Heterogeneous bedrock investigation for a closed-loop geothermal system: A case study
Introduction
The characterization of rock properties and of their heterogeneity is critical in many engineering geology applications, in particular for geothermal systems where thermal properties control the geothermal reservoir behaviour. This knowledge of the subsurface can be used to determine geothermal targets or design underground heat exchange systems at the regional or local scale (Fuchs and Förster, 2010). Lithological variations, mineralogy (in particular quartz content), uneven distribution of fractures, the presence of faults and tectonic structures and varying dip angle contribute to the heterogeneity of the bulk rock thermal properties in addition to variations in the water content and porosity (Guéguen and Palciauskas, 1994). Therefore, a detailed bedrock characterisation is crucial for understanding and predicting the thermal behaviour of the rock mass in-situ.
Several methods have been developed in order to characterise the bedrock at the laboratory scale, using rock samples or cuttings. Core samples or cuttings can be studied to obtain lithology and textural data (Bradbury et al., 2007), but provide limited or no information on fracturing. Several studies include measurement of thermal properties at rock samples to investigate the influence of various factors, such as mineral composition, porosity and degree of saturation (Clauser and Huenges, 1995, Popov et al., 1999, Pechnig et al., 2010). However, extrapolating those results to in-situ conditions remains challenging (Liebel et al., 2010).
On the other hand, borehole logging may provide more representative information of in-situ conditions. Borehole geophysical methods are used for site characterisation such as gamma-ray logging, resistivity logging, flow meter testing, seismic logging and borehole imaging (Monier-Williams et al., 2009). These methods can provide information on lithology, stratigraphy and fracturing (Keys, 1990) of the rock locally surrounding the borehole. Fujii et al. (2006) and Acuña (2010) investigated anisotropic thermal behaviour by conducting Distributed Thermal Response Tests (DTRT) in Borehole Heat Exchangers (BHEs). In both of these works, the authors analysed temperature profiles during the heat injection and recovery phase of the test to study the distribution of effective thermal conductivity through depth. Laloui et al. (2003) installed thermometers in a heat exchanger pile passing through different soil layers to study its thermo-mechanical behaviour. Fujii et al. (2009) used optical fiber sensors to record vertical temperature profiles in two bedrock case-studies in Japan and related these results with local geological and groundwater information, to verify the validity of the test and interpretation method. In the first case a permeable granite zone of 10 m thick was related to higher calculated thermal conductivity and quicker temperature recovery compared to non-permeable granite, as an effect of an active groundwater flow. In the second case lower thermal conductivity was related to weathered tuff, compared to unweathered tuff. Liebel et al. (2011) studied non-grouted wells in Norway and proposed taking temperature measurements four to five hours after the beginning of the recovery phase. They related faster temperature recovery to hydraulically active fractures and upcoming groundwater flow from confined artesian aquifers, as an effect of groundwater flow. They verified the existence of fractures by using flow measurement test data, televiewer imaging and/or drillers reports. The correlations provided in these studies concern distinct thick rock layers and/or are based on groundwater flow effects.
The objective of this paper is to characterise the heterogeneity of rock in-situ in the absence of high groundwater flow based on high-resolution temperature measurements and on rock thermal behaviour knowledge. We studied high-resolution temperature profiles, measured by fiber optics, in four BHEs (namely B1-B4) filled with grouting material, installed over a surface area of 32 m2 in an heterogeneous bedrock in Belgium. We located fractured zones more than one meter thick based on temperature profiles during hardening of the grouting material. We related temperature profiles measured during the recovery phase to thin rock layers of different mineral content (thickness more than 1.2 m) and determined the layer dip angle. We evaluated the correlations through a detailed geological description resulting from borehole logging measurements conducted in the four boreholes. Moreover, we measured the thermal conductivity of cuttings at the laboratory and correlated the measurements with the in-situ observations in terms of thermal conductivity and layering.
The remainder of the paper is organised as follows. First the geological background of the site is presented together with the materials and methods used in this study. Then a characterisation of the bedrock heterogeneity (including fracture characterisation, rock identification and layer dip angle determination) based on borehole logging measurements follows. The laboratory measurements of cuttings thermal conductivity are presented and their extrapolation to the in-situ conditions is discussed. The fiber optic measurements and their correlation to the rock characteristics as indicated by the borehole logging results follow. Finally conclusions are provided as well as a discussion on the accuracy of the fiber optic measurements analysis and its possible applications.
Section snippets
Geological settings and BHEs installation
The investigated site is located in the north-east side of the Dinant Synclinorium geological structure. The geological map of Sart-Tilman (Calembert et al., 1964) provides the most recent published geological interpretation of the bedrock for the studied area (Fig. 1). The site is also located on the North side of a local syncline. The synclinal axis has an E-W orientation. Based on the geological map, the boreholes cross Emsian (Lower Devonian) detrital sedimentary rocks, probably
Borehole logging
The televiewer measurements were conducted at depths beneath 15.55 m for B1, 13.50 m for B2, 10 m for B3 and 10.46 m for B4, since the boreholes were supported with casing at the first top meters to keep loose soil from collapsing into the borehole. The bottom depth was 98.67 m for B2, 102 m for B3 and 96.44 m for B4. For B1 the bottom depth was limited to 75.28 m, since collapsed rock pieces had blocked the borehole at that depth. The following results correspond to above depth intervals.
Discussion
Two different analyses are presented for the fracture characterisation: the borehole logging analysis and the fiber optic temperature profiles analysis. The extended fractured zones (thickness more than 1 m) location is limited between 22 m and 29 m deep for B1 and between 26 m and 34 m deep for B4, for both approaches. For B1, the middle of the fractured zone is located at a depth of 26 m for both approaches. The thickness of this fractured zone is 2 m based on the borehole logging analysis and
Conclusions
A bedrock heterogeneity investigation is presented based on in-situ and laboratory geophysical measurements. Temperature profiles during hardening of the grouting material allow us to locate extended fracture zones, more than one meter in this specific case. The profiles during hardening of the grouting material were obtained at different time period in B1 and B4 after injecting the grouting material. Hence a behaviour comparison between the two types of the grouting cannot be deduced by these
Acknowledgements
The work undertaken in this paper is supported by the Walloon Region project Geotherwal n° 1117492 and the F.R.S.-FNRS F.R.I.A. fellowship of Georgia Radioti. We also would like to thank the University service ARI for their support in the installation of the BHEs as well as our partners in the project, the ULB (Université Libre de Bruxelles) and the companies OREX and Geolys. We thank the company REHAU for providing the pipes and the commercial grouting materials.
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