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Characterizing and monitoring the highly variable near-surface underneath urban areas is crucial for understanding phenomena such as resource distribution and storage potential, changes in hydrological systems, and seismic hazard at the sub-kilometer scale. Our current understanding of the urban subsurface is limited by our ability to image its structure and temporal variations at high resolution using classical geophysical approaches. Distributed Acoustic Sensing (DAS) deployed on unused telecommunication fiber-optic cables (dark fibers) is an attractive alternative to classic seismological studies in urban areas, as it enables continuous recording of the seismic wavefield across long distances (10’s of km) at high spatial (~1 m) and temporal (down to seconds) resolution across a broad frequency range (mHz to kHz). Here, we explore the potential and challenges of using DAS deployed on dark fiber networks for characterizing and monitoring the near-surface beneath urban areas. We will discuss dark fiber experiments conducted in a variety of settings, including several km-long profiles crossing agricultural areas and densely populated centers in California, and experiments conducted within the city of Potsdam (Germany). We evaluate the usability of anthropogenic signals as passive sources for near-surface imaging and monitoring, as well as the potential for active source seismic imaging in complex noise environments. Challenges associated with poorly documented deployment and coupling conditions of dark fiber networks and the large data volumes generated in these studies will also be discussed. Ultimately, we aim at providing insights into the challenges and opportunities of using dark fiber DAS for understanding the urban subsurface., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Reliable well completion and monitoring technologies are a prerequisite to achieve safe, sustainable and efficient use of geothermal reservoirs. Since borehole completion is a high-risk process, we tested how distributed fiber optic sensing technologies can aid monitor the completion process continuously in space and real time. To evaluate the success of the operation conventional well logging tools and other methods can only be used after the completion and show snapshots of discrete points in time.We present results from a case study in Berlin, where a 450 m deep exploration well was completed for Aquifer Thermal Energy Storage (ATES) purpose. To simultaneously measure distributed temperature (DTS) and distributed elastic and static strain (DAS and DSS) a fiber optic cable was installed behind the production tubing. The combined evaluation of the different fiber optic technologies evidences the potential to observe previously untraceable processes, such as changes in subsurface flow paths due to clogging. Further, the unwanted process of caving can be identified and the rise of gravel or cement observed. This allows to react on site, and subsurface risks will be considerably mitigated., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Fiber optic sensing has gained importance for wellbore monitoring and reservoir characterization in geothermal fields as it allows continuous, spatially highly resolved measurements. Distributed acoustic sensing (DAS) and distributed temperature sensing (DTS) technologies, among others, enable monitoring of flow regimes and heat transport inside the wellbore to describe the dynamical behavior of the reservoir. The technically challenging installation of a permanent fiber optic monitoring system in a geothermal production well over the entire wellbore length was conducted for the first time at the geothermal site Schäftlarnstraße in Munich, Germany. One cable with two DAS fibers, two DTS fibers, and one fiber for a downhole fiber optic pressure/temperature gauge were clamped to ¾-in. sucker rods and installed to 3.7 km measured depth to collect data from the wellbore after drilling, during testing, and during operations. We present DTS profiles during 3 months of well shut-in and show the results of two cold water injection tests conducted to localize inflow zones in the reservoir and to test the performance of the fiber optic setup. A vertical displacement in temperature peaks of approximately 1.5 m was observed during the injection tests, presumably resulting from thermal contraction of the sucker rod–cable setup. This was verified by analyzing the strain information from the DAS records over 1 h of warm-back after cold water injection with the calculated theoretical thermal contraction of DTS of the same period. We further verified the flowmeter measurements with a gradient velocity analysis of DTS profiles during injection. Intake to the major inflow zone was estimated to 93.5% for the first injection test, respective 94.0% for the second, intake of flowmeter was calculated to 92.0% for the same zone. Those values are confirmed by analyzing DTS profiles during the warm-back period after the well was shut.

Für eine erfolgreiche Umsetzung der Wärmewende sind unter anderem kom- munale Energieversorger in der Verant- wortung. Deutschlands größte und 2021 in Betrieb genommene Geothermiean- lage »Schäftlarnstraße« der Stadtwerke München (SWM) mit drei geothermalen Dubletten in München-Sendling soll dazu beitragen, bis 2035 eine Klimaneutralität zu erreichen und den Bedarf an Wärme in München CO2-neutral zu decken. Zur strukturellen Überwachung der Bohrloch- konstruktion und zur Unterstützung einer sicheren und nachhaltigen Bewirtschaf- tung der Wärmequelle wurde bereits im Jahr 2019 ein Glasfaserkabel in einer der Produktionsbohrungen bis Endteufe eingebracht.

Casing failures in-high temperature geothermal wells occur if the casing is overstressed. The most common failures occur because of axial forces and/or differential pressures. For medium-enthalpy geothermal wells, thermal cycling has the potential to severely deteriorate the integrity of the cemented annulus and for higher enthalpy wells yielding of casings becomes a structural concern. The recently innovated patent solution, flexible couplings, aims to reduce thermal straining by allowing displacement into the connections of each casing segment at approximately 12 m intervals (API R3). In the GEOTHERMICA internationally co-funded GeConnect project a full-scale experiment is performed at a well site in a high-temperature geothermal field. The experimental set-up is composed of two concentric casings cemented together and the inner one equipped with a flexible coupling. The aim is to investigate effects of thermal cycling on well integrity and to test the function of cemented-in flexible couplings using a bypass of a producing well. Cement sheath integrity and cement-casing boundary are evaluated at moderate to high temperatures, e.g. by using fiber optic distributed sensing technology. Structural models are used to analyze casing-cement interactions and to evaluate prospects and potential improvements of well integrity by using the new technology of flexible couplings.

The fiber optic cable was installed down to 832 m behind the production casing of a 9 5/8" (445-2932 m) and 9 7/8" (0 - 445 m) production casing in well RN-15/DEEPEGS/IDDP-2 in the Reykjanes geothermal field, SW Iceland (depth reference: surface). Fiber optic distributed temperature data was acquired (campaign based) during cementation (09/2016) of the production casing, at the end of the cold fluid injection (09/2018) as well during the onset of well stimulation (10/2019-04/2020).

The integrity of casing and cement is of utmost importance in order to increase the lifecycle and to improve safe operations of geothermal wells. This contribution focuses on the potential of real-time downhole monitoring techniques along fiber optic cables which are permanently installed behind casing. Distributed fiber optic temperature and strain sensing technology are used to measure thermal as well as load signatures during the completion of a low-enthalpy well for geothermal energy storage (Gt BChb1/2015, ATES Fasanenstrasse, Berlin, Germany). Gravel and cement pumping was monitored with distributed strain sensing. The pumping of gravel leads to a density change in the annulus which results in a measureable strain reading on the fiber optic cable. A simultaneous measurement with a gamma-gamma density log shows that the strain data from the fiber indicates the position of the gravel head in the annulus. In addition, a delayed consolidation of the gravel packing was monitored with the fiber. During cement pumping, it was observed that fluid shear stresses generate a measureable strain on the cable. The magnitude of these forces can be used to estimate rheological parameters such as fluid density and viscosity of the pumped medium. An experimental study was conducted to validate the field observations. Using distributed strain sensing, we can extract relevant downhole information (such as fluid/material changes) in real-time without interfering with the operational schedule of a well.

Die Erfindung betrifft bevorzugt in einem ersten Aspekt ein Verfahren zur Messung und/oder Überwachung der Viskosität eines in einem Kanal fließenden Materials, wobei auf Basis einer ortverteilten Dehnungsmessung entlang eines Messkabels in dem Kanal mindestens abschnittsweise ein Dehnungsgradienten bestimmt wird mit dessen Hilfe die Messung und/oder Überwachung der Viskosität erfolgt. In einem weiteren Aspekt betrifft die Erfindung ein System zur bevorzugten Durchführung des Verfahrens umfassend ein dehnungssensitives Messkabel, eine Messeinheit sowie eine Datenverarbeitungsvorrichtung.

In January 2020, a swarm of earthquakes started under Thorbjorn volcano, Reykjanes, SW Iceland, associated to the uplift of up to 0.5 cm per day. Concern in Iceland is growing and the Iceland Meteorological Office suggested that possibly magma was intruded in the crust at shallow depth (3 to 9 km). The GFZ is starting a seismological Hazard and Risk Team (HART) in cooperation with IMO, ISOR and the University of Iceland. This dataset comprise a selection of waveforms recorded along a fibre length of 21 km. The interrogator is located in Grindavik and is connected to a standard telecom cable. Waveform data are available from the GEOFON data centre, under network code 5J.

In a first aspect, a method for monitoring a material and/or a device in a borehole using a fiber optic measurement cable is provided, and, in a second aspect, a system for carrying out the method is provided. The system includes a fiber optic measurement cable which is present introduced into a borehole, wherein a material and a device are present in the borehole

Common failure mechanisms in high-temperature geothermal wells are casing collapse and tensile joint rupture. For medium- to high-enthalpy geothermal wells, thermal cycling has the potential to severely deteriorate the integrity of the cemented annulus. Additionally, for higher enthalpy wells, yielding of casings becomes a structural concern. A recently innovated patented solution, flexible couplings, aim to reduce thermal strains by allowing displacement from thermally expanding casing segments at ~12 m intervals (API Range 3 casings). In the GeConnect project, a full-scale surface experiment composed of a 9⅝′′ casing equipped with a flexible coupling cemented into a 13⅜′′ casing is constructed and installed on a wellpad of an existing high-temperature geothermal well in Iceland. The aim is to investigate effects of thermal cycling on well integrity and to test the function of cemented-in flexible couplings. Along with testing the flexible coupling, cement sheath integrity and the cement-casing boundary behavior will be evaluated atmoderate to high temperatures. Structural models are used to analyze casing-cement interactions and to evaluate prospects and potential improvements of well integrity by using flexible couplings on experimental up to field scale.