Igor Faoro

Influence of shear and deviatoric stress on the evolution of permeability in fractured rock

[1] The evolution of permeability in fractured rock as a function of effective normal stress, shear displacement, and damage remains a complex issue. In this contribution, we report on experiments in which rock surfaces were subject to direct shear under controlled pore pressure and true triaxial stress conditions while permeability was monitored continuously via flow parallel to the shear direction. Shear tests were performed in a pressure vessel under drained conditions on samples of novaculite (Arkansas) and diorite (Coso geothermal field, California). The sample pairs were sheared to 18 mm of total displacement at 5 mm/s at room temperature and at effective normal stresses on the shear plane ranging from 5 to 20 MPa. Permeability evolution was measured throughout shearing via flow of distilled water from an upstream reservoir discharging downstream of the sample at atmospheric pressure. For diorite and novaculite, initial (preshear) fracture permeability is 0.5–1 � 10 � 14 m 2 and largely independent of the applied effective normal stresses. These permeabilities correspond to equivalent hydraulic apertures of 15–20 mm. Because of the progressive formation of gouge during shear, the postshear permeability of the diorite fracture drops to a final steady value of 0.5 � 10 � 17 m 2 . The behavior is similar in novaculite but the final permeability of 0.5 � 10 � 16 m 2 is obtained only at an effective normal stress of 20 MPa.

The signature and mechanics of earthquake ruptures along shallow creeping faults in poorly lithified sediments

Seismic slip episodically occurring along shallow creeping faults in poorly lithified sediments represents an unsolved paradox, largely due to our poor understanding of the mechanics governing creeping faults and the lack of documented geological evidence showing how coseismic rupturing overprints creep in near-surface conditions. Here we describe the signature of seismic ruptures propagating along shallow creeping faults affecting unconsolidated forearc sediments. Field observations of deformation band–dominated fault zones show widespread foliated cataclasites in fault cores, locally overprinted by sharp slip surfaces decorated by thin (0.5–1.5 cm) black gouge layers (herein, black gouge). Compared to foliated cataclasites, black gouges have much lower grain size, porosity, and permeability. Moreover, they are characterized by distinct mineralogical assemblages compatible with high temperatures (180–200 °C) due to frictional heating during seismic slip. Foliated cataclasites were also produced by laboratory experiments performed on host sediments at subseismic slip rates (≤0.1 m/s), displaying high residual friction (µf = 0.65) and strain-hardening behavior. Black gouges were produced during experiments performed at seismic (1 m/s) slip rates, displaying low residual friction (µf = 0.3) due to dynamic weakening. Our results show that black gouges represent a potential diagnostic marker for seismic faulting in shallow creeping faults. These findings can help understanding the time-space partitioning between aseismic and seismic behavior of faults at shallow crustal levels.

Stress- and Chemistry-Mediated Permeability Enhancement/Degradation in Stimulated Critically-Stressed Fractures

This work has investigated the interactions between stress and chemistry in controlling the evolution of permeability in stimulated fractured reservoirs through an integrated program of experimentation and modeling. Flow-through experiments on natural and artificial fractures in Coso diorite have examined the evolution of permeability under paths of mean and deviatoric stresses, including the role of dissolution and precipitation. Models accommodating these behaviors have examined the importance of incorporating the complex couplings between stress and chemistry in examining the evolution of permeability in EGS reservoirs. This document reports the findings of experiment [1,2] and analysis [3,4], in four sequential chapters.