Thomas Reinsch

Thermal, mechanical and chemical influences on the performance of optical fibres for distributed temperature sensing in a hot geothermal well

Structural well-bore integrity is an important issue for sustainable provision of geothermal energy. Raman scattering based fibre optic distributed temperature sensing (DTS) can help to monitor the status of a well and therefore help to optimize expensive work-over activities. This study reports on the installation of a fibre-optic cable in the cemented annulus behind the anchor casing in the high temperature geothermal well HE-53, Hellisheiði geothermal field, SW Iceland. Although the cable has been damaged during the installation, temperature data could be acquired during the entire length of installation down to 261.3xa0m. Temperature measurements were performed during the installation in spring 2009, during the onset of a flow test in summer 2009 and after a 8.5 month shut-in period in summer 2010. During the flow test, maximum temperatures of 230xa0°C were measured after 2xa0weeks fluid production. Using optical time domain reflectometry (OTDR), attenuation measurements at 850 and 1,300xa0nm enabled to identify mechanical, thermal, and chemical degradation along the optical fibre. The observed degradation led to erroneous temperature readings and limits, due to the optical budget of the DTS system, the accessible length of the fibre. The characteristics and the influence of the different degradation mechanisms on the accuracy of the DTS measurements are discussed and recommendations for an optimized installation are given.

Temperature-dependent characterization of optical fibres for distributed temperature sensing in hot geothermal wells

This study was performed in order to select a proper fibre for the application of a distributed temperature sensing system within a hot geothermal well in Iceland. Commercially available high temperature graded index fibres have been tested under in situ temperature conditions. Experiments have been performed with four different polyimide-coated fibres, a fibre with an aluminium coating and a fibre with a gold coating. To select a fibre, the relationship between attenuation, temperature and time has been analysed together with SEM micrographs. Based on these experiments, polyimide fibres have been chosen for utilization. Further tests in ambient and inert atmosphere have been conducted with two polyimide-coated fibres to set an operating temperature limit for these fibres. SEM micrographs, together with coating colour changes have been used to characterize the high temperature performance of the fibres. A novel cable design has been developed, a deployment strategy has been worked out, and a suitable well for deployment has been selected.

Simple feed-through for coupling optical fibres into high pressure and temperature systems

A best practice guide for assembling and testing a simple and inexpensive system feeding an optical fibre into a high pressure and temperature environment is presented. A standard Swagelok-type connector is tested together with different ferrule materials and a PEEK capillary tube as feed-through. The system proved to seal an optical fibre during several pressure and temperature cycling experiments up to 500 bar and 180 °C.

Multi-station Analysis of Surface Wave Dispersion Using Distributed Acoustic Fiber Optic Sensing

Multi-station analysis of surface wave dispersion to evaluate the near subsurface has been used in geotechnical applications for more than 15 years. A fiber optic cable used as array of distributed horizontal component seismic receivers for the analysis of surface wave dispersion is used in an extended setup here by the recording of active hammer blows. Their first analyses reveal low-frequency signal recordings with good continuity that can be used for further processing.

Dynamic strain determination using fibre-optic cables allows imaging of seismological and structural features

Natural hazard prediction and efficient crust exploration require dense seismic observations both in time and space. Seismological techniques provide ground-motion data, whose accuracy depends on sensor characteristics and spatial distribution. Here we demonstrate that dynamic strain determination is possible with conventional fibre-optic cables deployed for telecommunication. Extending recently distributed acoustic sensing (DAS) studies, we present high resolution spatially un-aliased broadband strain data. We recorded seismic signals from natural and man-made sources with 4-m spacing along a 15-km-long fibre-optic cable layout on Reykjanes Peninsula, SW-Iceland. We identify with unprecedented resolution structural features such as normal faults and volcanic dykes in the Reykjanes Oblique Rift, allowing us to infer new dynamic fault processes. Conventional seismometer recordings, acquired simultaneously, validate the spectral amplitude DAS response between 0.1 and 100u2009Hz bandwidth. We suggest that the networks of fibre-optic telecommunication lines worldwide could be used as seismometers opening a new window for Earth hazard assessment and exploration.Imaging the internal structure of faults remains challenging using conventional seismometers. Here, the authors use fibre-optic cables used for telecommunications to obtain strain data and identify faults and volcanic dykes in Iceland and suggest that fibre-optic cables could be used for hazard assessment.

Crustal Exploration and Monitoring Seismic Events with a Fibre-optic Cable Deployed at the Ground Surface in Iceland

We present new results of records from a 15 km long fibre-optic cable deployed at the surface in Iceland. We estimate the quality of records of seismic events with the cable for both exploration and monitoring.

New Methods in Geophysical Exploration and Monitoring with DTS and DAS

The development of new subsurface technologies often calls for measurements under extreme conditions and/or extended sensing requirements. Within recent years, continued developments in fiber-optic sensing have led to new possibilities for geophysical exploration and monitoring. These include several distributed methods, where data is recorded with high spatial and temporal resolution over long distances using the optical fiber as a sensor, exploiting different scattering mechanisms. Here we outline some new technologies in this context within case studies from different research projects including permanent installation of fiber-optic sensor cables behind casing, monitoring of high-temperature wells, a hybrid wireline logging system, and seismic recording using long-distance surface cables. We show that fiber-optic sensing opens up new possibilities for geophysical measurements with a broad range of applications in well logging and seismic exploration and monitoring. More time and cost effective deployment is possible, but continued research and development efforts are necessary to address remaining challenges.