Hydraulic Characterization of Fracture Networks at the Bedretto Underground Laboratory (BUL)

Project Framework

ETH is currently constructing an underground research laboratory in the Bedretto Adit of the Furka Base Tunnel (BUL) (Figure 1) within the framework of a multi-disciplinary collaboration of the Swiss Competence Center for Energy Research, Supply of Energy (SCCER_SoE). Completion and inauguration of this multi-million test facility operated by the Department of Earth Sciences at ETH is on May 18, 2019. The first large scale experiments carried out in this lab are related to enhanced geothermal systems, and will include drilling of a large number of long (250-300m) injection and monitoring boreholes. A hydrogeological model of the test volume is a fundamental component of this multi-disciplinary project and will be developed from a combination of measurements carried out during the drilling operations, and during a special characterization phase following the drilling phase.

The first tests after site characterization include a series of hydraulic stimulation and circulation experiments related to the understanding of formation and behavior of deep geothermal reservoirs (such as Basel). The envisioned testing volume at the scale of several 100 m includes faults in a weakly fractured crystalline rock reservoir of the Rotondo granite (Figure 1). The seismo-hydro-mechanical response to various stimulation strategies will be investigated, aiming at the most effective stimulation through well-understood fault slip activation and controlled induced seismicity. The experiment will be facilitated by a network of multi-disciplinary monitoring boreholes equipped with microseismic, fiber-optics strain, water pressure, temperature, resistivity, and tilt sensors. In this context, the hydrogeological characterization is needed to provide the required data for the design, execution and evaluation of the hydraulic stimulation phase and understanding the involved physical processes.

MSc Thesis Project Goals

The goals of this MSc thesis project (which could involve 2 students) are focused on the analysis of the three-dimensional hydrogeological properties of the BUL experimental rock mass volume before stimulation. This characterization includes locations, orientations, structural types, flow and transport characteristics and interconnections of preferential groundwater flow pathways (faults and joints) in the test volume (Figure 2).

Project Tasks and Methods

The first thesis will mainly focus on the analysis of hydraulic data collected by the drilling crew during borehole drilling in 2019 (Figure 3). This data set includes locations and hydraulic properties of preferential groundwater inflows to the subsurface boreholes, and cross-hole hydraulic interactions monitored in packed-off intervals of completed boreholes in response to the during drilling of subsequent boreholes (Figure 4). The testing protocol applied will follow the operations designed for pre-drillings the Lötschberg and Gotthard Base Tunnels (Masset and Loew 2013, Pesendorfer and Loew 2007, 2010). The field data will be interpreted with analytical models used in transient pressure test analysis. The hydrogeological interpretations will by compared with structural geology interpretations from a companion MSc thesis (David Jordan) and geophysical borehole investigations. The second student would be involved in the execution and analysis of the hydrogeological testing program following the drilling phase. This testing program includes single and cross-hole packer testing, heat tracer tests and possibly cross-well tracer tests with dyes. Only a selection of these tests will be supervised and analysed by the MSc student. Analysis of cross-hole responses might also include

sophisticated numerical modelling of discrete fracture networks (Figure 5). According to the current planning, the main data sets of both MSc projects will be collected in the second half of 2019.

The following tasks are required to be carried out in this thesis 1. Mapping and characterization of discontinuities (brittle faults, ductile shear zones, mesoscale

fractures, foliations) of new drill cores from the Rotondo granite in BUL 2. Comparison of geological structures with geophysical borehole surveys 3. Interactions with drilling crew during inflow logging and pressure build-up operations. Data

collection and quality control. 4. On-site support during single and cross hole packer testing operations 5. On-site support during heat and dye tracer testing operations 6. Analysis of single-hole transient pressure or flow responses with analytical solutions. 7. Analysis of hydraulic cross-hole and fracture network responses, possibly with a discrete fracture

network model (Fracman/Mafic). This analysis might be combined with an inversion process to generate the most representative hydraulically active fracture network.

MSc Thesis Project Supervisors and Organization

Main Supervisor: Prof. Simon Löw, Co-Supervisor Dr. Nima Gholizadeh. The expenses will be covered by the SCCER-SoE project.

References

Pesendorfer,M.andLoew,S.(2007)."Detectionefficiencyoflonghorizontalpredrillingstoidentifywaterconductivestructures."Felsbau25(4):18-27.

Pesendorfer,M.andLoew,S.(2010)."Subsurfaceexplorationandtransientpressuretestingfromadeeptunnelinfracturedandkarstifiedlimestones(LötschbergBaseTunnel,Switzerland)."InternationalJournalofRockMechanicsandMiningSciences47:121-138.

Masset,O.andLoew,S.(2013).Quantitativehydraulicanalysisofpre-drillingsandinflowstotheGotthardBasetunnel(SedrunLot,Switzerland).EngineeringGeology164:50-66.

Figure 1. Geological setting of the planned Bedretto Underground Laboratory (BUL)

Figure 2: Injection and Monitoring Boreholes drilled from the BUL test cavern

Figure 3: Two drill rigs in operation at the face of the Gotthard Base Tunnel

Figure 4: Inflows mapped to pre-drillings of the Gotthard Base Tunnel

Figure 5: Example of discrete fracture network model from the Grimsel ISC Experiment (Brixel et al. 2019)