Breakdown of the Gutenberg‐Richter relation for microearthquakes induced by hydraulic fracturing: influence of stratabound fractures

Abstract

A B S T R A C THydraulic fracturing, a powerful completion technique used to enhance oil or gas production from impermeable strata, may trigger unintended earthquake activity. The primary basis for assessment of triggered and natural seismic hazard is the classic Gutenberg-Richter (G-R) relation, which expresses scale-independent behaviour of earthquake magnitudes. Here we use a stochastic approach to simulate and test magnitude-distance trends expressed by microseismic catalogues derived from three hydraulic fracture monitoring programmes in North America. We show that a widely observed rapid fall-off in large-magnitude events, almost universally quantified using the G-R b value, may in our case be an artefact of the strongly laminated character of the stimulated oil and gas reservoirs. We also show that, for the three reservoirs considered, mechanical bed thickness can be approximated by a lognormal distribution. For a stratabound fracture network, this leads asymptotically to a Gaussian decay for induced magnitudes. We show that the stratabound model provides a more significant correspondence with our observations. If applicable in general, this result has important implications for determining the energy balance of hydraulic fracture systems (i.e. radiated seismic energy versus injected energy) as well as hazard assessments based on the probability of occurrence of anomalous seismic events.

Effects of long‐term fluid injection on induced seismicity parameters and maximum magnitude in northwestern part of The Geysers geothermal field

“…The first considered upper limit to the maximum magnitude observed in the analyzed seismicity cloud could be related to the geometrical constraints of the reservoir. This type of upper constraint to the magnitude has been reported recently [e.g., Eaton et al , ]. In our case, the thickness of the seismically active part of the reservoir in the area of Prati‐9 and Prati‐29 deduced from the vertical span of the relocated seismicity (cf.…”

Cryoseismology

“…It is reasonable to assume that the stress drop Δ σ is higher than the strength of snow (10 −4 MPa) and lower than that of ice (1.5 MPa).This relation, expressed in terms of M w and shown in Figure , can be applied only if we adopt a liberal definition of slip, in order to incorporate the diversity of icequake events. By “liberal” we mean that a classic shear model is used to represent a vast variety of sources, as, for example, it is commonly done for hydraulic fracture or for ice rift‐widening events [e.g., Eaton et al , ; Bassis et al , ]. Obviously, with a mixture of magnitudes and source mechanisms, which are sometimes different from the shear fault assumption [e.g., Walter et al , ], it is difficult to draw final conclusions.…”

The microseismic response at the In Salah Carbon Capture and Storage (CCS) site

“…The b-value, the slope of the frequency-magnitude distribution, is globally found to be ∼1.0 but larger b-values have been estimated up to ∼2.0 in volcanic regions and where fluid injection increases pore pressure and causes fracturing (McNutt, 2005;Bachmann et al, 2012;Eaton et al, 2014). We thus estimate bvalues for the In Salah data to test whether there is any variation in event generation with injection rates, over time or with event location.…”