Non–double‐couple mechanisms of microearthquakes induced by hydraulic fracturing

Abstract

[1] We have inverted polarity and amplitude information of representative microearthquakes to investigate source mechanisms of seismicity induced by hydraulic fracturing in the Carthage Cotton Valley, east Texas, gas field. With vertical arrays of four and eight three-component geophones in two monitoring wells, respectively, we were able to reliably determine source mechanisms of the strongest events with the best signal-to-noise ratio. Our analysis indicates predominantly non-double-couple source mechanisms with positive volumetric component consistent with opening cracks oriented close to expected hydraulic fracture orientation. Our observations suggest the induced events are directly the result of opening cracks by fluid injection, in contrast to many previous studies where the seismicity is interpreted to be primarily shearing caused by pore pressure diffusion into the surrounding rock or associated with shear stresses created at the hydraulic fracture tip.

On the link between stress field and small-scale hydraulic fracture growth in anisotropic rock derived from microseismicity

“…Double-couple mechanisms are also observed during HF treatments (e.g., Chitrala et al, 2013;Ishida, 2001;Dahm et al, 1999), although the primary dislocation mechanism during HF is thought to be tensile fracturing (i.e., propagation in mode I or opening mode). Detailed moment tensor analyses of the seismic waveforms have shown that most induced events involve a predominant double-couple mechanism with relatively few indicating an occasionally strong tensile component (Horálek et al, 2010;Guilhem et al, 2014;Šílený et al, 2009;Martínez-Garzón et al, 2017). The widespread observation of dominant shear source characteristics of HF-V. S. Gischig et al: Stress field, hydrofractures and microseismicity 41 induced microseismicity has been explained by fluid leak-off into small pre-existing fractures (Dusseault et al, 2011).…”

Focal mechanism determination of induced microearthquakes in an oil field using full waveforms from shallow and deep seismic networks

“…The focal mechanism can be represented by a 3 by 3 second order moment tensor with six independent components (Aki & Richards, 2003). Here we assume the focal mechanism of the small induced events can be represented by pure double couples (Rutledge & Phillips, 2002), though it is possible that a volume change or Compensated Linear Vector Dipoles (CLVD) part may also exist, especially in hydraulic fracturing cases, and the nondouble-couple components are informative for understanding the rock failure under highpressure fluid (Ross & Foulger, 1996;Jechumtálová & Eisner, 2008;Šílený et al, 2009;Song & Toksoz, 2010). The constraining of focal mechanism as double couple (DC) can eliminate the spurious non-DC components in the inversion raised by modeling the wave propagation in anisotropic medium with isotropic Green's functions or inaccuracy of the velocity model (Šílený & Vavryčuk, 2002;Godano et al, 2011).…”

Earthquakes and errors: Methods for industrial applications

“…These include several geothermal and hydrocarbon reservoirs areas and EGS stimulation experiments Ross et al, 1996;Julian et al, 1997;Miller et al, 1998;Šílený et al, 2009;Baig and Urbancic, 2010;Julian et al, 2010b).…”