Pierre Dick

In situ observations on the coupling between hydraulic diffusivity and displacements during fault reactivation in shales

Key questions in fault reactivation in shales relate to the potential for enhanced fluid transport through previously low-permeability aseismic formations. Here we explore the behavior of a 20 m long N0-to-170°, 75-to-80°W fault in shales that is critically stressed under a strike-slip regime (σ1 = 4 ± 2 MPa, horizontal and N162° ± 15°E, σ2 = 3.8 ± 0.4 MPa and σ3 = 2.1 ± 1 MPa, respectively 7–8° inclined from vertical and horizontal and N72°). The fault was reactivated by fluid pressurization in a borehole using a straddle packer system isolating a 2.4 m long injection chamber oriented-subnormal to the fault surface at a depth of 250 m. A three-dimensional displacement sensor attached across the fault allowed monitoring fault movements, injection pressure and flow rate. Pressurization induced a hydraulic diffusivity increase from ~2 × 10−9 to ~103 m2 s−1 associated with a complex three-dimensional fault movement. The shear (x-, z-) and fault-normal (y-) components (Ux, Uy, and Uz) =  (44.0 × 10−6 m, 10.5 × 10−6 m, and 20.0 × 10−6 m) are characterized by much larger shear displacements than the normal opening. Numerical analyses of the experiment show that the fault permeability evolution is controlled by the fault reactivation in shear related to Coulomb failure. The large additional fault hydraulic aperture for fluid flow is not reflected in the total normal displacement that showed a small partly contractile component. This suggests that complex dilatant effects estimated to occur in a plurimeter radius around the injection source affect the flow and slipping patch geometries during fault rupture, controlling the initial slow slip and the strong back slip of the fault following depressurization.

Elastic wave velocity evolution of shales deformed under uppermost‐crustal conditions

Conventional triaxial tests were performed on a series of samples of Tournemire shale along different orientations relative to bedding (0∘, 90∘). Experiments were carried out up to failure at increasing confining pressures ranging from 2.5 to 80 MPa, and at strain rates ranging between 3 × 10−7s−1 and 3 × 10−5s−1. During each experiment, P and S wave elastic velocities were continuously measured along many raypaths with different orientations with respect to bedding and maximum compressive stress. This extensive velocity measurement set up allowed us to highlight the presence of plastic mechanisms such as mineral reorientation during deformation. The evolution of elastic anisotropy was quantified using Thomsens parameters wich were directly inverted from measurement of elastic wave velocity. Brittle failure was preceded by a change in P wave anisotropy, due to both crack growth and mineral re-orientation. Anisotropy variations were largest for samples deformed perpendicular to bedding, at the onset of rupture. Anisotropy reversal was observed at the highest confining pressures. For samples deformed parallel to bedding, the P wave anisotropy change is weaker.

Strength anisotropy of shales deformed under uppermost‐crustal conditions

Conventional triaxial tests were performed on three sets of samples of Tournemire shale along different orientations relative to bedding (0∘, 45∘, 90∘). Experiments were carried out up to failure at increasing confining pressures ranging from 2.5 to 160 MPa, at strain rates ranging between 3 × 10−7s−1 and 3 × 10−5s−1. This allowed us to determine the entire anisotropic elastic compliance matrix as a function of confining pressure. Results show that the orientation of principal stress relative to bedding plays an important role on the brittle strength, with 45∘ orientation being the weakest. We fit our results with a wing crack micromechanical model [Ashby and Sammis, 1990] and an anisotropic fracture toughness. We found low values of internal friction coefficient and apparent friction coefficient in agreement with friction coefficient of clay minerals (between 0.2 and 0.3) and values of KIc comparable to that already published in the litterature. We also showed that strain rate has a strong impact on peak stress and that dilatancy appears right before failure and hence highlighting the importance of plasticity mechanisms. Although brittle failure was systematically observed, stress drops and associated slips were slow and deformation always remained aseismic (no acoustic emission were detected). This confirms that shales are good lithological candidates for shallow crust aseismic creep and slow slip events.

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

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.

Elastic Anisotropy Reversal During Brittle Creep in Shale

We conducted two brittle creep experiments on shale samples under upper crustal conditions (confining pressure of 80 MPa at 26 °C and 75 °C). We deformed the samples to failure, with bedding oriented perpendicular to the maximum compressive stress direction, using the stress-stepping methodology. In both experiments, the failure stress was ~64% higher than the short-term peak strength. Throughout each differential stress step, ultrasonic wave velocities initially decreased and then gradually increased with deformation/time. The magnitude of these variations depends both on the direction of measurement with respect to the bedding and the temperature, and it is largest for velocities measured parallel to the bedding and at high temperature. Elastic wave anisotropy was completely reversed at 75 °C, following a limited amount of axial strain (~0.6%). SEM investigation confirmed evidence of a time-dependent pressure solution, localized compaction, crack sealing/healing, and mineral rotation. Our observations reveal that elastic anisotropy can evolve rapidly in both time and space, which has implications on the stress state and its rotation near fault zones.

What Controls the Occurrence of Seismicity in Shale? Lessons Learned from a Meter-Scale Fluid-Injection Experiment

Clay formations are present in reservoirs and earthquake faults, but their mechanical behaviors are still poorly understood, as they can vary from plastic (aseismic) to brittle (seismic). A decametric scale experiment, which aims to reactivate a natural fault by fluid injection was performed in shale materials. The injection area monitored by sensors measuring pressure, deformation and ground motion. Few tens of events were recorded with magnitude ranging between -4.3 and -3.7. Their spatio-temporal distribution,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 predominates 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 impervious hydraulic barrier that favors fluids confinement and pressurization. Therefore, the seismicity seems to be driven by the stress changes induced by the pressurized volume and the hydraulic permeability heterogeneity strongly influences the seismicity distribution.