Interaction between Hydraulic and Natural Fractures in Enhanced Geothermal Systems (EGS) in Crystalline Rocks

Introduction: The power generation with Enhanced Geothermal Systems (EGS) is one of the proposed renewable sources of energy in this era that global warming poses an alarming threat to the environment. 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. Due to a high record of induced seismicity in multiple geothermal sites around the world (e.g. South Korea, Indonesia, Philippines, Japan, Kenya, North and South America, Australia, New Zealand and Switzerland) man-made earthquakes caused by hydraulic stimulation have become a public concern (Guglielmi et al 2015). On the other hand, an effective solution to controlling the induced seismicity, which can be applied with confidence, does not exist yet. The interaction between hydraulic fracture and preexisting natural fractures is a very common mechanism happening during hydraulic stimulation of enhanced geothermal systems and has been considered as the main mechanism responsible in increasing the productivity of the shale gas and EGS reservoirs (Mayerhofer et al. 2010). Upon intersection, the hydraulic fracture may cross, divert or be arrested by the natural fracture (Figure 1).

Figure 1: Different scenarios for the intersection between hydraulic and natural fractures, a) crossing the natural fracture; (b) arresting by the natural fracture and propagating off the end; (c) deflecting into the natural fracture and kinking off prior to reaching the end (Wu & Olson 2014).

It has been shown that differences of horizontal principle stress and fault angle are the most important factors controlling the interaction between hydraulic and natural fractures (Blanton 1982, Warpinski and Teufel 1987). Under high differential stress conditions and high angle of approach, hydraulic fractures cross pre-existing fractures. At intermediate and low differential stress and angles approaching the pre-existing fracture direction, the hydraulic fractures open the pre-existing fracture and divert the fracturing fluid or arrest propagation of the hydraulic fracture (Zhou et al 2008).

Natural fractures and faults in nature are usually filled with granular gouge materials that are created due to erosion and fragmentation. Olson et al (2012) stated that natural fractures may also be cemented (or healed) with calcite or quartz cements. This means that the natural fracture can be a fresh joint, a gouged fault or a cemented interface (Figure 2).

Figure 2: A core from Grimsel Test side containing natural fresh, quartz-cemented and

gouged filled fractures (Inj_001, Box012_Dry, Depth: 33.050-35.963) Objectives: Although several experimental and numerical studies have investigated these scenarios, still the induced seismicity and partitioning of the seismic sources during intact rock fracturing and hydraulic shearing along the fault are not clear, especially for crystalline rocks. The objectives of the proposed research are as follows:

1-­‐ To observe how hydraulic fracture interacts with fresh, gouge filled and quartz-cemented natural fractures

2-­‐ To understand how seismic properties (e.g., rate, magnitude, energy) change during hydraulic fracture initiation in the intact rock and how they change during slip initiation on the fault surface

3-­‐ To investigate source locations and focal mechanisms (shear, tensile, mixed) of the seismic signals during hydraulic fracturing in the intact rock and hydraulic shearing at the fracture surface

Methodology: In this MSc research, the interaction between hydraulic and natural fractures will be investigated experimentally and also numerically (depending on the time and interest of the candidate) in crystalline rocks from Bedretto Project. Fresh and quartz-cemented natural rock fractures (if there is any) from Bedretto research laboratory project for enhanced geothermal systems (EGS) are gathered to be tested in the laboratory. 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 fresh 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 3c). 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.

Figure 3: a) schematic representation of the experimental setup for a) hydraulic fracturing

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

and hydraulic shearing (current MSc proposal for MSc students of 2019-2020) References: 1. Blanton TL. An experimental study of interaction between hydrau- lically induced

and pre-existing fractures. SPE 10847, presented at the SPE/DOE unconventional gas recovery symposium, Pittsburgh, 16–18 May 1982.

2. Olson JE, Bahorich B, Holder J (2012) Examining hydraulic fracture—natural fracture interaction in hydrostone block ex- periments. In: SPE hydraulic fracturing technology conference, Society of Petroleum Engineers

3. Warpinski NR, Teufel LW. Influence of geologic discontinuities on hydraulic fracture propagation. J Petrol Technol 1987;February: 209–20.

4. Wu, K. & Olson, J. E. 2014. Mechanics Analysis of Interaction Between Hydraulic and Natural Fractures in Shale Reservoirs. Unconventional Resources Technology Conference (URTeC), Denver, Colorado, USA

5. Zhou J, M. Chen, Y. Jin, G.Q. Zhang. 2008. Analysis of fracture propagation behavior and fracture geometry using a tri-axial fracturing system in naturally fractured reservoirs. J Rock Mech Min Sci, 45, pp. 1143-1152

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

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