Fault structure, stress, or pressure control of the seismicity in shale? Insights from a controlled experiment of fluid‐induced fault reactivation

Abstract

Clay formations are present in reservoirs and earthquake faults, but questions remain on their mechanical behavior, as they can vary from ductile (aseismic) to brittle (seismic). An experiment, at a scale of 10 m, aims to reactivate a natural fault by fluid pressure in shale materials. The injection area was surrounded by a dense monitoring network comprising pressure, deformation, and seismicity sensors, in a well‐characterized geological setting. Thirty‐two microseismic events were recorded during several injection phases in five different locations within the fault zone. Their computed magnitude ranged between −4.3 and −3.7. Their spatiotemporal distribution, compared with the measured displacement at the injection points, shows that most of the deformation induced by the injection is aseismic. Whether the seismicity is controlled by the fault architecture, mineralogy of fracture filling, fluid, and/or stress state is then discussed. The fault damage zone architecture and mineralogy are of crucial importance, as seismic slip mainly localizes on the sealed‐with‐calcite fractures which predominate in the fault damage zone. As no seismicity is observed in the close vicinity of the injection areas, the presence of fluid seems to prevent seismic slips. The fault core acts as an impermeable hydraulic barrier that favors fluid confinement and pressurization. Therefore, the seismic behavior seems to be strongly sensitive to the structural heterogeneity (including permeability) of the fault zone, which leads to a heterogeneous stress response to the pressurized volume.

Aseismic Motions Drive a Sparse Seismicity During Fluid Injections Into a Fractured Zone in a Carbonate Reservoir

“…The same pattern was observed in the Mayet de la Montagne experiments (Scotti & Cornet, ) where water injections in a 800 m long borehole mainly drove aseismic deformation. Aseismic motion was also observed in controlled in situ fluid injection experiments at meter scale in limestone (Guglielmi et al, ) or in shale (De Barros et al, ). At the laboratory scale, Goodfellow et al () performed fluid injection experiments on granite samples under triaxial stresses and showed that most of the deformation is aseismic during the fluid pressurization.…”

“…Other processes triggering seismicity can be fracture propagation through hydrofracturing (Cornet, ) and poroelastic stress perturbation transferred through failure away from a pressure source (Goebel et al, ). Nevertheless, recent observations (De Barros et al, ; Guglielmi et al, ; Zoback et al, ) showed that fluid pressure first drives an aseismic fault rupture. The seismicity appears as a secondary process and a response of the surrounding medium to the aseismic rupture.…”

“…Hence, more accurate observations of seismicity, deformation, and fluid pressure near and within faults are required in order to improve understanding of these interconnected mechanisms and to bridge the gap between small‐scale (cm) laboratory experiments and large‐scale (km) observations. To this aim, controlled fluid injection experiments at a meter to tens of meters have already been developed in limestones by Guglielmi et al () and Derode et al () as well as in shales (De Barros et al, ; Rivet et al, ). Here we have performed a series of fluid injection experiments with a proximal (meters) array of 31 high‐sensitivity seismic sensors within the damage zone of the same fault zone previously tested in Guglielmi et al (), but with a limited seismic network (only three sensors).…”

Seismic velocity changes associated with aseismic deformations of a fault stimulated by fluid injection

“…The presence of phyllosilicate minerals strongly reduces the frictional strength of faults [Ikari et al, 2009;Faulkner et al, 2011;Kohli and Zoback, 2013]. Several factors can explain this situation: (1) the failure of shales is complex due to the dependences of their mechanical properties to the mineralogy and the water content [Ikari et al, 2009;Kohli and Zoback, 2013;De Barros et al, 2016] and (2) laboratory experiments on shales are not fully representative of this complex behavior. However, assessing deformation and failure processes in shales is challenging.…”

Scientific Exploration of Induced SeisMicity and Stress (SEISMS)

“…Armed with new technology in fault zone drilling and geodesy, a new-generation earthquake experiment could more directly measure fault slip and fluid pressures within both the fault core as well as the surrounding damage zone, which should enhance our ability to determine where and when failure will occur. Recent borehole experiments have successfully induced small earthquakes (−4.5<M w < − 3) in a controlled way (Derode et al, 2015;Guglielmi et al, 2015;De Barros et al, 2016). Observations of the induced earthquakes have demonstrated that the physical processes that lead to runaway slip are complex and depend on the hydromechanical and frictional characteristics of both the fault and the surrounding rock.…”