Giorgia Dalla Santa

Induced thermal compaction in cohesive sediments around a borehole heat exchanger: laboratory tests on the effect of pore water salinity

In closed-loop geothermal systems, the operation of a Borehole Heat Exchanger (BHE) affects the thermal equilibrium of the surrounding subsoil. This study was part of a large laboratory program undertaken to understand how the cyclic thermal stress induced by a BHE in the subsoil can change sediments’ mechanical properties, if the BHE works in extreme running conditions which induce both freeze–thaw cycles and heating processes in the ground. This paper presents the results from an experimental laboratory investigation on the effects of different pore water salinity concentrations on the vertical strain induced in a thermally loaded silty clay. The considered study case is the historical center of Venice (Italy), which provides an example of typical conditions of densely urbanized areas inserted in coastal or transitional environments, with abundance of cohesive layers and brackish conditions in the subsoil. Results show that the irreversible compaction effect induced on sediments increases with higher salinity concentration, despite the fact that increasing salt content lowers the sediment freezing point, thereby protecting the soil from freezing processes.

Soil thermal conductivity from early TRT logs using an active hybrid optic fibre system

At the Molinella test site in Italy, a monitoring well has been equipped with a hybrid active optical fibre cable down to the depth of 100 m sealed with geothermal grouting. The cored borehole provided a full stratigraphic sequence of unconsolidated alluvial deposits. The cored material has been classified from a geotechnical point of view, and the thermal parameters of the most relevant lithologies have been directly measured. Active heating of the optical fibre cable has been provided by a constant heating power injected through copper wires contained within the cable structure. This way, not only the equivalent thermal conductivity of the entire stratigraphy but also the thermal conductivity at a spatial resolution of 1 m have been obtained. For each investigated layer, the thermal conductivity obtained from the distributed temperature measurements and the ones acquired using direct measurements are interpreted and compared.

Modelling an induced thermal plume with data from electrical resistivity tomography and distributed temperature sensing: a case study in northeast Italy

Groundwater tracer tests are often used to improve aquifer characterization, but they present several disadvantages, such as the need to pour solutions or dyes into the aquifer system and alteration of the water’s chemical properties. Thus, tracers can affect the groundwater flow mechanics and data interpretation becomes more complex, hindering effective study of ground heat pumps for low enthalpy geothermal systems. This paper presents a preliminary methodology based on a multidisciplinary application of heat as a tracer for defining the main parameters of shallow aquifers. The field monitoring techniques electrical resistivity tomography (ERT) and distributed temperature sensing (DTS) are noninvasive and were applied to a shallow-aquifer test site in northeast Italy. The combination of these measurement techniques supports the definition of the main aquifer parameters and therefore the construction of a reliable conceptual model, which is then described through the numerical code FEFLOW. This model is calibrated with DTS and validated by ERT outcomes. The reliability of the numerical model in terms of fate and transport is thereby enhanced, leading to the potential for better environmental management and protection of groundwater resources through more cost-effective solutions.RésuméLes essais de traçage des eaux souterraines sont souvent utilisés pour améliorer la caractérisation d’un aquifère, mais ils présentent plusieurs inconvénients, tels que la nécessité d’injecter des solutions ou colorants dans l’aquifère et l’altération des propriétés chimiques de l’eau. Ainsi, les traceurs peuvent affecter les écoulements d’eau souterraine, rendant l’interprétation des données plus complexe, entravant l’étude des pompages à chaleur pour les systèmes géothermaux de basse enthalpie. Cet article présente une méthode préliminaire basée sur une application pluri-disciplinaire de la chaleur comme traceur pour définir les principaux paramètres d’aquifères superficiels. Les mesures de tomographie électrique (ERT) et la détection des températures distribuées (DTD) sont non invasives et ont été appliquées sur un site test d’aquifère peu profond dans le Nord Est de l’Italie. La combinaison de ces techniques de mesure satisfait la définition des principaux paramètres de l’aquifère et donc l’élaboration d’un modèle conceptuel fiable, qui est ensuite décrit à l’aide du code numérique FEFLOW. Ce modèle est paramétré avec DTD et validé par les sorties ERT. La fiabilité du modèle numérique en termes du devenir et du transport est par là renforcée, pouvant conduire à une meilleure gestion de l’environnement et à une meilleure protection des ressources en eau souterraine à l’aide de solutions d’un meilleur rapport coût-efficacité.ResumenLos ensayos con trazadores en agua subterránea se utilizan a menudo para mejorar la caracterización del acuífero, pero presentan varias desventajas, como la necesidad de verter soluciones o colorantes en el sistema acuífero y la alteración de las propiedades químicas del agua. Por lo tanto, los trazadores pueden afectar la mecánica del flujo del agua subterránea y la interpretación de los datos se torna más compleja, lo que dificulta el estudio efectivo de las bombas de calor terrestre para los sistemas geotérmicos de baja entalpía. Este artículo presenta una metodología preliminar basada en una aplicación multidisciplinaria del calor como trazador para definir los principales parámetros de acuíferos poco profundos. Las técnicas de monitoreo de campo, la tomografía de resistividad eléctrica (ERT) y la detección de temperatura distribuida (DTS) no son invasivas y se aplicaron a un sitio de prueba en un acuífero somero en el noreste de Italia. La combinación de estas técnicas de medición respalda la definición de los principales parámetros del acuífero y, por lo tanto, la construcción de un modelo conceptual confiable, que luego se describe a través del código numérico FEFLOW. Este modelo está calibrado con DTS y validado por los resultados de ERT. Por lo tanto, se mejora la confiabilidad del modelo numérico en términos de destino y transporte, lo que permite una mejor gestión ambiental y protección de los recursos de aguas subterráneas a través de soluciones más rentables.摘要常常采用地下水示踪试验提高含水层特征描述水平,但是其也显示出几个缺点,诸如需要把溶剂或者燃料灌入含水层系统,改变水的化学特性。因此,示踪剂可影响地下水流机理,数据解译变得更加复杂,阻碍了低热焓地热系统地下水热泵的有效研究。本文展示了基于多学科应用热量作为示踪剂确定浅层含水层主要参数的初步方法。野外监测技术如电阻断层摄影和分布式温度传感技术为非侵入式的,应用于意大利北部一个浅层含水层中。这些测量技术结合一起可以支持主要含水层参数的定义,因此,进而支持建立一个可靠的概念模型,然后通过数值编码FEFLOW对模型进行描述。这个模型利用分布式温度传感技术进行校正,利用电阻断层摄影技术进行验证。因此在热羽和传输方面,提高了数值模型的可靠性,就有可能通过更加合算的解决方法对地下水资源进行更好地进行环境管理和保护。ResumoOs testes de rastreamento de águas subterrâneas são frequentemente utilizados para melhorar a caracterização do aquífero, mas apresentam várias desvantagens, como a necessidade de derramar soluções ou corantes no sistema aquífero e a alteração das propriedades químicas da água. Assim, os traçadores podem afetar a mecânica do fluxo de águas subterrâneas e a interpretação dos dados torna-se mais complexa, dificultando o estudo efetivo de bombas de calor terrestres para sistemas geotérmicos de baixa entalpia. Este artigo apresenta uma metodologia preliminar baseada em uma aplicação multidisciplinar de calor como um traçador para a definição dos principais parâmetros dos aquíferos rasos. As técnicas de monitoramento de campo, tomografia de resistividade elétrica (TRE) e detecção de temperatura distribuída (DTD), não são invasivas e foram aplicadas em um local de teste de um aquífero raso no nordeste da Itália. A combinação dessas técnicas de medição sustenta a definição dos principais parâmetros do aquífero e, portanto, a construção de um modelo conceitual confiável, que então é descrito através do código numérico FEFLOW. Este modelo é calibrado com DTD e validado pelos resultados de TRE. A confiabilidade do modelo numérico em termos de destino e transporte é, assim, aprimorada, conduzindo ao potencial para uma melhor gestão ambiental e proteção dos recursos de águas subterrâneas através de soluções mais viáveis economicamente.