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.
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
Results and discussion
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.
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