Reference : Fractured bedrock investigation by using high-resolution borehole images and the Dist...
Scientific congresses and symposiums : Paper published in a book
Engineering, computing & technology : Energy
Engineering, computing & technology : Geological, petroleum & mining engineering
Fractured bedrock investigation by using high-resolution borehole images and the Distributed Temperature Sensing technique
Radioti, Georgia mailto [Université de Liège > Département ArGEnCo > Géomécanique et géologie de l'ingénieur >]
Delvoie, Simon mailto [Université de Liège > Département ArGEnCo > Géomécanique et géologie de l'ingénieur >]
Radu, Jean-Pol mailto [Université de Liège > Département ArGEnCo > Département ArGEnCo >]
Nguyen, Frédéric mailto [Université de Liège > Département ArGEnCo > Géophysique appliquée >]
Charlier, Robert mailto [Université de Liège > Département ArGEnCo > Géomécanique et géologie de l'ingénieur >]
ISRM Congress 2015 Proceedings - Int’l Symposium on Rock Mechanics
ISRM CONGRESS 2015: The 13th international congress of rock mechanics
du 10 mai 2015 au 13 mai 2015
[en] Fracture detection ; Layer dipping ; Bedrock heterogeneity ; Ultrasonic borehole imager ; Distributed Temperature Sensing technique
[en] In order to investigate the fracturing of the bedrock and its possible heterogeneous distribution in situ, four boreholes equipped with double-U geothermal pipes of 100 m long were installed on the campus of the University of Liege (Liege, Belgium) over a surface area of 32 m². The bedrock, which starts at a depth approximately of 8 m, is quite fractured and consists mainly of siltstone and shale interbedded with sandstone.

Different geophysical methods are applied at two different phases, after drilling the boreholes and after injecting the grouting material. The first approach consists in lowering an ultrasonic borehole imager (borehole televiewer; Zemanek, Glenn, Norton, & Caldwell, 1970), an instrument that acts as an ultrasonic transducer and receiver, into the boreholes to obtain high-resolution, continuous images with 360° coverage of the local geology and fracturing. Moreover gamma-ray logs of the four boreholes are obtained and inclinometry is conducted.

After drilling the boreholes fiber optic cables are attached along the pipe loops and the double-U pipes are installed inside the boreholes. Then the grouting material is injected. The second approach consists in measuring the temperature along the fibers by applying the Distributed Temperature Sensing technique (Soto, Sahu, Faralli, Bolognini, Di Pasquale, Nebendahl, & Rueck, 2007). A laser pulse is injected into the optical fiber and the temperature along the fiber is determined by the intensity of Raman stokes and anti-stokes reemitted signals. Temperature evolution is measured during hardening of the grouting material. Local maxima of the temperature curve are probably due to a local lower thermal conductivity and/or a local larger quantity of grouting material due to gathering of fractures.

A detailed fracture characterisation (position, opening, orientation, dip angle) is obtained based on the acoustic signal travel time and amplitude. The fractures are characterised by the same dipping and orientation but significantly vary in number and location in the four boreholes, despite the close distance between them. Gamma-ray data and observation of the cuttings during drilling result in rock identification through depth as well as in determination of the layer dipping. The inclination of the four boreholes tends to be perpendicular to the dipping. The combination of the two geophysical methods as presented provides information useful for the hydro-thermo-mechanical behaviour of the bedrock. The contribution of the thermal behaviour of borehole heat exchangers to bedrock investigation will be further studied by conducting Distributed Thermal Response tests (Fujii, Okubo, & Itoi, 2006). During the tests we will measure the temperature variation thanks to the installed fiber optics. These data will allow us to correlate any anisotropic thermal behaviour to the geological characteristics. The available information could be used for a detailed numerical model.

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