Abstract
Abstract We conducted three-dimensional coupled fluid-flow and geomechanical modeling of fault activation and seismicity associated with hydraulic fracturing stimulation of a shale-gas reservoir. We simulated a case in which a horizontal injection well intersects a steeply dipping fault, with hydraulic fracturing channeled within the fault, during a 3-h hydraulic fracturing stage. Consistent with field observations, the simulation results show that shale-gas hydraulic fracturing along faults does not likely induce seismic events that could be felt on the ground surface, but rather results in numerous small microseismic events, as well as aseismic deformations along with the fracture propagation. The calculated seismic moment magnitudes ranged from about −2.0 to 0.5, except for one case assuming a very brittle fault with low residual shear strength, for which the magnitude was 2.3, an event that would likely go unnoticed or might be barely felt by humans at its epicenter. The calculated moment magnitudes showed a dependency on injection depth and fault dip. We attribute such dependency to variation in shear stress on the fault plane and associated variation in stress drop upon reactivation. Our simulations showed that at the end of the 3-h injection, the rupture zone associated with tensile and shear failure extended to a maximum radius of about 200 m from the injection well. The results of this modeling study for steeply dipping faults at 1000 to 2500 m depth is in agreement with earlier studies and field observations showing that it is very unlikely that activation of a fault by shale-gas hydraulic fracturing at great depth (thousands of meters) could cause felt seismicity or create a new flow path (through fault rupture) that could reach shallow groundwater resources.
Highlights
The rapid increase in North American shale-gas energy production has been made possible through new technology development, including extended-reach horizontal drilling and multistage hydraulic-fracture stimulation
(10 by 10 m) that can rupture in one instance. This means that in our modeling, we model the relatively larger microseismic events occurring from shear slip along the fault plane, whereas in the field, there are numerous smaller-magnitude events perhaps occurring as a result of slip in small-scale fractures in the host rock surrounding the fault, and these are not resolved in our modeling
The modeling results of repeated small microseismic events and aseismic slip and fracture opening is consistent with field observations—that the energy emitted from microseismic events represents only a small fraction of the energy input or the energy to open the fracture (Warpinski et al, 2012)
Summary
The rapid increase in North American shale-gas energy production has been made possible through new technology development, including extended-reach horizontal drilling and multistage hydraulic-fracture stimulation. The hydraulic fracturing data showed that all microseismic events occurred less than 600 m above well perforation, most were very much closer, and the farthest were usually associated with faults These studies indicated that shale-gas hydraulic fracturing at great depth (thousands of meters) could not create flow paths for leakage to reach shallow groundwater resources. The third case of felt seismicity occurred at Etsho and Kiwigan fields in Horn River, Canada, where 19 events between ML = 2 and 3 occurred having a clear temporal correlation with the shale-gas operation; the largest (and felt) event, occurring in May 2011, had a magnitude of ML = 3.8 (BC Oil and Gas Commission, 2012; Davies et al 2013) Each of these three cases of felt seismicity have been associated with reactivation of faults. In 2D plane-strain simulations, it is difficult to estimate a representative injection rate, and some assumptions have to be made about the shape of the rupture area
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