Well Proximity Governing Stress Drop Variation and Seismic Attenuation Associated With Hydraulic Fracturing Induced Earthquakes

Abstract

We use broadband seismic data collected within 3 km of a hydraulic fracturing (HF) well in northeast British Columbia, Canada, to estimate the stress drop values of HF-induced earthquakes and their spatial variation. Applying both spectral ratio and clustered single-spectra fitting methods to 484 induced earthquakes (M-1.0 to 3.0), we find that earthquakes close to the injection well have lower stress drop values than those at greater distance. Stress drop values are generally invariant within clusters either proximal (~0.1-1 MPa) or distal (~1-10 MPa) to the well, suggesting that dynamic ruptures in rocks with similar rheological properties tend to have relatively constant stress drop values. Clustered single spectrum fitting also suggests that the seismic quality factor (Q) is lower proximal to the well. We interpret the lower stress drop values and higher seismic attenuation proximal to the well as a result of higher fracture density and/or elevated pore pressure in the rock matrix due to hydraulic stimulation. Plain Language Summary Earthquake static stress drop is the average difference in stress across the fault surface before and after an earthquake rupture. It is a measure of the stress released by fault slip. Investigating the stress drop of induced earthquakes helps us understand the causal relation between hydraulic fracturing (HF) and earthquake source properties. Here we study the source parameters of 484 HF induced earthquakes in the Montney Play of northeast British Columbia. We find that stress drop increases with distance to the injection well but is roughly constant within the respective proximal or distal groups of events. We also find 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 larger magnitudes of stress at distances approximately <1 km from the well.

Stress drops of hydraulic fracturing induced microseismicity in the Horn River basin: challenges at high frequencies recorded by borehole geophones

“…For this reason, we only consider the Q = 180 model further. This Q value falls within the uncertainty of Q = 226 ± ∼70 calculated by Yu et al (2020) for seismic events recorded in the crystalline basement at a hydraulic fracturing pad 240 km away from this study, which adds further credibility to our estimate.…”

“…Our absolute estimates of stress drop are very similar to the closest and most similar study (at a pad ∼240 km from the pad in this study) by Yu et al (2020). Compared with our estimated mean of 1.6 ± 1.2 MPa, Yu et al (2020) calculate an average stress drop of ∼1 MPa from their best results of proximal seismic events using the spectral ratio method. Proximal events are mostly confined to the sedimentary layer in Yu et al (2020), whereas most events in this study occur in the underlying crystalline formation.…”

“…We use borehole geophones in comparison to the surface array used by Yu et al (2020). Using a relatively simple fitting procedure we are able to obtain similar stress drops to the values obtained by Yu et al (2020). While it is certainly likely that our absolute estimates are smaller than actual values, because of the effects of high frequency attenuation, it is encouraging to see that a borehole array can produce similar results to the closest study, which uses a more sophisticated spectral ratio method.…”

“…We use borehole geophones in comparison to the surface array used by Yu et al (2020). Using a relatively simple fitting procedure we are able to obtain similar stress drops to the values obtained by Yu et al (2020).…”

“…Compared with our estimated mean of 1.6 ± 1.2 MPa, Yu et al (2020) calculate an average stress drop of ∼1 MPa from their best results of proximal seismic events using the spectral ratio method. Proximal events are mostly confined to the sedimentary layer in Yu et al (2020), whereas most events in this study occur in the underlying crystalline formation. The similarity of stress drops (both using a Madariaga constant) between our study and Yu et al (2020) at similar depths and both induced by hydraulic fracturing strongly suggests that the rupture slip to length-scale ratio of seismic events in these two data sets is similar.…”