How to Reduce Fluid-Injection-Induced Seismicity

  • Arno Zang
  • Günter Zimmermann
  • Hannes Hofmann
  • Ove Stephansson
  • Ki-Bok Min
  • Kwang Yeom Kim
Original Paper
  • 26 Downloads

Abstract

The recent growth in energy technologies and the management of subsurface reservoirs has led to increased human interaction with the Earth’s crust. One consequence of this is the overall increase of anthropogenic earthquakes. To manage fluid-injection-induced seismicity, in this study, we propose to use an advanced fluid-injection scheme. First, long-term fluid-injection experiments are separated from short-term fluid-injection experiments. Of the short-term experiments, enhanced geothermal systems stimulations have shown a higher propensity to produce larger seismic events compared to hydraulic fracturing in oil and gas. Among the factors discussed for influencing the likelihood of an induced seismic event to occur are injection rate, cumulative injected volume, wellhead pressure, injection depth, stress state, rock type, and proximity to faults. We present and discuss the concept of fatigue hydraulic fracturing at different scales in geothermal applications. In contrast to the conventional hydraulic fracturing with monotonic injection of high-pressure fluids, in fatigue hydraulic fracturing, the fluid is injected in pressure cycles with increasing target pressure, separated by depressurization phases for relaxing the crack tip stresses. During pressurization phases, the target pressure level is modified by pulse hydraulic fracturing generated with a second pump system. This combination of two pumps with multiple-flow rates may allow a more complex fracture pattern to be designed, with arresting and branching fractures, forming a broader fracture process zone. Small-scale laboratory fluid-injection tests on granite cores and intermediate-scale fluid-injection experiments in a hard rock underground test site are described. At laboratory scale, cyclic fluid-injection tests with acoustic emission analysis are reported with subsequent X-ray CT fracture pattern analysis. At intermediate scale, in a controlled underground experiment at constant depth with well-known stress state in granitic rock, we test advanced fluid-injection schemes. The goal is to optimize the fracture network and mitigate larger seismic events. General findings in granitic rock, independent of scale, are summarized. First, the fracture breakdown pressure in fatigue hydraulic testing is lower than that in the conventional hydraulic fracturing. Second, compared to continuous injection, the magnitude of the largest induced seismic event seems to be systematically reduced by cyclic injection. Third, the fracture pattern in fatigue testing is different from that in the conventional injection tests at high pressures. Cyclic fracture patterns seem to result from chiefly generated low energy grain boundary cracks forming a wider process zone. Fourth, cyclic injection increases the permeability of the system. A combination of cyclic progressive and pulse pressurization leads to the best hydraulic performance of all schemes tested. One advantage of fatigue testing is the fact that this soft stimulation method can be applied in circumstances where the conventional stimulation might otherwise be abandoned based on site-specific seismic hazard estimates.

Keywords

Crack tip stresses Fluid-injection-induced seismicity Fracture process zone Hydraulic fracturing 

Abbreviations

CHF

Cyclic hydraulic fracturing

EGS

Enhanced geothermal system

FBP

Fracture breakdown pressure

FHF

Fatigue hydraulic fracturing

FIP

Fracture initiation pressure

FPP

Fracture propagation pressure

FPZ

Fracture process zone

ISRM

International Society for Rock Mechanics and Rock Engineering

PHF

Pulse hydraulic fracturing

RFP

Refrac pressure or reopening pressure

ISIP

Instantaneous shut-in pressure

dc/dt

Crack growth rate

dc/dN

Crack growth per cycle

K or KI

Stress intensity factor

Pp

Pore pressure

SV

Vertical stress

Sh

Minimum horizontal stress

SH

Maximum horizontal stress

T0

Tensile strength

Q

Flow rate

ΔPf

Fluid pressure

Vinj

Fluid volume injected

Notes

Acknowledgements

The in situ experiment at Äspö Hard Rock Laboratory (HRL) was supported by the GFZ German Research Center for Geosciences (75%), the KIT Karlsruhe Institute of Technology (15%), and the Nova Center for University Studies, Research and Development Oskarshamn (10%). We thank Gerd Klee, MeSy Solexperts, and Hana Semikova, ISATech Ltd for performing the hydraulic fracturing experiments and Göran Nilsson, GNC for arranging the diamond drillings. We thank Katrin Plenkers and Thomas Fischer (GMuG) for the implementation of acoustic emission sensors and accelerometer, and for managing the continuous and triggered recording system during the hydraulic fracturing experiments. An additional in-kind contribution of the Swedish Nuclear Fuel and Waste Management Co (SKB) for using Äspö HRL as test site for geothermal research is greatly acknowledged. Hannes Hofmann is currently funded by the EU Horizon 2020 project DESTRESS (Grant agreement No. 691728). Ki Bok Min is supported by the Korea-EU Joint Research Support Program of the National Research Foundation of Korea (NRF) through a grant funded by the Korean Government’s Ministry of Science, ICT and Future Planning (No. NRF-2015 K1A3A7A 03074226). We like to thank Graeme Weatherill for commenting an earlier version of this manuscript. Finally, we like to acknowledge the constructive comments and valuable input of two anonymous reviewers and experts in the field of hydraulic fracturing.

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Copyright information

© Springer-Verlag GmbH Austria, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Section 2.6 Seismic Hazard and Risk DynamicsGFZ—German Research Centre for GeosciencesPotsdamGermany
  2. 2.Section 6.2 Geothermal Energy SystemsGFZ—German Research Centre for GeosciencesTelegrafenberg, PotsdamGermany
  3. 3.Department of Energy Resources Engineering and Research Institute of Energy ResourcesSeoul National UniversitySeoulSouth Korea
  4. 4.Korea Institute of Civil Engineering and Building Technology (KICT)GoyangSouth Korea

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