Fluid Injection and Fault Reactivation in Enhanced Geothermal Systems (EGS) in Crystalline Rocks

Introduction: High consumption of fossil fuels and the production of CO2 contributing to global warming have forced politicians, researchers, and engineers to seek other sources of energy that are renewable. The geothermal power generation with Enhanced Geothermal Systems (EGS) is one of the proposed renewable sources of energy for Switzerland. The technique involves three mechanisms 1) hydraulic fracturing, 2) hydraulic shearing along faults and natural fractures, and 3) a combination of hydraulic fracturing and hydraulic shearing in which the hydraulic fracture initiates and propagates first and then it activates the faults and the natural fractures. Despite considerable advancement in this area, the goal of sufficiently enhancing permeability has not yet been obtained in a sustained way and fracture induced seismicity sometimes obstructs the EGS projects, e.g. in Basel and Saint Gallen (Figure 1a). In this MSc research, the second mechanism (hydraulic shearing) will be investigated experimentally and also numerically (depending on the time and interest of the candidate). Several studies have been done to investigate the effect of stress states, pore-pressure, fluid properties, injection volume, injection rate, fault geometry, and fault roughness on the sliding mechanisms and induced seismicity of the faults during fluid injection (Frohlich et al., 2015; Horton, 2012; Kim, 2013; Brodsky and Lajoie, 2013; McGarr, 2014; Weingarten et al., 2015). The mentioned studies have shown both seismic and aseismic faulting. These contradictory results indicate that additional experimental work is required to address this issue. Objectives: In this research, we are going to conduct an experimental investigation on sliding fault activations and assess the effect of different fluid pressurization paths (monotonic, cyclic, progressive) on fault slip and associated induced seismicity in crystalline rocks (Figure 1b). We will develop a seismic–controlled injection path in which the seismicity stays lower than a predefined threshold as “traffic-light” system to stop large earthquakes from happening (Deichmann and Giardini, 2009, Ellsworth, 2013). By conducting this research, we hope to be able to answer the following questions:

1. How does shear reactivation of bare faults differ from gouged faults? 2. Is fault reactivation under fluid injection in crystalline rocks seismic or aseismic?

How do seismic signals change prior to fault slip? Can we predict the main-shock by the seismic precursors?

3. How do different fluid injection paths affect the rate and magnitude of the induced earthquake? What is the optimal injection path?

4. How do source locations and focal mechanisms (shear, tensile, mixed) of the seismic signals change under different fluid injection paths?

5. What is the upper-limit of the magnitude-detection threshold in which the failure is controllable?

Figure 1: a) map of induced earthquakes around the world (http://www.seismo.ethz.ch.), b) schematic representation of fault reactivation by fluid injection and failure in Mohr-Coulomb envelope due to reduction in effective normal stress (Scuderi and Collettini, 2018), and c) A schematic view of a fault structure (Lin & Yamashita, 2013)

Methodology Natural rock fractures from Bedretto research laboratory project for enhanced geothermal systems (EGS) are gathered to be tested in the laboratory. Faults in nature are usually filled with granular gouge materials that are created due to erosion and fragmentation (Figure 1c). Gouged specimens are also prepared by putting gouge material on the saw-cut surfaces of the intact rock. The sizes of the specimens are 63 mm (diameter) * 126 mm (height). Before and after each experiment, the surfaces of the fractures are scanned and the roughness properties are measured using the photogrammetry system at the Rock Mechanics Lab of the Engineering Geology group at ETH. The specimens are tested inside a triaxial Hoek cell (Figure 2b). The confining pressure can go up to 70 MPa using a hydraulic pump. A 2,000 kN servo controlled uniaxial press (Walter and Bai AG,

Switzerland) is used to apply axial loading to the specimens. Fluid injection is controlled with a syringe pump (Teledyne ISCO, Model 260D). The pump is used to inject fluid into the fault surface with a constant flow rate of 5 mL/min. A TraNET® EPC Continuous Data Acquisition system with 16 wideband acoustic emission (AE) sensors is used to detect induced seismicity caused by the fluid injection. The injection processes and the fault activation will be controlled by the AE system so that the level of the induced seismicity stays lower than the pre-defined seismic threshold.

Figure 2: a) schematic representation of the experimental setup for a) hydraulic fracturing

(ongoing research of MSc students of 2018-2019), b) hydraulic shearing (Current MSc proposal for MSc students of 2019-2020), and c) interaction between hydraulic fracturing

and hydraulic shearing (joint MSc proposal for MSc students of 2019-2020) In this research project, several innovations will be presented comparing to what has been done so far. In the experimental part, a continuous data acquisition without data loss will be used in comparison to the already used hit-based methods where a significant amount of the data is lost. In addition, 16 wide-band AE sensors with flat frequency response from 10 kHz to 2 MHz will be attached to the specimens. The high number of the AE sensors and their state of the art characteristics help to minimize the data loss considerably. This advanced data acquisition setup will help to interpret the fault damage mechanisms properly and to develop a new seismic “traffic-light” system based on the optimized injection path. References: - Brodsky, E., and L. Lajoie (2013), Anthropogenic seismicity rates and operational

parameters at the Salton Sea geothermal field, Science, 341(6145), 543–546. - Ellsworth, W. (2013), Injection-induced earthquakes, Science, 341(6142), 1225942. - Frohlich, C., J. I. Walter, and J. F. W. Gale (2015), Analysis of transportable array

(USARRAY) data shows earthquakes are scarce near injection wells in the Williston Basin, 2008–2011, Seism. Res. Lett., 86(2A), 492–499.

- Horton, S. (2012), Disposal of hydrofracking waste fluid by injection into subsurface aquifers triggers earthquake swarm in central Arkansas with potential for damaging earthquake, Seismol. Res. Lett., 83(2), 250–260.

- Lin, A., & Yamashita, K. (2013). Spatial variations in damage zone width along strike-

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slip faults: An example from active faults in southwest japan. J. Struct. Geol., 57, 1-15. - Kim, W.-Y. (2013), Induced seismicity associated with fluid injection into a deep well

in Youngstown, Ohio, J. Geophys. Res. Solid Earth, 118, 3506–3518. - McGarr, A. (2014), Maximum magnitude earthquakes induced by fluid injection, JGR.

Solid Earth, 119, 1008–1019. - Deichmann N, Giardini D, 2009, Earthquakes induced by the stimulation of an

enhanced geothermal system below Basel (Switzerland). Seismological Research Letters 80 (5): 784-798.

- Scuderi MM, Collettini CC, 2018. Fluid Injection and the Mechanics of Frictional Stabilityof Shale-Bearing Faults. JGR: Solid Earth, 123, 8364-8384.

- Weingarten, M., S. Ge, J. W. Godt, B. A. Bekins, and J. L. Rubinstein (2015), High-rate injection is associated with the increase in U.S. mid-continent seismicity, Science, 348(6241), 1336–1340.

Supervisor: Dr. Omid Moradian (omid.moradian@erdw.ethz.ch)