Summary
Many studies have pointed out a correlation between either the cumulative or the maximum seismic moment and the injected fluid volume when analyzing global datasets of fluid injection-induced earthquake sequences. However, those correlations become quite uncertain when looking at individual episodes, mainly because of the large aseismic component of the induced deformation. If natural swarms are thought to result from the same physical processes as sequences from anthropogenic origin, little is still known about them as observations are limited by the depth of the active zone and the moderate deformations. In this work, we make profit of the similarity between both natural and injection-induced swarms. To this aim, we develop new relations between seismic observables and hydraulic attributes by using a global compilation of injection-induced earthquake catalogs, leading to two methods to estimate the injected fluid volume based solely on earthquake catalogs. Once the precision of our approaches is validated, we estimate the volume and flowrate of fluids circulating in diverse natural swarms, shedding a new light on the fluid dynamics that trigger them.
Summary
The Opalinus Clay (OPA) is a clay-rich formation considered as a potential host rock for radioactive waste repositories and as a caprock for carbon storage in Switzerland. Its very low permeability (10−19 to 10−21 m2) makes it a potential sealing horizon, however the presence of faults that may be activated during the lifetime of a repository project can compromise the long-term hydrological confinement, and lead to mechanical instability. Here, we have performed laboratory experiments to test the effect of relative humidity (RH), grain size (g.s.) and normal stress on rate-and-state frictional properties and stability of fault laboratory analogues corresponding to powders of OPA shaly facies. The sifted host rock powders at different grain size fractions (< 63 μm and 63 < g.s. < 125 μm), at room (∼25 per cent) and 100 per cent humidity, were slid in double-direct shear configuration, under different normal stresses (5 to 70 MPa). We observe that peak friction, μpeak, and steady-state friction, μss, depend on water vapor content and applied normal stress. Increasing relative humidity from ∼25 per cent RH (room humidity) to 100 per cent RH causes a decrease of frictional coefficient from 0.41 to 0.35. The analysis of velocity-steps in the light of rate-and-state friction framework shows that the stability parameter (a-b) is always positive (velocity-strengthening), and it increases with increasing sliding velocity and humidity. The dependence of (a-b) on slip rate is lost as normal stress increases, for each humidity condition. By monitoring the variations of the layer thickness during the velocity steps, we observe that dilation (Δh) is directly proportional to the sliding velocity, decreases with normal stress and is unaffected by humidity. Microstructural analysis shows that most of the deformation is accommodated within B-shear zones, and the increase of normal stress (σn) promotes the transition from strain localization and grain size reduction to distributed deformation on a well-developed phyllosilicate network. These results suggest that: (1) the progressive loss of velocity dependence of frictional stability parameter (a-b) at σn > 35 MPa is dictated by a transition from localized to distributed deformation; (2) water vapor content does not affect the deformation mechanisms and dilation, whereas it decreases steady-state friction (μss), and enhances fault stability.
Abstract.A 2D travel-time inversion algorithm that uses a recently developed fast-sweeping eikonal solver is presented. The algorithm is fast, extremely easy to implement, and robust. It is applied to borehole radar data collected in the karst aquifer at LSBB (Laboratoire Souterrain à Bas Bruit) and the recovered permittivity profile is shown to be consistent with borehole televiewer and material measurements.
Abstract. We performed high-pressure water injections in boreholes set in a fractured porous limestone at 250 m-depth in the LSBB URL in order to investigate the coupling relationships between induced seismicity and the rock permeability changes. The seismic events and tilts of the free wall of the gallery were monitored at different locations by two 3-component accelerometers and two 2D tiltmeters located from 1.5 to 2 m from the injection chamber. Changes in fluid pressure and flow rate were simultaneously monitored in the chamber. A damage-induced permeability increase of a factor of 2.2 occurred after a 10 seconds long swarm of complex seismic events caused by a pressure peak injection of 3.5 × 10 6 Pa and a sudden increase in the injected flow rate from 0 to to 5.3 × 10 −5 m 3 · s −1 . The various angular directions observed on both tiltmeters and 3D accelerometric components indicate that the source of the deformation remains complex and evolves over the time of fluid pressure diffusion in the fractures. At the beginning, signals appear dominated by the source geometry, the rock mass boundary and stress conditions. After the damage and during the pressure decay, tangential slipping of the fracture behavior dominates stress relaxation in the medium. Finally, tilt monitoring coupled with seismo-acoustic measurements present a promising way to quantitatively estimate the relationships between the changes in fractured rocks permeability and seismicity induced by fluid pressurization.
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