Towards numerical modelling of triggered and induced seismicity D. Kühn (
[email protected]), T. Dahm , M. Wangen and S. Heimann 1
1
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3
2
NORSAR, Kjeller, Norway; 2GFZ, Potsdam, Germany; 3 IFE, Kjeller, Norway
Introduction
Human-induced earthquakes are more and more brought into the focus of public attention. Ellsworth (2013) attracts notice to the dramatic increase of the number of earthquakes in the central and eastern United States over the past few years (Fig. 1): whereas the average rate of M ≥ 3 earthquakes per year was as low as 21 in the period between 1967 and 2000, the rate increased to more than 300 earthquakes with M ≥ 3 in the period 2010 to 2012, whereof 188 events took place in 2011 alone.
Modelling to understand spatial and temporal distribution of seismicity
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What are we trying to accomplish? Time July 2011 (Days)
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r, distance from well (m)
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Fig. 2: Left: map view of relocated seismicity occurring during stimulation of an EGS in Paralana, Australia, coloured according to origin time; black square: well head, red square: injection point at depth. Right: r-t plot illustrating distance of events from injection point (both figures taken from Albaric et al., 2013)
3-D FEM hydrofracture model Fig. 1: Cumulative number of earthquakes with M ≥ 3 in Central and Eastern United States from 1967 to 2012; dashed line: long-term rate of 21.2 EQ/a; inset: geographical distribution (figure after Ellsworth 2013, taken from www.usgus.gov/research/induced) Infamous examples: - Wilmington oil field (California): 8.8 m subsidence accompanied by damaging earthquakes between 1947 and 1961 (Kovach, 1974) - Gazli gas field (Uzbekistan): two magnitude 7 earthquakes under discussion to be triggered by production in 1976 and 1984 (Simpson and Leigh, 1985) - Waste water injection induced 1967 Mw 4.8 event close to Denver (Healey et al., 1986) and potentially, the 2011 Mw 5.7 central Oklahoma earthquake (Kerenan et al., 2013) - Impoundment of water reservoirs causing earthquakes with magnitudes > 6 (Gupta, et al., 1972): Koyna (India), Kariba (Zimbabwe), Kremasta (Greece) - Mining: mb 5.5 rock burst in potash mine near Völkershausen (Germany; Knoll, 1990) - Basel (Switzerland): EGS stopped after 2006 M 3.4 event, leading to 2300 damage claims (Aegerter and Bosshardt, 2007) - Groningen (Netherlands): 16 Aug 2012 M 3.6 gas-field induced event
Induced, triggered and natural seismicity Definition after Dahm et al. (2012): - induced earthquake: entirely controlled by human-induced stress change - triggered earthquake: human-induced stress change causes only event nucleation, but not whole failure process Until now, the discrimination between natural, triggered and induced earthquakes is challenging and no clear rules or scientific methods are established (Dahm et al. 2010a).
=Rock
=Fracture
Fits for: hydrofractures in high permeability formations/low injection pressure (leak-off) - based on poroelastic Biot equations (decoupled for pressure and displacement) - fracture criterion: strain at element centres - initial stress field (gravity) & additional stress field due to pressure build-up (injection) see Wangen (2013)
=Fractured element
Fig. 3: Top: FEM representation of fracture and rock matrix; top and bottom right: fracture developing in homogeneous/heterogeneous material, respectively
2-D analytical hydrofracture model
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Why bother? - Occurrence near engineering activities and shallow depth: even earthquakes of small magnitude may cause damage and concern among public - Fluid pressure effects from injection operations may extend well beyond the range of actual fluid migration (Nicholson and Wesson, 1992) - Harmless small earthquakes may deliver information on larger, potentially damaging earthquakes (McGarr et al., 2002) - Indications as to the changes in hydromechanics that triggered them (McGarr et al., 2002) Provided that the fundamental processes are understood!
}
long wing
} short wing
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hydrofrac length
Fits for: hydrofractures in low permeability formations/high injection pressure (sealed) - based on elementary crack solutions - Griffith fracture criterion (stress intensity factor) - effect of stress and pore pressure gradients on fracture growth - 1-D laminar viscous flow within fracture - combined with Dieterich (1994) rate-and-state seismicity model see Dahm et al. (2010b; 2012)
Fig. 4: Left: time-distance plot showing asymmetrical bi- and unidirectional fracture growth during injection (grey area); top: observed time-dependent seismicity rate; bottom: modelled rate (density plots)
Elementary Green’s functions
Acknowledgements
This work was supported by the Norwegian Research Council (SafeCO2, project no. 189994) and by the industry partners of the SafeCO2 project (Statoil, Lundin, Octio, and READ).
References
Aegerter, A. and Bosshardt, O. [2007]. Schadenfälle Erdbeben vom 8.12.2006, Perimeterdefinition. Technischer Bericht, 13.07.2007, Ingenieurbüro A. Aegerter & Dr. O. Bosshardt AG, Basel, Möhlin. Albaric, J., Oye, V., Langet, N., Hasting, M., Lecomte, I., Iranpour, K., Messeiller, M., Reid and P. [2013]. Monitoring of induced seismicity during the first geothermal reservoir stimulation at Paralana, Australia. Geothermics, in press, doi:10.1016/j.geothermics.2013.10.013. Dahm, T., Becker, D., Bischoff, M., Cesca, S., Dost, B., Fritschen, R., Hainzl, S., Klose, C.D., Kühn, D., Lasocki, S., Meier, T., Ohrnberger, M., Rivalta, E., Wegler, U. and Husen, S. [2012]. Recommendation for the discrimination of human-related and natural seismicity. J. Seismol., doi: 10.1007/s10950-012-9295-6 Dahm, T., Hainzl, S. and Fischer, T. [2012]. Stimulation induced seismicity: fracture combined with rate-and-state model. 3rd Annual Meeting AIM, 10-12 Oct 2012, Smolenice, Slovakia (Abstract). Dahm, T., Hainzl, S., Becker, D., Bischoff, M., Cesca, S., Dost, R., Fritschen, R., Kühn, D., Lasocki, S., Klose, C.D., Meier, T., Ohrnberger, M., Rivalta, R., Shapiro, S. and Wegler, U. [2010a]. How to discriminate induced, triggered and natural seismicity. In: Ritter, J. and Oth, A. (Eds.) Proceedings of the workshop “Induced seismicity”, Nov. 1-17, 2010, Luxembourg, Cahier du Centre Europeen de Geodynamique et de Sismologie, 30, 69-76. Dahm, T., Hainzl, S. and Fischer, T. [2010b]. Bidirectional and unidirectional fracture growth during hydrofracturing: role of driving stress gradients. J. Geophys. Res., 115(B12322). Dieterich, J. [1994]. A constitute law for rate of earthquake production and its application for earthquake clustering. J. Geophys. Res., 99, 2601-2618. Ellsworth, W.L. (2013). Injection-induced earthquakes. Science, 341, 1225942, doi:10.1126/science.1225942. Gupta, H.K., Rastogi, B.K. and Narain, H. [1972]. Some discriminatory characteristics of earthquakes near the Kariba, Kremasta, and Koyna artificial lakes. Bull. Seismol. Soc. Am., 62(2), 493-507. Healy, J.H., Rubey, W., Griggs, D.T. and Raleigh, C.B. [1968]. The Denver Earthquakes. Science, 161, 1301-1310. Kerenan, K.M., Savage, H.M., Abers, G.A. and Cochran, E.S. [2013]. Potentially induced earthquakes in Oklahoma, USA: links between wastewater injection and the 2011 Mw 5.7 earthquake sequence. Geology, 41, 699–702. Kovach, R.L. [1974]. Source mechanisms for Wilmington oil field, California, subsidence earthquakes. Bull. Seismol. Soc. Am., 64, 699-711. McGarr, A., Simpson, D., and Seeber, L. (2002). Case histories of induced and triggered seismicity. In: Int. Handbook of Earthquake and Eng. Seismol., 81(a), 647-661. Nicholson, C. and Wesson, R.L. [1992]. Triggered earthquakes and deep well activities. Pure Appl. Geophys., 139(3/4), 561-578. Simpson, D.W. and Leith, W. [1985]. The 1976 and 1984 Gazli, U.S.S.R. earthquakes – were they induced? Bull. Seismol. Soc. Am., 75, 1465-1468. Wang, R. and Kümpel, H.-J. [2003]. Poroelasticity: efficient modelling of strongly coupled, slow deformation processes in a multilayered half-space. Geophysics, 68(2), 705-717. Wangen, M. [2013]. Finite element modeling of hydraulic fracturing in 3D. Comp. Geosci., 17(4), 647-659.
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Fig. 5: Left: representation of irregular geometries by elementary Green’s functions; right: testing summation of elementary Green’s functions: pore pressure after 10 s and 3000 s for line source (top), line source assembled from 4 elementary sources (middle) and line source assembled from 13 elementary sources (bottom)
Fits for: fast simulation of a large range of different hydrofrac-seismicity scenarios to better constrain hydrofrac kinematics -time-dependent elementary Green’s functions for deformation of porous rock, diffusion of pressure and fluids, stress changes (POEL; Wang and Kümpel, 2003) - implementation of routines to compute, store, retrieve and combine Green’s functions: Pyrocko framework (http://emolch.github.com/pyrocko) within Kiwi Tools (http://kinherd.org/)
