RESEARCH PROJECTS

a.  Physical mechanisms of fluid injection induced earthquakes

Hydraulic fracturing (HF) through fluid injection alters stresses near injection sites as the volume of fluid increases, thus produces earthquakes in some cases. While HF injection induced earthquakes (IIE) are typically minor (M<4), some recent events have exceeded M4. To understand the properties of induced earthquakes associated with hydraulic fracturing (HF), and the interaction between the injected fluids and the shale rock formation, we designed an experiment to isolate the effect of HF by deploying eight broadband seismometers at close proximity to the well pad for a period of pre-, co- and post-HF stimulation, in the Montney Play, BC, Canada. In collaboration with Prof. Rebecca Harrington at Ruhr University Bochum, Prof. Yajing Liu at McGill University, and postdoc fellow Bei Wang at Victoria University, we use multi-station matched filter detection and double-difference earthquake relocation methods to develop a catalog of 350 events associated with HF stimulation (M ~ -2.8 to 1.8) [Yu et al., 2019]. Our work finds that pore pressure change and poroelastic stress transfer could both be dominant mechanisms responsible for producing HF induced earthquakes, depending on proximity to the well and the elapsed time.
Moreover, earthquake static stress drop, as a measure of the stress released by fault slip, helps us quantitatively understand the causal relation between hydraulic fracturing (HF) and earthquake source properties. With the additional contribution from Prof. Honn Kao (Natural Resources Canada and University of Victoria) and Prof. Rachel E. Abercrombie (Boston University), we apply both spectral ratio and clustered single‐spectrum fitting methods to aforementioned induced earthquakes [Yu et al., 2020]. We find that stress drop increases with distance to the injection well but is generally invariant within clusters either proximal (~0.1–1 MPa) or distal (~1–10 MPa) to the well. Clustered single spectrum fitting also suggests that seismic energy loss during wave propagation (seismic attenuation) is higher near the well. The observations lead us to interpret that either the higher fracture density and/or elevated pore pressures near the well prevent the crustal rocks from storing and releasing a larger amount of stress at distances approximately <1 km from the well.
However, with the classical concept of pore pressure and/or poroelastic stress change, it is challenging to explain the occurrence of M3+ earthquakes induced over the short operational period of hydraulic fracturing stimulations but located several kilometers away from the wellbore. Aseismic slip loading has recently been proposed as a complementary mechanism, which is validated by laboratory work and numerical modeling. However, aseismic/slow slip signals linked to fluid injection-induced earthquakes remain largely undocumented to date. We find a new type of induced seismic signal in Montney Play that possibly manifests the source process that bridges the slow (aseismic) slip inferred by recent modeling and observations near the wellbore to seismic slip at greater distances. This signal consists of an impulsive broadband onset followed by protracted low-frequency ringing. They have broader P and S-pulses (implying longer source durations) and lower corner frequencies (implying either slower rupture speeds, lower stress drop values, or a combination of both), which are identical to low-frequency earthquakes found in plate boundary zones.