôlées par les prélèvements anthropiques plutôt que par des taux de recharge variables associées au changement climatique.ResumenTanto la creciente aridez como el crecimiento demográfico hacen énfasis en los recursos de agua dulce en las zonas semiáridas, como Jordania. La segunda mayor gobernación del país, Irbid, con más de 1 millón de habitantes, ya sufre un déficit hídrico anual de 25 millones de metros cúbicos (MCM). Se espera que la población se duplique en los próximos 20 años. Incluso sin el gran número de refugiados de Siria, el déficit probablemente aumentará a más de 50 MCM por año para 2035. El recurso exclusivo de la gobernación es el agua subterránea, que se extrae en los extensos campos de pozos de Al Arab y Kufr Asad. Este estudio presenta el primer modelo tridimensional transitorio de flujo de agua subterránea de todo el Wadi al Arab para responder a preguntas importantes sobre la dinámica, calidad y la disponibilidad de agua dentro de la cuenca. Se hace hincapié en el cálculo y validación de la dinámica de la recarga del agua subterránea, derivada de un enfoque multi-proxy, que incluye: (1) un modelo hidrológico que cubre un conjunto de datos de 30 años; (2) medidas de nivel de agua subterránea y (3) información acerca de los manantiales. El modelo permite evaluar el impacto de la extracción sobre el régimen de flujo y el balance del agua subterránea del recurso. Los análisis de sensibilidad de los parámetros de control indican que la intensa extracción en la parte sur del sistema de Wadi al Arab puede resultar en descensos de nivel de agua de 10 m a una distancia de 16 km de los pozos de producción. Por otra parte, los resultados del modelado sugieren que las fluctuaciones observadas en la carga hidráulica están fuertemente controladas por la extracción antropogénica más que por las tasas variables de recarga debido a los cambios climáticos.摘要日益增加的干旱和人口增长使半干旱地区的淡水资源倍感紧张,诸如约旦。该国的第二大省伊尔比德人口超过100万,早已经受着每年缺水2500万立方水的痛苦。预计未来20年内人口是现在的两倍。即使没有大量来自叙利亚的难民,到2030年每年的水短缺将超过5000万立方。省内的唯一资源是地下水,被广阔的Al Arab 和 Kufr Asad井场抽取。本研究提出了整个Al Arab 和 Kufr Asad地区第一个三维瞬时区域地下水流模型,以回答有关流域内水的动态水质及可用量的重要问题。强调了根据多代理方法得到的动态地下水补给量的计算和验证结果,多代理方法包括:(1)包含30年数据集的水文模型;(2)地下水位测量及(3)有关泉的信息。模型能够评价抽水对水流场及资源的地下水平衡的影响。控制参数的灵敏度分析表明,干谷al Arab系统南部的强烈抽水可导致距抽水井16公里的地方水位下降10米。此外,模拟结果显示,观测到的水头波动受到人为抽水的强烈控制,而不是受到由于气候变化导致的变化着的补给量的控制。ResumoTanto a crescente aridez quanto o crescimento populacional estressam fortemente os recursos de água doce em áreas semiáridas, como a Jordânia. A segunda maior província do país, Irbid, com mais de 1 milhão de habitantes, já sofre com um déficit hídrico anual de 25 milhões de metros cúbicos (MMC). Espera-se que a população dobre nos próximos 20 anos. Mesmo sem o grande número de refugiados da Síria, o déficit provavelmente aumentará para mais de 50 MMC por ano até 2035. O recurso exclusivo da província é agua subterrânea, abstraída pelos extensivos campos de poços Al Arab e Kufr Asad. Esse estudo apresenta o primeiro modelo transiente regional tridimensional de fluxo das águas subterrâneas de todo o Wadi al Arab para responder importantes questões a respeito da qualidade dinâmica e disponibilidade de água dentro da bacia hidrográfica. A ênfase é dada ao cálculo e validação da recarga dinâmica das águas subterrâneas, derivada de uma abordagem multi-proxy, incluindo (1) um modelo hidrológico incluindo um conjunto de dados de 30 anos, (2) medições de nível das águas subterrâneas e (3) informações sobre nascentes. O modelo permite a avaliação do impacto da abstração no regime de fluxo e no balanço de águas subterrâneas do manancial. A análise de sensibilidade de parâmetros controlados indica que a extração intensa na parte sul do sistema Wadi al Arab pode resultar em quedas críticas do nível freático de 10 m a uma distância de 16 km dos poços de produção. Além disso, os resultados da modelagem sugerem que as flutuações das cargas observadas são fortemente controladas pela abstração antropogênica ao invés de taxas de recarga variáveis devido a alterações climáticas.

Coupled THM-Processes

Hydro-Mechanical (HM) and transient Thermo-Hydro-Mechanical (THM) simulations in a faulted aquifer are presented. Both 2D and 3D scenarios are illustrated.

HT (Convection) Processes

In a geothermal system, unstable fluid density profiles due to temperature variations can trigger the onset and development of free thermal convective processes (J.W. Elder. Transient convection in a porous medium. (Elder in J Fluid Mech 27: 609–623 , 1967, Elder 1967). Early studies on the problem showed that the development of free thermal convection in the Earth’s crust require a relatively high permeability of the porous rocks (Lapwood in Math Proc Camb Philos Soc, 44:508–52, 1948, Lapwood 1948). Since the permeability inside the damaged area of major fault zones can far exceed the permeability of the enclosing rocks (Wallace, Morris in PAGEOPH, 124:107–125, 1986, Wallace and Morris 1986), one can expect the development of free thermal convective instabilities to occur in such tectonically perturbed rocks. The onset of thermal convection of a single-phase fluid in a vertical fault enclosed in impermeable rocks was considered in a full 3D approximation by Wang, Kassoy, Weidman (Int J Heat Mass Trans, 30:1331–1341, 1987, Wang et al. (1987)). A fundamental result of those investigations was that highly permeable faults allow for onset of free thermal convection even under a normal (e.g. 30 \(^{\circ }\mathrm{C}\cdot \mathrm{km}^{-1}\)) geothermal gradient. In contrast to simple homogenous 1D and 2D systems, no appropriate analytical solutions can be derived to test numerical models for more complex 3D systems that account for variable fluid density and viscosity as well as permeability heterogeneity (e.g. presence of faults). Owing to the efficacy of thermal convection for the transport of thermal energy and dissolved minerals in the moving fluid, a benchmark case study for density/viscosity driven flow is crucial to ensure that the applied numerical model accurately simulates the physical processes.

Coupled 2D and 3D Numerical Simulations to Quantify Regional-scale Observations of Heat and Salt Distributions Around the Lake Tiberias, Israel

2D and 3D numerical simulations of thermally driven flow show that the interaction between free convection and topographically driven regional flow is responsible for the upsurge of deep-seated hot salty waters along the shores of Lake Tiberias and the high temperature gradient of 46 °C/km in the Lower Yarmouk Gorge (LYG). Buoyant flow develops in permeable faults which hydraulic conductivity is estimated to vary between 30 m/yr and 140 m/yr. The thermohaline simulations support the conclusions derived from hydrochemical investigations that the derivatives of relic seawater brines are the major source of salinity. Deep brines leaching salt diapirs are too heavy to be transported toward the surface by thermally-induced buoyancy forces. However, the presence of local shallower salt bodies below the lake may potentially contribute to the salinity of the western spring and well waters, though in very small amounts. 3D simulations reveal additional aspects of mixed convective patterns that 2D simulations could not account for. In 3D, the flow patterns are more complex, allowing buoyant flow to develop not only in the fault plane but also perpendicular to it. As a result, the location of discharge areas are not restricted to a single fault, in agreement with the observed springs. 3D convective cells are much wider than those inferred from 2D simulations and can reach basement depths. In this regard, further geophysical and geochemical studies are needed to better characterize faults, so far modeled either as equivalent porous media (this case) or as planar surface features. Besides being of importance for understanding the hydrogeological processes that salinize Lake Tiberias, the presented simulations provide a scenario illustrating large-scale fluid patterns due to buoyancy in a faulted basin.