- Research Article
- Open Access
Induced seismicity in EGS reservoir: the creep route
© Schmittbuhl et al.; licensee Springer. 2014
- Received: 26 June 2014
- Accepted: 20 October 2014
- Published: 16 December 2014
Observations in enhanced geothermal system (EGS) reservoirs of induced seismicity and slow aseismic slip ruptures on related faults suggest a close link between the two phenomena.
We base our approach on the case study of the EGS site of Soultz-sous-Forêts where seismicity has been shown in particular during the 1993 stimulation to be induced not only by fluid pressure increase during stimulation but also by aseismic creeping effects. We propose an interpretation of the field observations of induced seismicity using a laboratory experiment that explores, in great detail, the deformation processes of heterogeneous interfaces in the brittle-creep regime. We track the evolution of an interfacial crack over 7 orders of magnitude in time and 5 orders of magnitude in space using optical and acoustic sensors.
We show that a creep route for induced seismicity is possible when heterogeneities exist along the fault. Indeed, seismic event occurrences in time and space are in strong relation with the development of the aseismic motion recorded during the experiments. We also infer the statistical properties of the organization of the seismicity that shows strong space-time clustering.
We conclude that aseismic processes might drive seismicity besides the classical effects related to fluid pressure and show that a creep route for induced seismicity is possible.
- Induced seismicity
- Fault asperities
- Asperity pinning
The exploitation of a geothermal reservoir can lead to the occurrence of an abundant seismicity, particularly during phases of hydraulic stimulation. This seismicity has a strong societal impact, as it can be felt by the population, especially in densely populated area as in Europe, e.g., M L =3.4, Basel, 2006 ([Häring et al. 2008]). Reducing this impact is thus one of the main challenges in the development of geothermal energy production. In order to understand the mechanical processes responsible for seismic activity, all possible mechanisms for induced seismicity have to be studied ([Majer et al. 2007]). Among them the link between earthquakes and aseismic deformations is only at a starting state ([Cornet et al. 2007]; [Bourouis and Bernard 2007]). Deciphering this link between seismicity and aseismic motion can not only help to mitigate the risk posed by seismicity but also help to monitor and model the evolution of the geothermal reservoir. The mechanism relating earthquakes and aseismic processes is still elusive due to the difficulty of imaging these phenomena with large spatiotemporal variability at depth. However, a good example of joint seismic and aseismic deformation has been obtained at the enhanced geothermal system (EGS) site of Soultz-sous-Forêts (France) ([Genter et al. 2010]) in particular during the 1993 water injection experiment for stimulation of the naturally fractured granite reservoir below 2,500 m.
Fluid-induced seismicity at Soultz-sous-Forêts
Aseismic slip at Soultz-sous-Forêts
Micro-seismicity during aseismic slip at Soultz-sous-Forêts
([Bourouis and Bernard 2007]) re-explored 10 years after these outstanding observations trying to conduct a fine analysis of the micro-seismicity during the injection period. They used a multiplet approach and relocated up to 400 events within the fault zone where aseismic slip was observed. The location accuracy was of the order of 1 m and they obtained 30 multiplets or families of similar events within the fault zone. An interesting observation is that the rupture size for all these events is of the order of d=10 m. From their observations, they inferred three important conclusions. First, events within a multiplet were located within the same rupture zone which showed that the same asperity was reloaded and broken several times during the injection period. The second conclusion is that several asperities along the fault were ruptured at the same time during the injection. Third, the cumulative slip at each asperity through several ruptures was consistent with the borehole offset measurement. The conclusion is a clear image of the fault behavior during loading: the fault undergoes a large aseismic slip which triggers multiple local asperity failures.
The goal of the present paper is to propose a mechanical model of this fault behavior. To do so, we developed an experimental approach to mimic the response of a single fault when submitted to a global slow rupture propagation but locally unstable at asperities where micro-seismic activity is triggered. The model is analogous and incorporates a large space and time dynamics: 7 orders of magnitude in time and 5 orders of magnitude in space. A numerical approach would be difficult with such a large range of timescales and wavelengths ([Kaneko et al. 2010]).
An analogous fault model with random asperities
Samples are made of transparent Plexiglas which provides optical access to the rupture propagation. The analogous fault model is obtained by annealing two plates of 20×10×1 cm3 and 23×2.8×0.5 cm3 at 190°C for 45 min which is significantly above the glass transition of the material but below the melting point. Under normal load, the two plates get in close contact and stick together along a relatively weak plane (weaker than the bulk) ([Schmittbuhl and Måløy 1997]). The goal is to study the collective behavior of multiple asperities when a slow crack propagates and locally pins. For this, we sandblast one of the plates before the annealing procedure to induce random local toughness fluctuations ([Lengliné et al. 2011b]). Sandblasting is obtained using (180 to 300 μ) particles, an air pressure of 3 bars for 4 min at a blowing distance of 20 to 40 cm ([Grob et al. 2009]).
A subcritical rupture propagation
Optical monitoring of the fracture front
Acoustic emissions during fracture propagation
Combining aseismic and seismic events
An imposed loading perturbation To go one step further in understanding the link between acoustic emission and local creep events, we took advantage of the experimental configuration to introduce a perturbation in the loading and look for the response of the system. Figure 4 shows how this perturbation is introduced in the loading rate. We observed that the average position of the fracture front follows exactly the perturbation of the loading frame after a small shift in time. The force decreases as the front velocity increases which is a velocity weakening effect.
An interesting observation is that local velocities are distributed as a unique power law on a broad range of velocities (more than 3 orders of magnitude): P(v)∝v−2.55 ([Måløy et al. 2006]), suggesting that most of the energy is dissipated by creep events consistently with observations from ([Gross et al. 1993]) who showed that only 3% of the fracture energy is radiated and from ([Kanamori and Anderson 1975]) for earthquake dynamics where the radiated energy is always negligible in front of the seismic moment (M0/E≈1/20,000, where M0 is the seismic moment describing the total dissipated energy and E the radiated energy).
Observations at the Soultz-sous-Forêts EGS site of aseismic slips that are synchronous with micro-seismic events located in multiplets ([Bourouis and Bernard 2007]; [Cornet et al. 1997]) suggest that both processes co-exist within the same fault zones during a fluid injection period. Here we proposed an experimental model that directly supports this observation. Our model is built on the interfacial failure along a heterogeneous weak plane and combines creep failure and brittle rupture without fluid. Experimental observations are numerous and consistent with large-scale measurements. They provide clear hints on the processes involved at the asperity scale. Our conclusion is therefore that fluids are not necessarily the driving force of the fault activity in terms of pore pressure. They might have rather a role on local creep acceleration because of their effect on subcritical crack growth. An interesting perspective would be to estimate the evolution of the permeability owing to aseismic slip.
We thank J. Elkhoury, J.P. Ampuero, R. Toussaint, A. Cochard, M. Bouchon, H. Karabulut, K.J. Maloy, A. Stormo, A. Hansen, M. Grob, and G. Daniel for fruitful discussions; A. Steyer for technical support; and two anonymous reviewers for very constructive comments. This work has been published under the framework of the LABEX ANR-11-LABX-0050-G-EAU-THERMIE-PROFONDE and benefits from a funding from the state managed by the French National Research Agency as part of the ‘Investments for the Future’ program.
- Abercrombie R, Leary P: Source parameters of small earthquakes recorded at 2.5 km depth, Cajon Pass, Southern California: implications for earthquake scaling. Geophys Res Lett 1993, 20(14):1511–1514. 10.1029/93GL00367View ArticleGoogle Scholar
- Bourouis S, Bernard P: Evidence for coupled seismic and aseismic fault slip during water injection in the geothermal site of Soultz (France), and implications for seismogenic transients. Geophys J Int 2007, 169(2):723–732. 10.1111/j.1365-246X.2006.03325.xView ArticleGoogle Scholar
- Burov EB: Plate rheology and mechanics. In Watts AB (ed) Crust and lithosphere dynamics. Treatise on geophysics. Elsevier, Amsterdam; 2009:99–151.Google Scholar
- Charléty J, Cuenot N, Dorbath L, Dorbath C, Haessler H, Frogneux M: Large earthquakes during hydraulic stimulations at the geothermal site of Soultz-sous-Forêts. Int J Rock Mech Min Sci 2007, 44(8):1091–1105. 10.1016/j.ijrmms.2007.06.003View ArticleGoogle Scholar
- Cornet F, Bérard T, Bourouis S: How close to failure is a granite rock mass at a 5 km depth. Int J Rock Mech Min Sci 2007, 44(1):47–66. 10.1016/j.ijrmms.2006.04.008View ArticleGoogle Scholar
- Cornet FH, Helm J, Poitrenaud H, Etchecopar A: Seismic and aseismic slips induced by large-scale fluid injections. Pure Appl Geophys 1997, 150: 563–583. 10.1007/s000240050093View ArticleGoogle Scholar
- Cuenot N, Dorbath C, Dorbath L: Analysis of the microseismicity induced by fluid injections at the EGS site of Soultz-sous-Forêts (Alsace, France): implications for the characterization of the geothermal reservoir properties. Pure Appl Geophys 2008, 165(5):797–828. 10.1007/s00024-008-0335-7View ArticleGoogle Scholar
- Dorbath L, Cuenot N, Genter A, Frogneux M: Seismic response of the fractured and faulted granite of Soultz-sous-Forêts (France) to 5 km deep massive water injections. Geophys J Int 2009, 177(2):653–675. 10.1111/j.1365-246X.2009.04030.xView ArticleGoogle Scholar
- Genter A, Castaing C, Dezayes C, Tenzer H, Traineau H, Villemin T: Comparative analysis of direct (core) and indirect (borehole imaging tools) collection of fracture data in the hot dry rock Soultz reservoir (France). J Geophys Res Solid Earth 1997, 102(B7):15419–15431. 10.1029/97JB00626View ArticleGoogle Scholar
- Genter A, Evans K, Cuenot N, Fritsch D, Sanjuan B: Contribution of the exploration of deep crystalline fractured reservoir of Soultz to the knowledge of enhanced geothermal systems (EGS). Comptes Rendus Geoscience 2010, 342(7):502–516. 10.1016/j.crte.2010.01.006View ArticleGoogle Scholar
- Grob M, Schmittbuhl J, Toussaint R, Rivera L, Santucci S, løy KJM: Quake catalogs from an optical monitoring of an interfacial crack propagation. PAGEOPH 2009, 166(5–7):777–799. 10.1007/s00024-004-0496-zView ArticleGoogle Scholar
- Gross SP, Fineberg J, Marder M, McCormick W, Swinney HL: Acoustic emissions from rapidly moving cracks. Phys Rev Lett 1993, 71(19):3162. 10.1103/PhysRevLett.71.3162View ArticleGoogle Scholar
- Häring MO, Schanz U, Ladner F, Dyer BC: Characterisation of the basel 1 enhanced geothermal system. Geothermics 2008, 37(5):469–495. 10.1016/j.geothermics.2008.06.002View ArticleGoogle Scholar
- Helm, J (1996) The natural seismic hazard and induced seismicity of the european HDR (hot dry rock) geothermal energy project at Soultz-sous-Forêts, France. PhD thesis.Google Scholar
- Huenges E, Ledru P: Geothermal energy systems: exploration, development, and utilization. John Wiley & Sons, Weinheim; 2010.View ArticleGoogle Scholar
- Jaeger JC, Cook NG, Zimmerman R: Fundamentals of rock mechanics. Backwell Publishing, Australia; 2009.Google Scholar
- Kanamori H, Anderson DL: Theoretical basis of some empirical relations in seismology. Bull Seismol Soc Am 1975, 65(5):1073–1095.Google Scholar
- Kaneko Y, Avouac J-P, Lapusta N: Towards inferring earthquake patterns from geodetic observations of interseismic coupling. Nat Geosci 2010, 3(5):363–369. 10.1038/ngeo843View ArticleGoogle Scholar
- Lengliné, O, Toussaint R, Schmittbuhl J, Elkhoury JE, Ampuero J, Tallakstad KT, Santucci S, Måløy KJ (2011a) Average crack-front velocity during subcritical fracture propagation in a heterogeneous medium. Phys Rev E 84(3): 036104.Google Scholar
- Lengliné, O, Schmittbuhl J, Elkhoury J, Ampuero J-P, Toussaint R, Måløy KJ (2011b) Downscaling of fracture energy during brittle creep experiments. J Geophys Res Solid Earth 116(B8).Google Scholar
- Lengliné O, Elkhoury J, Daniel G, Schmittbuhl J, Toussaint R, Ampuero J-P, Bouchon M: Interplay of seismic and aseismic deformations during earthquake swarms: an experimental approach. Earth Planet Sci Lett 2012, 331: 215–223. 10.1016/j.epsl.2012.03.022View ArticleGoogle Scholar
- Majer EL, Baria R, Stark M, Oates S, Bommer J, Smith B, Asanuma H: Induced seismicity associated with enhanced geothermal systems. Geothermics 2007, 36(3):185–222. 10.1016/j.geothermics.2007.03.003View ArticleGoogle Scholar
- Måløy KJ, Santucci S, Schmittbuhl J, Toussaint R: Local waiting time fluctuations along a randomly pinned crack front. Phys Rev Lett 2006, 96(4):045501. 10.1103/PhysRevLett.96.045501View ArticleGoogle Scholar
- Sausse J, Dezayes C, Dorbath L, Genter A, Place J: 3d model of fracture zones at Soultz-sous-Forêts based on geological data, image logs, induced microseismicity and vertical seismic profiles. Comptes Rendus Geoscience 2010, 342(7):531–545. 10.1016/j.crte.2010.01.011View ArticleGoogle Scholar
- Schmittbuhl J, Delaplace A, Maloy KJ, Perfettini H, Vilotte JP: Slow Crack Propagation and Slip Correlations. Pure Appl Geophys 2003, 160: 961–976. 10.1007/PL00012575View ArticleGoogle Scholar
- Schmittbuhl J, Måløy KJ: Direct observation of a self-affine crack propagation. Phys Rev Lett 1997, 78(20):3888. 10.1103/PhysRevLett.78.3888View ArticleGoogle Scholar
- Shapiro SA, Audigane P, Royer J-J: Large-scale in situ permeability tensor of rocks from induced microseismicity. Geophys J Int 1999, 137(1):207–213. 10.1046/j.1365-246x.1999.00781.xView ArticleGoogle Scholar
- Vilarrasa V, Carrera J, Olivella S: Hydromechanical characterization of co 2 injection sites. Int J Greenhouse Gas Control 2013, 19: 665–677. 10.1016/j.ijggc.2012.11.014View ArticleGoogle Scholar
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