Research ArticleGEOPHYSICS

The role of aseismic slip in hydraulic fracturing–induced seismicity

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Science Advances  28 Aug 2019:
Vol. 5, no. 8, eaav7172
DOI: 10.1126/sciadv.aav7172
  • Fig. 1 Global occurrences of hydraulic fracturing–induced seismicity and potential models.

    (A) Map of global reported seismicity induced by hydraulic fracturing with Mw > 3.0 (i.e., events that can likely be felt at surface). Study area is located in the Duvernay region. (B) Three models proposed for fault activation due to hydraulic fracturing, including our new model (3) [adapted from (27)].

  • Fig. 2 Data used to derive rate-state model parameters.

    (A) Stratigraphic column from the study area (blue, limestone and dolostone; gray, shale and mudstone; orange, sandstone; pink, evaporites). (B) West-east slice through a 3D seismic volume (vertical exaggeration, 4.25; see Fig. 3 for location), showing the location of the Mw 4.1 earthquake (red star). TVD, total vertical depth from surface. Several north-south–oriented faults can be distinguished (black lines), including one coincident with the location of the earthquake. C, possible channel visible as a “bowtie” structure; SR, edge of Swan Hills Formation reef structure. (C) Histogram of seismic event locations versus depth for 20-m bins (red asterisk, Mw 4.1 earthquake; red dashed line, treatment well depth). (D) Estimated TOC + clay weight percentage derived from well log data.

  • Fig. 3 Microseismic monitoring data from a treatment associated with a Mw 4.1 earthquake.

    (A) Depth slice through the 3D seismic volume showing normalized seismic reflection amplitude at the level of the Beaverhill Lake Group (at 3386-m depth; Fig. 2), which directly underlies the Duvernay Formation (gray line, treatment well location; black solid line, profile in Fig. 2; red star, Mw 4.1 earthquake). Various structural features can be interpreted, including the edge of the Swan Hills reef structure and a series of roughly north-south lineaments. Seismic event locations are overlain, colored by depth. (B) Enlargement of (A) around the treatment well [triangles, injection stages (numbered at intervals)]. Focal mechanism of the Mw 4.1 event (27) is shown as a beach ball. Approximate orientation of maximum horizontal stress is labeled (SHmax); hydraulic fractures should grow parallel to this direction (1). (C) Depth of the seismic events versus time for the events in the immediate vicinity of the mainshock [red dashed lines, stage start times; black dotted lines, stratigraphy according to a well log ~3 km north (BhL, Beaverhill Lake; Dv, Duvernay; Irtn, Ireton); red star, Mw 4.1 earthquake].

  • Fig. 4 Foreshocks and aftershocks of the Mw 4.1 earthquake over time in west-east cross section.

    Timing of the Mw 4.1 earthquake is highlighted in red. Treatment well (red line), stage locations (black triangles), stratigraphy (black dotted lines), and estimated positions of the steeply dipping fault plane and region of increased pore pressure due to hydraulic fracturing (HF) are shown. Creep outpaces the pore pressure diffusion (PPD) front on the fault plane. The mainshock is believed to be located on the same fault plane as the other events but is slightly mislocated horizontally because of the lower frequency content.

  • Fig. 5 Displacement evolution on a vertical strike-slip fault intersecting the injection zone (Duvernay).

    (A) Snapshot of overpressure on a vertical fault intersected by hydraulic fractures at the shut-in of stage 23 (numbered black rectangles, stages 20 to 23). Model includes pre-existing overpressure field that developed during hydrocarbon maturation. (B) Evolution of effective normal stress, averaged over the along-strike fault section centered on x = 0. Dashed contours show long-term stress evolution (adjusted to the present-day depth), while solid contours show the short-term increase due to hydraulic fracturing. Stage 27, end of stage 26. (C) Fault slip shown by gray dashed lines every 10 million years (Ma; long-term geologic creep), by continuous gray lines every 5.5 hours starting from the beginning of stage 23 (accelerated creep), and by red continuous lines every 0.01 s (coseismic slip). Total hydraulic fracturing–induced aseismic slip before the earthquake nucleation shown by the shaded area. (D) Moment release as a function of time; onset of accelerated aseismic slip at the beginning of stage 23 leads to a Mw 4.1 earthquake early into stage 24.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/8/eaav7172/DC1

    Fig. S1. Broadband seismic event locations.

    Fig. S2. Uranium-to-TOC and thorium-to-clay correlations for a well ~17 km SE of the treatment wellpad.

    Fig. S3. Model of a vertical strike-slip fault intersecting the Duvernay.

    Fig. S4. Modeling of pore pressure diffusion along a vertical fault intersected by hydraulic fracture stages within the Duvernay formation.

    Fig. S5. Geologic creep of a vertical strike-slip fault intersecting the Duvernay over a 50-Ma window of reservoir pore overpressure generation.

    Fig. S6. Steady-state friction of carbonate-bearing fault accounting for flash heating at asperity contacts.

    Fig. S7. Evolution of shear stress, slip rate, and slip along the fault induced by hydraulic fracturing (encapsulated in the evolution of the fault effective stress normal in Fig. 5B) shown by continuous gray lines every 5.5 hours starting from stage 23 (accelerated creep), and by red continuous lines every 0.01 s (coseismic slip).

    Fig. S8. Numerical example of accelerated creep on a frictionally stable fault with homogeneous properties driven by 1D pore pressure diffusion from a point source of constant overpressure.

    Table S1. Completions data for each stage.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Broadband seismic event locations.
    • Fig. S2. Uranium-to-TOC and thorium-to-clay correlations for a well ~17 km SE of the treatment wellpad.
    • Fig. S3. Model of a vertical strike-slip fault intersecting the Duvernay.
    • Fig. S4. Modeling of pore pressure diffusion along a vertical fault intersected by hydraulic fracture stages within the Duvernay formation.
    • Fig. S5. Geologic creep of a vertical strike-slip fault intersecting the Duvernay over a 50-Ma window of reservoir pore overpressure generation.
    • Fig. S6. Steady-state friction of carbonate-bearing fault accounting for flash heating at asperity contacts.
    • Fig. S7. Evolution of shear stress, slip rate, and slip along the fault induced by hydraulic fracturing (encapsulated in the evolution of the fault effective stress normal in Fig. 5B) shown by continuous gray lines every 5.5 hours starting from stage 23 (accelerated creep), and by red continuous lines every 0.01 s (coseismic slip).
    • Fig. S8. Numerical example of accelerated creep on a frictionally stable fault with homogeneous properties driven by 1D pore pressure diffusion from a point source of constant overpressure.
    • Table S1. Completions data for each stage.

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