Heat Dilution Testing in Deep Underground Excavations

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Master Thesis Heat Dilution Testing

in Deep Underground Excavations

Presented at the Department of Earth Science at ETH Zurich Institute of Geology

Supervised by

Prof. Dr. Simon Löw, Geological Institute, ETH Zurich

Dr. Nima Gholizadeh Doonechaly, Geological Institute, ETH Zurich

Submitted by Matthias Meier meiermat@student.ethz.ch

14-924-476

Zurich, 16.08.2020

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ABSTRACT

This thesis presents new test data and analysis methods for the localization and characterization of preferential groundwater pathways in crystalline rocks at the Bedretto Underground Laboratory for Geothermal Energies (BULG) in Switzerland. The methods applied include flow logging and pressure build-up monitoring during drilling of long inclined boreholes drilled from BULG, and the development of a new heat dilution test protocol.

The heat dilution test aims to measure ambient flow in a borehole in combination with thermal properties and thus provides a method for determining hydrological properties in deep underground excavations of fractures that intersect a borehole in saturated rock. By changing the water temperature through heating and subsequent cooling, a better contrast between inflow and borehole water temperature is reached. A hybrid cable (simultaneous distributed temperature sensing and distributed heating) lowered into the borehole is used to measure the temperature along the cable in the borehole with an interrogator over time and heat the water in the borehole along the cable using a power supply. The resulting heat dilution test protocol can be used for an analysis with the temporal and spatial development of the temperature and provides information on the location, inflow rates, inflow temperature and transmissivity of fractures.

For boreholes drilled from deep underground excavations it can be assumed that water from the fractures is flowing along the borehole to the borehole head. A new transient heat- transport model is developed as part of this study with radial symmetry, composed of a centralized hybrid cable, borehole filled with water and borehole wall of granite with waterfilled fractures. Thus, the heat transport in and around the borehole before, during and after the heat dilution test is simulated. The obtained results provide a basis for the design and analysis of such tests with a manual iterative procedure. It is assumed, that the new method can be used to measure flow velocities in boreholes in the order of 10^{- 4} m/s. The development of an automatized method for the evaluation of the heat dilution test would further support the development of this new method which can determine the location of inflow zones with their inflow velocities, inflow temperature and transmissivity.

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1 INTRODUCTION

The focus of global energy transition includes substantial energy savings, greater energy efficiency and the broad-based promotion of new renewable energies. Therefore, the Energy Strategy 2050 passed by the Federal Council in 2011 is intended to ensure security of energy supply in Switzerland after phasing out of nuclear power. In Switzerland, no electricity is currently produced from geothermal energy. However, the potential for geothermal electricity production in Switzerland is very high. This inexhaustible, clean and constant energy source potentially offers several highly attractive advantages. The main obstacles to the development of this technology are the lack of knowledge about local underground conditions, the required stimulation technology, as well as uncertainty about the associated seismic risks.

The Swiss Competence Center for Energy Research (SCCER) – Supply of Electricity (SoE) focuses research on the technology of geothermal energy generation and production. It investigates whether it is possible to generate 5 to 10 percent of the electricity required in Switzerland safely and at competitive prices using deep geothermal energy. Researchers of this center are working on a profound understanding of the processes involved in hydro-fracking and hydro-shearing of crystalline rock to develop deep geothermal reservoirs.

From a depth of 10 to 20 metres below the earth's surface, the temperature no longer depends on the recent climate, the time of day or the season. From this depth, the temperature increases following a geothermal temperature gradient. The geothermal gradient in Switzerland is typically between 3 to 3.5 °C per 100 metres. (Swiss Federal Office of Energy, 2019) and therefore favourable temperature can be reached for geothermal energy exploitation at a depth 4 – 5 kilometers.

Geothermal energy uses the heat that is present in rock at great depth, usually in crystalline rock. The permeability of the crystalline rock is artificially increased by hydraulic stimulation to create a fractured and interconnected geothermal reservoir (Swiss Federal Office of Energy, 2017). One technique of hydraulic stimulation is hydraulic shearing. In this process, natural, already existing fracture systems are isolated in borehole intervals to activate fractures in shear, whereby the pore pressure is increased less than in hydraulic fracking, i.e. the locally prevalent rock pressure is generally not exceeded (Gischig and Preisig, 2014). Such a geothermal reservoir can be used to create a cycle. Cold water is injected, is heated underground and on the surface the hot water can emit its energy and start the cycle again. In this context one also speaks of an enhanced geothermal system (EGS).

In most cases earthquakes have a natural cause, but they can also be triggered by the construction of facilities such as tunnels, dams, mining or stimulated geothermal systems. In Switzerland, the last two events related to deep geothermal projects were minor earthquakes that occurred in Basel in 2006 (local magnitude 3.4) and in St. Gallen in 2013 (local magnitude 3.5) (SED, 2020).

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The above-mentioned process to increase permeability in the rock results in numerous microquakes that are not perceived by the local population. However, they provide valuable information on the size and extent of the geothermal reservoir. Systems for recording and monitoring microseismic events are available, so that geothermal activities could be interrupted if the frequency and intensity of microquakes increases (Swiss Federal Office of Energy, 2017).

However, it is currently not possible to completely exclude these types of events or to predict them sufficiently in advance.

The In-situ Stimulation and Circulation (ISC) project of the SCCER-SoE aimed at better understanding the hydro-seismo-mechanical coupled processes and associated induced seismicity. These laboratory scale (several centimetres) to medium scale (several dozen meters) experiments were performed at the Grimsel Test Site (GTS). The project was divided into three main phases, a characterization, a stimulation and a circulation phase. The first phase included geophysical and geological mapping of faults and fractures and determination of the actual stress field. In the second phase, while monitoring the associated permeability creation, pore pressure propagation, deformations and seismicity, the fault zone was pressurized until slip occurred and the massive rock mass between the faults was hydro-fissured, allowing the third phase to be carried out (SCCER – SoE, 2020).

In order to enable deep geothermal energy to be exploited on an even larger scale in Switzerland, the technologies for increasing permeability in crystalline rock must be further improved and validated in order to reduce the risk of major seismic events to a minimum. On the basis of the results of the ISC, the large-scale Bedretto Reservoir Project has been established.

The Bedretto Reservoir Project is part of the Pilot and Demonstration Projects of SCCER- SoE and focuses on questions of the validation of stimulation methods and the sustainable use of heat exchangers in the deep underground. These projects are the main component of the strategy that the integrated approaches developed for geoenergy, as well as those in developed innovative technologies, will be implemented in projects with industry to demonstrate the feasibility of the proposed approaches (SCCER – SoE, 2020). The Bedretto project intends to show that the construction of a sustainable heat exchanger can be planned and controlled using hydraulic stimulation methods. Compared to the Grimsel ISC project, the experiments will be carried out on a larger scale (hundreds of meters). This scale allows more realistic experiments with different stimulation concepts.

As a result of the Swiss Energy Strategy 2050, the Matterhorn Gotthard Railway leases the Bedretto tunnel to ETH Zurich for 8 years for the development of a geothermal underground laboratory (Bedretto Lab, 2020). With the support of the SCCER - SoE, the Department of Earth Sciences of ETH Zurich has established the Bedretto Underground Laboratory for Geoenergies (BULG) in the Bedretto tunnel, since its geology and accessibility make it a suitable location for a research laboratory (Figure 1.1). This unused tunnel, located in the Central Swiss Alps, connects the Ticino with the Furka Base Tunnel. The overburden of 1 km on average at the test site, represents to some extent the is-situ conditions of a typical geothermal

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reservoir, compared with already established underground laboratories around the world. It offers ideal conditions for experiments focusing on geothermal processes.

Several research projects on different topics have been carried out previously in the Bedretto tunnel, under the leadership of the Chair of Engineering Geology of ETH Zurich. For example, research has been carried out on detailed investigations of the groundwater flow systems in the Rotondo granite (Ofterdinger, 2001), the formation and hydrogeologic properties of brittle-ductile fault zones (Lützenkirchen, 2002) and geological characterization of the BULG (Jordan, 2019). Further work was done by Kissling et al. (1978), Huber (2004), Zangerl et al. (2006), Lützenkirchen and Loew (2011) and Meier (2017) in the tunnel area.

The first large scale experiments to be carried out in the BUL are related to enhanced geothermal systems and involve the drilling of long (200 – 400 m) injection and monitoring wells. A fundamental part of this multidisciplinary project is a hydrogeological model of the test volume. The hydraulic characterization is used to predict the rock behaviour during stimulation. For the main experiment permeability and transmissivity of the water-bearing structures are very important. Connected flow paths are to be found, since it is assumed that the stimulation propagates in this direction. If flow paths are connected, a larger volume needs to be stimulated immediately. This would cause further difficulties for a stimulation experiment.

This master thesis will develop and test new advanced methods of borehole fluid logging to improve the current state of the art in the characterization of borehole flow zone identification and at the same time to collect basic information for the hydrogeological characterization for the BULG project. This was achieved by flow and pressure logging during drilling, heat dilution testing and the application of existing analytical and new numerical simulation tools to the field data collected in the first three long boreholes intersecting the future test volume.

Figure 1.1: Impression of the Bedretto Underground Laboratory for Geoenergies (BULG) of ETH Zurich during drilling of Characterization Boreholes (Werner Simens-Stiftung, 2020).

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1.1 Motivation

Hydraulic head differences in fractures intersecting boreholes create (ambient) fluid flow in a borehole. The water enters the borehole at the higher head fracture intersection and flows to the lower head or out of the borehole. The composite head in an open borehole is the sum of the transmissivity weighted heads of the individual fracture zones and is dominated by the head of the most transmissive zone (USGS, 2020). The various hydrogeological units located in the deep underground have a wide range of groundwater storage and transmission capabilities (USGS, 2020). Borehole logging is an extremely important means of characterising the geological and hydrogeological conditions through which the boreholes pass.

The United States Geological Survey (USGS, 2020) describes some borehole flow logging methods in use such as flowmeter logs, fluid resistivity logs, temperature logs and heat pulse flowmeter:

Flowmeter logs directly record the direction and velocity of 1D flow in the borehole.

Borehole flow rates can be calculated from measurements of borehole velocity and borehole diameter recorded by the calliper log. Conventional used impeller flowmeters generally cannot resolve velocities of less than 1.5 m/min. High-resolution flowmeters have lower measurement limits around 0.03 m/min.

Fluid resistivity logs record the specific electrical resistance of the water in the borehole.

Variations in fluid resistivity reflect differences in the concentration of dissolved solids in water. Resistivity logs are useful for identifying water bearing zones and small inflows into a borehole.

Temperature logs record downhole water temperature and are useful for delineating water bearing zones and identifying ambient flow between zones of different hydraulic head along a borehole. Flow in the borehole between zones is indicated by temperature gradients that are lower than the regional geothermal gradient.

Another application of temperature logging brings measurements with heat pulse flowmeter. A pulse heating of the water at a specific depth is performed. If there is a flow in the borehole, the heated water package moves with the flow towards the upper or lower sensor.

Temperature difference between the sensors is measured over time. The instrument measures the time from the first activation of the heating until the moment when the largest temperature change is detected by the sensors. This information is used to calculate the flow rate at particular locations and time.

In this study the heat dilution test using two separate and a hybrid Heating-Distributed Temperature Sensing (DTS) cable is further developed for boreholes drilled from deep underground excavations. The method is based on deriving flow profiles in the borehole from recorded fluid temperature time histories along the borehole. From the temporal evolution of the temperature of the borehole water, the velocity of the water and the corresponding inflow rates and inflow temperatures can be estimated. In this study we use both the increase of the

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water temperature during heating and the subsequent cooling to analyse borehole flow a fracture inflow. We used a free DTS cable lowered into the entire borehole, to continuously record the temporal evolution of the temperature before, during and after the heating phase. A new numerical model is developed to derive the flow rates and temperatures of the inflows.

1.2 Research Goals

In order to understand the hydraulic conditions in the boreholes, data on water inflow and pressure build-up during drilling are collected in cooperation with the drilling team during drilling in the second half of 2019. In addition, interactions with the drilling team during inflow acquisition and pressure build-up are part of this work and serve to support data acquisition and quality control. This data set includes locations and hydraulic properties of the preferred water inflows into the newly drilled boreholes as well as the hydraulic interactions between the different boreholes observed during the drilling of the following boreholes.

From the drilled cores from the Rotondo granite, mapping on a scale of 1:100 and characterization of discontinuities are carried out in collaboration with Andri Münger (Münger, 2020). The measurements during drilling are compared with structural-geological observations of the drill cores.

In order to obtain further information about the hydraulic conditions in the underground, heat dilution tests are carried out in two boreholes. In this test, the temporal evolution of temperature to artificially induced temperature disturbance is measured. Using fibre optic distributed temperature sensing and active heating along the same (hybrid cable measurement) or a second cable (dual cable measurement) two methods are compared to optimize the heat dilution test.

The collected data from heat dilution test are used to create a new numerical model capable to analyse measurements from continuous heat dilution tests. The model calculates the temporal development of the temperature in a 2D radially symmetric system, including the effects of the hybrid cable, the steady state inflow from fractures, the water flow along the borehole, and the heat conduction in the formation. The combined analysing the data from flow/pressure logging during drilling and the heat dilution test contributes to a hydrogeological characterization of the test volume of the BULG.

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2 SITE CHARACTERIZATION 2.1 Site Description

The Bedretto tunnel was built between 1972 and 1982, simultaneously with the Furka Railway Base tunnel which connects Oberwald (Valais, Switzerland) with Realp (Uri, Switzerland) (Figure 2.1). It was claimed that the construction was due to long and difficult constructional reasons (as an intermediate point of attack), which results from both the geological structure and the timetable for the construction of the entire Furka Railway Base tunnel project (Labhart, 2005). The real reason, however, was to create a railway connection to the Val Bedretto against the will of the Parliament. In the end, due to political complications the tunnel could not be used for a railway connection between Oberwald, Realp and Ronco (Sieber, 2004), but was only used to transport the excavated material and as a supply adit during the construction of the Furka Railway Base tunnel. The Bedretto tunnel has been used for ventilation purposes since 1982 and has not been developed further (Bedretto Lab, 2020). The 5’221 m long Bedretto tunnel was built conventionally, i.e. by drilling and blasting and has a horseshoe shaped cross section (Keller and Schneider, 1982). In this generally unsupported rock tunnel, support with steel sets, rock bolts or wire mesh was only carried out in increased deformation zones.

The following statements were presented by Keller and Schneider (1982) after the completion of the tunnel. The toppling section in the first hundred metres of the Bedretto tunnel shows high permeability. At the time of construction, it was and is still accordingly water bearing. In the 4 km long Rotondo granite section water inflow is concentrated generally on fractures and faults. In a zone of 35 m length around tunnel metre (TM) 2’835, peaks of up to 57 l/s were measured immediately after the excavation. After the completion of the tunnel in 1982, 90 – 100 l/s were discharged at the portal of the tunnel. Peaks of up to 140 l/s were measured during snow melting or after heavy rainfalls.

The Bedretto Underground Laboratory for Geoenergies (BULG) is located 2 km from the southern entrance (Figure 2.1). It is in a 100 m long niche, where the width of the tunnel is enlarged to a width of about 6 m. In the section of the research laboratory (TM 2’000 – TM 2’100) the overburden varies between approx. 1’000 and 1’030 m (Figure 2.2). A cross section along the tunnel axis shows the highest overburden of 1’650 m below Piz Rotondo (at approx. TM 3’100). Geological characterizations of the niche and its vicinity were carried out by Lützenkirchen (2002), Lützenkirchen and Löw (2011), Meier (2017) and Jordan (2019).

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2.2 Geological Setting

2.2.1 Regional Setting

A detailed geological map (Figure 2.1) and a geological cross section (Figure 2.2) of the Bedretto tunnel (Fenster Bedretto) were created by Keller and Schneider (1982). At the southeast portal the tunnel starts in the Tremola-Formation with some

"Hornblendegarbenschiefer", which are dominated by biotite gneiss and mica schists. Towards northwest the tunnel consists of the Prato Formation, which comprises a zone of stratified amphibolite and three different gneisses i.e. a stratified gneiss, a light mica gneiss and a mesocratic biotite gneiss. After the Prato Formation until the Furka Base Tunnel is encountered, the geology of the tunnel is dominated by Rotondo granite.

Figure 2.1: Geological map of the Furka Base tunnel and the Bedretto tunnel / Fenster Bedretto (After Keller and Schneider, 1982). The location of the BULG is shown in red.

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Figure 2.2: Geological cross section of the Bedretto tunnel (After Keller and Schneider, 1982). Location of the BULG is shown in red.

The granite was named by von Fritsch (1873) after the Piz Rotondo and is one of the most interesting intrusions of the central massifs of the Swiss Alps (Labhart, 1977). In a late phase of the Variscan orogeny (294.3 ± 1.1 Ma), the Rotondo granite penetrated the Paleozoic polymetamorphic crystalline bedrock of the Gotthard massif (Sergeev and Steiger, 1995).

During the Variscan metamorphosis, parts of it underwent an overprint in the amphibolite facies (Nunes and Steiger, 1974; Oberhänsli, 1986). The total surface area of the Rotondo granite is about 25 – 30 km². The flat plunge of the granite massif in the west as well as in the east should reflect its original dome shape (von Fritsch, 1873).

The Rotondo granite is relatively uniformly formed: generally a bright equigranular, often undirectional granite with greenish feldspars, brown quartz and remarkably low content of biotite, with hardly any foliation present (Labhart, 2005). The mineral composition of the Rotondo granite was analysed in detail by Hafner (1958) and Steck (1976) and is divided into the following minerals: Quartz 25 – 35%, alkaline feldspar 20 – 40%, plagioclase 10 – 25%, biotite 3 – 8%. Further: garnet, phengite, epidote, apatite, opaque minerals, zircon. The garnet, which is almost always macroscopically visible in the handpiece, can be of particular interest as its content varies between 0.01 and 1% (Labhart, 2005). It should be considered that almost the entire mineral content was recrystallized during the alpine deformation (Steck, 1976).

Although not very common, the Rotondo granite contains a few aplitic and lamprophyric (dark coloured) dykes, although the latter are unlikely to be very thick and extensive (Jordan 2019).

In a few zones the granite is schisted or gneissed. However, the Rotondo granite is intersected by fault zones at intervals varying between 1 and 100 m (Labhart, 2005). According to Lützkirchen (2002), encountered fault zone rocks can be described as followed based on

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typical main structural areas: The core of the fault zone, which has been deformed the most, shows intensely tectonized, fractured and crushed rock, surrounded by a fracture zone, which is characterized by a network of major fractures and connecting structures. Outcrops of fault zones further indicate the close relationship between ductile (mylonites or clusters of slates with biotite-phengite or biotite-chlorite) and brittle (non-cohesive material, which preferentially occur alongside older mylonites or lampophyric veins) structures, as the fault core usually is located in an area pre-dominated by a ductile shear zone. A classification scheme for brittle - ductile deformed zones developed by Lützenkirchen (2002) is based on the relative ratio of ductile and brittle areas within a fault zone. With the help of this scheme, critical structures in fault zones can be identified and potentially high permeable fault zone types can be distinguished. High permeable single structures are mainly found in fault zone types and fault zone areas which are characterized by the absence of ductile structures (Lützenkirchen, 2002).

2.2.2 Test Site of the Bedretto Underground Laboratory for Geoenergies

The 100 m long niche, where the BULG is located, is characterized by a very homogeneous Rotondo granite. Both sidewalls are completely unsupported and show the Rotondo granite in a very fresh state. Along this niche only sporadic fractures with slickensided surface occur (Meier, 2017). Isolated quartz veins are visible on the ceiling and walls, which are rarely slightly displaced by fractures.

Several fault zones can be observed in the Rotondo Granite, although this frequency decreases in the vicinity of the tunnel. Close to the cavern (TM 1’980) only one brittle-ductile fault zone can be found. The result of the ductile deformation can be described as mylonitic, as the grain size is very small and recrystallized well. It can be assumed, that the ductile portion is older than brittle deformation. The brittle deformation results in a cataclastic fault breccia with gauge (Lützenkirchen, 2002).

In the BULG, three boreholes define a test volume which is to be characterized geologically and hydrogeologically by the Bedretto team as well as by this and an accompanying master thesis (Münger, 2020). The three boreholes are drilled using coring techniques. The core logging process is important to obtain first information about the geological and geotechnical conditions of the test volume and possible water inflow zones.

Three characterization boreholes (CB1, CB2 and CB3) with lengths of 302 m, 222 m and 191 m and inclinations of 45°, 50° and 40° respectively (Appendix 2.1) have been drilled into the southwestern wall of the BULG. The boreholes have a larger diameter of 222 mm in the first meters, in which standpipes were installed (cemented). In the case of CB1 the larger diameter is 15 m long, in CB2 15.7 m and in CB3 16 m. From August to November 2019 the three boreholes were drilled by the Austrian company Züblin Spezialtiefbau GmbH using double tube wireline coring (CB1 and parts of CB2 and CB3) and counter flushing (CB2 and CB3) (core diameter 96 mm). The obtained core is divided into 1 m long sections and placed in core boxes.

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2.2.3 Core Logging

Core logs (Appendix 2.2 – 2.4) for the boreholes CB1, CB2 and CB3 have been prepared at the scale of 1:1’000 in collaboration with Andri Münger (accompanying master thesis in the BULG). Raw data at the scale of 1:150 of the core logs, as well as additional photos of the cores can be obtained from Münger and Meier (2020).

As basis for the core logging serve the classifications of the Rotondo granite and the associated structures (Appendix 2.6) of Laws (2001), Lützenkirchen (2002) and Jordan (2019) as well as a manual by Dr. Peter Guntli (Sieber Cassina + Handke AG, 2019). The core log is intended to provide a geological and geotechnical overview of the test volume, showing the most important structures and the variability of the rock mass along and between the boreholes.

Further, the cores will be used to identify features which might dominate the hydrogeological behaviour of the test volume.

The composite core logs for the boreholes CB1 (Figure 2.3), CB2 (Figure 2.4) and CB3 (Figure 2.5) includes a simplified geological core log, rock type, cumulative water inflow, RQD, discing, core loss, number of open fractures and porosity. In the simplified geological core log, the mapped geological structures are presented in a schematic way, to give an idea about the appearance and the most important features in the core (Figure 2.6). The rock type column indicates the depths at which the different types can be found. In the appendix 2.2 – 2.4 the geotechnical description with the texture, mineralogy and discontinuities is noted in this column. The presented field estimates of uniaxial compressive strength are used from Hoek and Brown (1997). In the cumulative water inflow column, the flow measurements during drilling are shown in l/min. The RQD (Deere and Deere, 1988) value is determined for each metre, whereby the drilling induced fractures and discing is not considered, thus, the core is considered to be intact at these positions. In the log the range from the minimum to the maximum value within 10 metres is indicated. Discing (Obert and Stepheson, 1965) is recorded, when the core is broken in many thinner disks (0.5 – 4 cm). It can be assumed that these discs are a result of high in situ stress magnitudes relative to the rock strength (Lim and Martin, 2010) in combination with the orientation of the borehole. The core loss column indicates where no core could be recovered. In this case, core loss is most likely caused by the drilling process (and not by a fault zone), whereby a core piece could have jammed during drilling and grinded the following rock. This conclusion is based on the shape of the lowest core piece before the core loss section and the study of the ATV log (Bedretto Team, 2019) which does not show any fault zone like structures in the corresponding section of the borehole. The number of open fractures is given for sections of 10 metres in length. As open fractures are counted slickensided fractures, fractures along which newly formed minerals are found as well as discontinuities with vuggy porosity. Two types of hydrothermal porosity are found in the cores, "unconnected" and

"connected" porosity. Unconnected porosity stands for small pores (0.5 – 3 mm) which are more or less regularly dispersed in the rock. Connected porosity stands for several pores with diameter of ~1 mm which have oriented themselves in the core along a plane and are partially

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Figure 2.3: Composite core log of CB1. Legend shown in Figure 2.6.

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Figure 2.6: Legend for simplified geological core log column of Figure 2.3 – 2.5.

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The three cores show sections with strong similarities. In order to provide a better comparison between the three boreholes four rock types have been elaborated. The types are the Intact Granite (Type 1 and 2) and the Foliated Granite (Type 1 and 2). These are described in more detail in the following sections. In all boreholes the Intact Granite (Type 1) is encountered first and then the Foliated Granite (Type 1 and 2). The Intact Granite (Type 2) occurs in the lowest section of the boreholes CB1 and CB2. A graphical representation of the depths of the rock types in each borehole can be seen in description column of the corresponding core log.

The Intact Granite (Type 1) is found in CB1 at depths of 0 – 143 m, in CB2 from 0 – 130 m and in CB3 from 0 – 120 m. Figures 2.7 A and B show the Intact Granite (Type 1) at a depth of 84 m in CB1. The Intact Granite (Type 1) can be described as homogeneous, massive, light-grey, medium to coarse-grained granite with plagioclase (often saussuritized), alkaline feldspar, quartz, white mica, biotite (in nests) and garnet (1 – 2 mm diameter). This Intact Granite is unweathered and extremely strong. Existing healed fractures have a thickness of 1 – 3 mm (sporadically 1 cm) and are usually containing biotite, muscovite (or phengite), partly chlorite, epidote or calcite (Figure 2.7 A). The healed fractures have a frequency of 0.2 – 4 per m and no systematic orientation is recognized. However, the core tends to fail along these healed fractures. Existing slickenside fractures have chlorite on the sliding surface, but no systematic orientation or rake is recognized (Figure 2.7 B). In this rock type the frequency of the slickensided fractures is >/~0.1 per m (depending on the borehole). Few quartz veins and few aplitic zones (Figure 2.7 C) are present in this rock type. During drilling, no water inflow has been recorded along the entire length of this rock type. The RQD is almost constantly 100, occasionally it decreases to 95. The Intact Granite (Type 1) very rarely shows discing, connected or unconnected porosity.

The Foliated Granite (Type 1) is found in CB1 at depths of 143 – 207 m, in CB2 from 130 – 140 m and in CB3 from 120 – 174 m. The Foliated Granite (Type 1) can be described as foliated (gneissic), grey, medium grained granite (Figure 2.7 E) and has a similar mineralogy as Intact Granite (Type 1). In CB1 and CB3 the foliated granite is intersected by homogenous massive granite (10 – 100 cm long sections). Foliation occurs by oriented mica and fine quartz layers (1 – 3 mm thick). The distance of (shear) foliation is 0.1 – 1 cm (sericitization). The foliated granite is very strong (locally decreased). Occurring healed fractures of the foliated granite are analogue to Intact Granite (Type 1). The existing open joints have an increased frequency (1 – 3 per m) compared to Intact Granite (Type 1) or are even forming a fracture zone with more than 10 per m (Figure 2.7 D). The open joints often occur along the foliation. The surfaces are often hydrothermally altered and show porosity (Figure 2.7 F). In few open fractures fault gauge is visible. The slickensided fractures are rare compared to Intact Granite (Type 1) and have properties analogue to Intact Granite (Type 1). In this rock type, ductile shear zones appear. These are characterized by very fine grained (mylonitic), matrix dominated, recrystalized, dark grey-green sections, sometimes accompanied by little fault gauge. Ductile shear zones are found at depths of 143 m and 172 m in CB1, between 130 and 140 m in CB2

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foliated (gneissic) sections, located near ductile shear zone and with little fault gauge. In the CB3 from 155 – 160 m and 163 – 165 m there are brittle fault zones (Figure 2.7 G), which can be described as cohesive cataclasites with grey matrix and rock fragments (quartz and plagioclase) with sizes of <1 cm (rarely up to 3 cm). These zones are weak to medium strong.

It can be assumed that during drilling this rock type caused the first inflow to each borehole.

The RQD is often 100, but sometimes it is decreased to 75 and once to 0. Discing was frequently encountered in the Foliated Granite (Type 1). From this rock unit onwards, connected and unconnected porosity is frequently encountered. This is possible due to the increased water content in this rock type and the associated hydrothermal weathering of the rock.

The Foliated Granite (Type 2) is found in CB1 at depths of 207 – 254 m, in CB2 from 140 – 215 m and in CB3 from 174 – 191 m. The Foliated Granite (Type 2) is a massive granite, intersected by foliated, gneissic Granite (similar to Foliated Granite (Type 1)). The mineralogy is similar to Intact Granite (Type 1) with addition of newly formed white mica on schistosity and granitic matrix (sericitization). The Foliated Granite (Type 2) is very strong to extremely strong. The healed fractures are similar to Intact Granite (Type 1). Existing open joints have a decreasing frequency (0 – 1 per m) relative to the Foliated Granite (Type 2). Some surfaces are hydrothermal altered and show newly formed minerals (Figure 2.7 F). The RQD is typically 100, occasionally it is decreased to 65. For the Foliated Granite (Type 2), discing was only recorded in CB2. Both connected and unconnected porosity was found in the foliated gneissic sections. The unconnected porosity is often found in the intact sections.

The Intact Granite Type 2 is found in CB1 at depths of 254 – 302 m and in CB2 at depths of 215 – 222 m. This type of granite was not found in CB3. The Intact Granite (Type 2) is a massive granite with a mineralogy similar as Intact Granite (Type 1). It has still muscovite as in Foliated Granite (Type 2). Relative to the Intact Granite (Type 1), the plagioclase is more often saussuritized. The Intact Granite (Type 2) is extremely strong. The healed fractures are analogue to the Intact Granite (Type 1). In CB1 the open joints have an opening of 0 – 10 mm and are often hydrothermally altered and show newly formed minerals. However, they are not always penetrative continuous. The open joints show a frequency of 0 – 2 per m. Slickensided surfaces are rarely found. The RQD is typically 100, occasionally it is decreased to 90. No discing was recorded in the Intact Granite (Type 2). Connected porosity was encountered regularly and almost always unconnected porosity.

The test volume in the BULG can be assumed to be very heterogeneous and the thickness of the structures can change significantly over a small distance (in order of metres). In particular, this is evident in the brittle-ductile shear zone, which occurs in CB3 but is not as prominent in the boreholes CB1 and CB2. It can be assumed that the same structures (aplitic zones, mylonites or shear zones (ductile and brittle)) are encountered in different boreholes.

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Figure 2.7: Photos of specific features in the cores of the test volume in BULG. A: Broken healed fracture, B: Slickensided fracture with chloritic and sericitic infilling (Core diameter for scale), C: Aplitic dike, D: Open joint, E: Foliated Granite Type 1 with ductile shear zones, F: Hydrothermal

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2.3 Hydrogeological Setting

In crystalline rock such as the Rotondo granite, the permeability of the rock matrix is often negligible compared to the permeability of fractures. Therefore, preferential water flow occurs along conductive structures such as brittle fractures, damage zones or single joints (Barton et al., 1995; Brace, 1980; Caine and Forster, 1999; Evans et al., 1997). Furthermore, brittle rocks, such as granites, generally do not show a significant decrease in hydraulic conductivity with depth (Masset and Löw, 2013). Lützenkirchen and Löw (2011) describe that the major part of the total measured water inflow in the Bedretto tunnel comes from a few highly conductive fault zones. Therefore, the spatial distribution and connectivity of the fractures are of great importance. An alternation of strongly fractured and unfractured zones can be observed in the Rotondo granite and the water bearing structures are not uniformly distributed within the Rotondo granite (Lützenkirchen, 2002; Jordan, 2019).

The cavern in the Rotondo granite (see Chapter 2.2.1) in which the BULG is located has no major water inflow. Sporadic dripping points are present and the rock is moist in the absence of ventilation. The brittle-ductile shear zone near the cavern at TM 1’980 is characterized by hydrothermal weathering and has a flow rate of 1.95 ± 0.2 l/min at a fixed measuring station (end of 2019). At approximately TM 2’175 another small inflow zone is located along a fracture zone striking almost parallel to the tunnel, but distributed over a length of several metres, so that it is not easily measurable. In this case, the water bearing structure can be identified as a slickensided fracture.

Due to the overburden of about 1’000 metres, the initial head at the location of the BULG was high (close to the land surface, i.e. 2’400 m), and then dropped to atmospheric pressure conditions during tunnel excavation. This has created a massif pressure drawdown “cylinder”

around the BULG and it can be assumed that the hydraulic head increases strongly from the atmospheric state (head equal to tunnel elevation plus 1 bar; 1’480 m) in the cavern into the surrounding rock. Thus, connections between conductive zones with different hydraulic heads are created by drilling several hundred metre long boreholes (CB1, CB2, CB3) from the tunnel.

The hydraulic head is assumed to increase radially with distance to the tunnel as the BULG is located far below the groundwater table. Additionally, an open borehole as new hydraulic sink with head equal to the tunnel elevation generates hydraulic gradient towards the borehole.

Therefore, all water is expected to flow from the formation towards the boreholes. Due to this hydrological setting, water is generally entering the boreholes at depth and flowing upwards towards the tunnel.

In the different rock types different structures are assumed to be water-bearing. The core log (see Chapter 2.2.3) shows that open joints and brittle fault zones are mainly found in the Foliated Granite (Type 1 and 2) and open joints with vuggy porosity are occasionally found in the Intact Granite (Type 2). The Intact Granite (Type 1) contains primarily slickensided structures. It is assumed that the mentioned structures represent the most transmissive structures for the respective type.

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The geochemical properties of granite rocks are influenced by hydrothermal alteration (Boyce et al., 2003). In the geological environment, the frequency, geometry and filling of fractures are important factors that control the migration of solutes as well as fracture transmissivity and flow (Yoshida et al., 2000). Hydrothermal altered zones are generated by the circulation of hydrothermal fluids and thus provide evidence of past fluid travel paths (Shanks, 2012). A direct evidence of past fluid circulation patterns is the chemical and mineralogical distributions of hydrothermal altered zones (Shanks, 2012). Newly formed minerals such as calcite, pyrite, muscovite and saussuritized plagioclase are observed in the cores described in Chapter 2.2.3. Additionally, vugs with openings of up to 1 cm were found, which therefore can be regarded as preferred groundwater pathways. In the boreholes such alteration occurs mainly in the deeper areas (> 100 m). Especially the Foliated Granite (Type 1 and 2) and Intact Granite (Type 2) show zones with hydrothermal altered joints.

2.4 Borehole Flow and Pressure Logging During Drilling

The principle of hydrological tests is to disturb the pressure conditions in the rock by controlled water injection or withdrawal from a borehole interval, and to record the resulting temporal development of the pressure conditions continuously and precisely. For radial borehole flow conditions, pressure transients from drawdown and recovery phases of the tests can be analysed for transmissivity using the semi-logarithmic linear method of Cooper and Jacob (1946) based on Theis (1935). Based on the drawdown and recovery of observation boreholes, storativity values can be determined according to the classical approaches of Cooper and Jacob (1946) and Ballukraya and Sharma (1991).

In deep underground excavations different types of hydrological tests can be carried out in subsurface exploration boreholes (or pre-drillings) during drilling. These tests can be used to estimate aquifer parameters such as transmissivity, storativity and initial formation pressure (e.g. Pesendorfer and Löw, 2010). Pesendorfer and Löw (2010) describe transient pressure drawdown tests in open exploration boreholes of the Lötschberg Base Tunnel. A basic estimation of the initial formation pressure from the pressure recovery phase is possible if monitoring intervals are short (only intersect one dominant transmissive structure) and boreholes intersect virgin ground. Pesendorfer and Löw (2010) used such pressures, when the pressure change in the recovery phase was less than 0.05 bar during a measuring time of 200 s.

Masset and Löw (2013) describe the detailed procedures applied for transient pressure test in 50 – 100 m long pre-drillings with cemented stand pipes (of 10 – 20 m length) of the Gotthard Base Tunnel:

1. Pressure buildup and stabilization with closed valves phase during about 1.5 hours.

2. Subsequently, the gate valve can be partially opened to generate a constant outflow rate and pressure drawdown over 1 to 2 hours (drawdown test). A complete opening

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sharp drop in the flow rate. Empirically it can be said that the flow rates should be about 1/5 of the initially open borehole flow rate to maintain a constant rate outflow during the pressure drawdown phase.

3. After shut-in, the pressure increase is measured during 1 to 2 hours (recovery phase).

The transient pressure is monitored at an interval of 1 second by a high resolution pressure transmitter mounted on the borehole head.

Pesendorfer (2006) and Masset and Löw (2013) describe the procedure for recording flow data in active pre-drillings of Alpine tunnels: If possible, the flow rate is determined after each drilling cycle, i.e. after reaching an additional depth of a rod length, or when significant changes in the flow rates are observed at the borehole head. The measured position of an inflow into the borehole is therefore known with an accuracy of about one rod length or depends on the experience and sensitivity of the drilling team. If a water-bearing structure is intersected by the borehole, the water flows from the structure along the annulus (small open space) to the drill bit, enters the drill bit and flows back through the drill rod. Outflow out of the borehole is measured using a normalized tub and clock at the end of the drill string or directly at the borehole head without rods in the borehole, in this case the water can flow out of the borehole directly. The flow rate should be measured three times and then averaged. It should be noted that a measured flow rate at the borehole head is always the sum of all flow rates up to the actual drilling depth. The flow rate of a single inflow point can be determined from the difference inflow rate between two logged inflow points.

In BULG hydraulic data (flow and transient pressure tests) were also collected during the drilling phase of the first three exploration boreholes. In comparison to Pesendorfer (2006) as well as Masset and Löw (2013), a new design of the measurement system was designed and used for the measurements in this master thesis. In addition, transient pressure tests (constant head) and constant flow tests (drawdown tests) are performed. This data set is also an important supplement to the heat dilution tests. The collected datasets of the constant head and constant flow tests are not analysed any further in this master thesis. They are used as an indication of specific depths at which water inflows into the borehole occur. It is recommended to analyse the data in a further step with different standard analytical methods developed by Theis (1935), Cooper and Jacob (1946) and Horner (1951).

2.4.1 Methodology and Experimental Setup

Systematic measurements of the water inflow into the borehole were carried out during drilling of boreholes CB1, CB2 and CB3 in the second half of 2019 (see Chapter 2.2.2). The test design is similar to the workflows described by Pesendorfer (2006) and Masset and Löw (2013). The data was collected in collaboration with the drilling team using a digital pressure and flow meter, measurements were carried out during drilling interruptions (i.e. during breaks and shift changes). Because of the temporal urgency, it was not possible to carry out the measurement after each individual drill rod (3 metres).

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A schematic illustration and a photo of the measurement setup for pressure and flow measurement are shown in Figures 2.8 and 2.9. By sealing at the borehole head, by the blowout preventer (which normally protects against overpressure during drilling) and by the flushing supply it was possible to measure a pressure stabilization with the pressure sensor connected to the computer. The expected pressures of the individual inflows should be spatially variable. In a flow measurement the water entered the borehole directly at the intersection of the borehole with a water bearing structure and flowed along the drill string to the borehole head. The flow measurement system attached to the borehole head was able to measure the outflowing water.

The casing could cause a restriction of the free flow compared to the borehole, whereby the limiting diameter of the electric valve would be 2.1 cm.

Figure 2.8: Schematic illustration of the experimental setup for pressure and flow measurement.

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Figure 2.9: Photo of the experimental setup of the pressure and flow measurement (Bedretto Team, 2019).

Due to the high pressure difference from the pressure in the aquifer to the atmospheric pressure at the end of the flexible tube, a flow measurement correspond to a constant head test (Figure 2.10). After a short stabilization of a few minutes, the electric valve was opened for 10 minutes and the constant head phase started. During the 10 minutes of flow, the amount of water was measured with the flow meter connected to the data logger. It was possible to reach a constant flow rate within the 10 minutes. From the following pressure recovery period, the data have been analysed based on Cooper and Jacob (1946).

Furthermore, the drilling team used a classical bucket measurement with stopwatch to record the flow rates during drilling, to compare the flow rates with the flow meter measurements. In these manual measurements, the water flowed from the structure along the annulus to the drill bit, enters the drill bit and flows through the drill rods to the surface.

During longer interruptions, constant rate tests were performed to gather more information about the aquifer type and parameters. For this test the same measuring setup was used as for the constant head test (Figure 2.8). The electric valve was only be partially opened by manually interrupting the opening process. After the electric valve has been opened, manipulation of the valve during the pressure drawdown phase should be avoided. With a reduced flow rate of 50%, the outflow rate tends to be constant during the pressure drawdown phase (Figure 2.11). With the pressure drawdown and the following pressure recovery, the data can be analysed based on the recovery method from Theis (1935).

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Figure 2.10: Idealized pressure reaction of a constant head test with short stabilization, open borehole and recovery phase. The time axis is schematic and therefore has no exact duration. (Löw, personal communication, August 21, 2019).

Figure 2.11: Idealized pressure reaction of a constant rate test with stabilization, drawdown and recovery phase including proposed time durations - After Pesendorfer 2006 and Löw (personal communication, August 21, 2019).

If an additional borehole is drilled into a structure that is connected to a shut-in observation borehole, a change in pressure might be detected in this borehole. For this reason, the pressure measurements at the head of completed (and shut-in) boreholes were continued during the drilling of the subsequent boreholes. Using the drilling protocol of the drilling team, the approximate depth of the structure in the newly drilled borehole could be estimated.

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2.4.2 Results from Flow and Pressure Logging

During the pressure build-up measurements at the first borehole (CB1) the blow-out preventer was damaged. After its reparation it was no longer possible to make pressure build- up measurements during drilling at the other boreholes (CB2 and CB3), as the preventer could have been damaged again.

Figure 2.12 shows an example from CB1 at 202 m borehole depth of a pressure reaction of a constant head test with short stabilization, open borehole and recovery phase.

Approximately 30 such measurements have been made in collaboration with the drilling team during the drilling of the boreholes CB1, CB2 and CB3. The duration of the stabilization phase is highly dependent on the person who performed the measurement. The constant head test had a duration of 10 minutes each. The duration of the recovery phase was variable, determined by the continuation of the drilling program.

Figure 2.12: Example of a pressure reaction of a constant head test from CB1 at 202 m borehole depth.

Only a few constant rate tests are performed due to the long duration (several hours).

Figure 2.13 shows an example of a pressure and flow reaction of a constant rate test from CB1 at 260 m borehole depth. It shows a pressure increase, as well as a first constant rate test, which however does not show a perfect constant outflow due to the handling of the electrical valve.

The electrical valve was manually stopped during the opening process and it was tried to achieve a constant flow rate until the valve was closed again.

The maximum measured pressures at the borehole head at CB1 with respect to borehole depth are shown in Figure 2.14. The first data point is based on an unaccompanied measurement, where someone from the drilling team had shut-in the borehole. The subsequent measurements are based on the method described in Chapter 2.4.1.

0 5 10 15 20 25 30 35

0 1000 2000 3000 4000

F lo w [ l/ m in ]

P re ss u re [ b ar ]

Time [s]

CB1 Depth 202 m - September, 11, 2019, 07:03 pm

Pressure [bar]

Flow [l/min]

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Figure 2.13: Example of a pressure and flow reaction of a constant rate test from CB1 at 260 m borehole depth.

The borehole outflow measurements were mostly taken manually by the drilling team and are also shown in Figure 2.14. The reported measurements are outflow rates relative to the drilling progress of the respective borehole in l/min. However, the measurements do not imply that an inflow is present at the exact location where an increase in the inflow rate occurs, but somewhere between the two measurement locations. In CB1 the last log entry without flow is at a depth of 140 m, in CB2 at a depth of 122.5 m and in CB3 at a depth of 150 m. CB1 shows a maximum flow rate of less than 10 l/min up to a depth of 250 m, whereby the rate increases in steps and no continuous increase can be detected. A strong increase occurs at a depth of 280 m. The maximum flow rate rises to 26.5 l/min in this measurement. CB2 and CB3 show a maximum flow rate of less than 10 l/min up to their final lengths (CB2 222 m and CB3 191 m).

The rate increases stepwise and no continuous increase can be observed as well.

0 2 4 6 8 10

0 10 20 30 40 50

31000 36000 41000 46000

F lo w [ l/ m in ]

P re ss u re [ b ar ]

Time [s]

CB1 Depth 260 m - September, 18, 2019, 04:55 am

Pressure [bar]

Flow [l/min]

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Figure 2.14: Pressure and flow measurements during drilling in CB1, CB2 and CB3.

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Figure 2.15: Pressure decrease measured in CB1 when CB2 reached a depth of 159 m.

Further the data set includes cross-hole interactions between all three boreholes. The interactions were monitored in completed, closed boreholes reacting to subsequent drilling from additional boreholes. During the drilling of borehole CB2, the boreholes CB1 and CB3 were closed. A pressure decrease was measured in CB1 when CB2 reached a depth of approximately 159 m (Figure 4.15). The decrease stopped further pressure build-up in CB1 and allowed the pressure to decrease from 45.4 bar to 44.4 bar within 24 hours. In CB3 two pressure decreases were measured when CB2 reached depths of 132 and 159 m. The decrease at 132 m is from 40 bar to 39.7 bar within 24 hours. The decrease at 159 m is from 39.7 bar to 38.5 bar in 48 hours.

44.5 44.6 44.7 44.8 44.9 45.0 45.1 45.2 45.3

0 20000 40000 60000 80000

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Time [s]

CB1 completed - October, 14, 2019, 09:32 am

Pressure in CB1 [bar]

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2.4.3 Discussion of Methodology and Results

It is recommended that the experimental set-up for the constant flow test be adapted, since the electric valve could only be partially opened. It is likely that the flow will decrease over time due to pressure decrease. Therefore, it is recommended to either use a mechanical version of the valve or to install an electrical valve where the specific opening or flow rate can be predefined.

The use of multiple electrical devices adds a lot of extra work to the measurement of the data and makes communication with a drilling team more difficult. It has been tried to keep the interaction of the drilling team with the instrument as short and simple as possible. However, it is suggested that a manual valve should be used for further measurements during drilling, as not much more time would be required by the drilling team.

Data collected with the pressure sensor show very sensitive data in small time intervals over long periods of time. To simplify data processing, it is suggested to increase the measurement interval from 1 to 5 s (or more). However, there is nothing to criticise about the collected data itself.

The electric flow meter is challenging to use because its sensitivity depends on the flow rate. Using the classic bucket measurements has proven to be efficient in borehole CB2 and CB3. Regarding constant flow measurements, Masset and Löw (2013) showed that even a few measurements during the test are sufficient for the following data analysis. Conclusions about the use of the existing data set could be drawn from a more detailed data analysis.

By comparing the optical and acoustic tele-viewer data (Bedretto Team, 2019) and the expected hydrogeological structures from the core descriptions (see Chapter 2.2.3), the results show that an inflow increase is measured within 10 metres after reaching a certain structure. It is assumed that flow rate measurements often represent an inflow into the borehole a few metres after reaching a specific inflow zone. An originally planned accuracy of one drill rod (3 metres) was not achieved. In order to obtain more accurate results, the drilling team would have to be instructed to measure the outflow from the borehole after every metre. Interruptions in an ongoing process are difficult to integrate, so a more precise indication of an inflow zone by flow logging during drilling cannot be expected. Nevertheless, the approximate depth and the approximate ratio of the different inflows can be estimated based on the measurements.

The recorded flow measurements (Figure 2.14) always reflect a condition dependent on the drilling progress and are therefore independent of each other in time. It is assumed that the decrease in flow rate observed at some depths is caused by the depletion of the upper layers over time, so that their flow may decrease.

Meier (2020) provides a data set of about 30 constant head tests and some constant flow tests in the test volume of the BULG. This data set has not been quantitatively analysed so far, and the use is limited to the location of inflow zones and the corresponding approximate flow rates.

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3 HEAT DILUTION TEST

3.1 Introduction

For a hydrogeological characterisation of a test volume in crystalline rock, knowledge of the location of flow zones and their hydraulic properties is essential. Most significant flow zones in crystalline rocks are expected in brittle faults, damage zones and individual joints as they act as preferred water conducting structures (Brace, 1980; Barton et al., 1995;

Evans et al., 1997; Caine and Forster, 1999). Often such knowledge is gained by drilling boreholes penetrating the fractured rock and characterizing it by various classical tests as mentioned in the introduction (see Chapter 1.1). Of the various borehole methods to determine flow, new methods have been developed in recent years in which heat is used as a tracer. Since the first applications (Selker et al., 2006; Pehme et al., 2010; Leaf et al., 2012;

Klepikova et al., 2014; Bense et al., 2016; Read, 2016), these methods have only been applied to boreholes drilled from the surface. In this study we apply heat dilution tests to inclined boreholes of up to 300 m length, which have been drilled from the underground lab BULG with an overburden of about 1’000 m. As discussed in Section 2.3 the hydraulic gradients and ambient flows in such boreholes are much larger than in surface drilled boreholes.

Heat dilution tests have been carried out in the two of the characterization boreholes CB1 and CB2, whereby the water inside the borehole was actively heated and the change in temperature was measured continuously with a Fibre-Optic Distributed Temperature Sensing (FO-DTS) system. In order to compare and optimize the heat dilution test two different methods, i.e. tests with a hybrid heating-monitoring cable and tests with a separate heating cable were performed in one of the boreholes. Due to the long and inclined boreholes, the tests were faced with further difficulties, for example how it is possible to lower a 300 m long hybrid cable and recover it undamaged after the measuring phase. These technical issues are also covered in this contribution.

At the beginning of this chapter the principles of distributed temperature sensing and the state of the art are introduced. Then the experiment carried out with the new devices and the corresponding setup of the two different test configurations (hybrid and dual cable) are presented. Finally, the results with the corresponding visualization of measurements are discussed and a first visual analysis is explained. The quantitative numerical analysis of the test data is presented in Chapter 4.

3.1.1 Principles of Distributed Temperature Sensing

The term Fibre-Optic Distributed Temperature Sensing refers to the measurement of temperature along a fibre optic cable using light. To determine the temperature along the fibre optic cable, a base unit/interrogator is used that emits laser pulses and measures the distance over the travel time of the light and the temperature-dependent Raman backscatter signal

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(Bense et al., 2016). Due to the inelastic interaction of the incident light with the glass crystal lattice structure, Raman backscattering is generated (Rogers, 1999). Using the ratio of the intensity of the photons returning at specifically higher (antiStokes, IaS) and specifically lower frequency (Stokes, IS), the temperature can be calculated as a function of the distance along the fibre (Farahani and Gogolla, 1999; Hausner et al., 2011). Based on the refractive index of the optical fibre, the distance to the place of scattering is calculated from the two-way propagation time of the light. The FO-DTS analyses the Stokes and anti-Stokes intensities after the emission of many repeated short laser pulses, which are integrated over predefined time and space intervals along the cable (Bense et al., 2016).

The FO-DTS offers many advantages over conventional technologies, although supporting characterization results are required for the final interpretation of the FO-DTS data (Leaf et al., 2012). The ability of FO-DTS to provide temperature profiles in a borehole without disturbing the fluid column and the ease of interpretation are advantages over conventional wired temperature loggers (Read, 2016). Therefore, FO-DTS can be used to measure temperatures along fibre optic cables rapidly and in detail (Selker et al., 2006).

3.1.2 State of the Art

The application of this technique in open boreholes is often prevented by vertical flow between permeable horizons, especially in heterogeneous and fractured rock environments (Pehme et al., 2010; Maurice et al., 2011). Even more dramatic is vertical or ambient flow in boreholes drilled from a deep underground excavation. Therefore, it is more convenient to trace the migration of a tracer along an open, flowing borehole to obtain flow rates and locate zones of inflow (Leaf et al., 2012). For a number of hydrogeological processes, heat is widely recognised as a useful tracer (Saar, 2011). The advantage of using heat compared to other tracers is the omnipresence of temperature and the possibility of measuring in-situ easily and economically (Read, 2016). On the other hand, heat is not at all a conservative tracer, and the rapid heat loss (or gain) caused by heat diffusion into or from the formation poses significant limits to this technology.

In the borehole, heat transport is influenced by advection, dispersion, free convection (buoyancy effects) and heat transfer between the borehole and the surrounding rock (Leaf et al., 2012). Since a few years, FO-DTS has become more accessible to researchers in the geosciences and allows temperatures to be measured at small intervals and over long distances along fibre optic cables (Read, 2016).

The use of simultaneous temperature monitoring and electrical heating is an evolving aspect of Active-DTS (A-DTS), where distributed temperature sensing along the fibre optic cable is applied simultaneously by a distributed heat source integrated in the same cable or by another cable installed in parallel (Read et al., 2014). Commonly available DTS cables are often already armoured with steel so that they can be used for A-DTS by passing an electric current through the metallic cable materials (Read, 2016). Fluid inflow zones as well as ambient flows in boreholes can be monitored using hybrid cable (simultaneous distributed heating and

Heat Dilution Testing in Deep Underground Excavations

Master Thesis Heat Dilution Testing

in Deep Underground Excavations

Presented at the Department of Earth Science at ETH Zurich Institute of Geology

Supervised by

Prof. Dr. Simon Löw, Geological Institute, ETH Zurich

Dr. Nima Gholizadeh Doonechaly, Geological Institute, ETH Zurich

Submitted by Matthias Meier meiermat@student.ethz.ch

14-924-476

Zurich, 16.08.2020

ABSTRACT

TABLE OF CONTENTS

1 INTRODUCTION

1.1 Motivation

1.2 Research Goals

2 SITE CHARACTERIZATION 2.1 Site Description

2.2 Geological Setting

2.3 Hydrogeological Setting

2.4 Borehole Flow and Pressure Logging During Drilling

0 5 10 15 20 25 30 35

0 5 10 15 20 25 30 35

0 1000 2000 3000 4000

F lo w [ l/ m in ]

P re ss u re [ b ar ]

Time [s]

CB1 Depth 202 m - September, 11, 2019, 07:03 pm

Pressure [bar]

Flow [l/min]

0 2 4 6 8 10

0 10 20 30 40 50

31000 36000 41000 46000

F lo w [ l/ m in ]

P re ss u re [ b ar ]

Time [s]

CB1 Depth 260 m - September, 18, 2019, 04:55 am

Pressure [bar]

Flow [l/min]

44.5 44.6 44.7 44.8 44.9 45.0 45.1 45.2 45.3

0 20000 40000 60000 80000

P re ss u re [ b ar ]

Time [s]

CB1 completed - October, 14, 2019, 09:32 am

Pressure in CB1 [bar]

3 HEAT DILUTION TEST

3.1 Introduction