[1] 3D numerical modeling has been used to investigate how the variations of mechanical properties in sedimentary layered sections affect the development of normal faults. We calculated the distribution of the Coulomb stress to assess the proximity of the layers to failure through an elastic layered section. The simulation of various combinations of rock properties allowed us to compare the effect of the stiffness and strength contrasts, which promote or inhibit faulting in the stiff layer, respectively. For rock systems showing little variation in strength, nucleation of the fault occurs in the stiff layer (e.g., limestones or sandstones), whereas it occurs in the compliant layer (e.g., clay-rich rocks) if the stiff layer has a high cohesion. Considering a mean strength profile of the carbonate sequences, nucleation occurs in limestones if the ratio of Young's moduli between the limestone and clay-rich rock is greater than 2; otherwise, clay-rich layers fail first. We also showed that nucleation is promoted in sandstones or limestones if these layers are thinner than the clayey layers. In a second set of simulation, using a slip on a fault, we examined the conditions needed to overcome the restriction of the fault propagation. Our results suggest that the lateral propagation of the fault, within a layer, produces increasingly favorable conditions for vertical propagation. A maximum aspect ratio of width to height of 13 is predicted for faults in limestone-clay sequences, and this maximum aspect ratio is expected to decrease as the contrast in the rock properties decreases.Citation: Roche, V., C. Homberg, and M. Rocher (2013), Fault nucleation, restriction, and aspect ratio in layered sections: Quantification of the strength and stiffness roles using numerical modeling,

The ability to generate deep flow in massive crystalline rocks is governed by the interconnectivity of the fracture network and its permeability, which in turn is largely dependent on the in-situ stress field. The increase of stress with depth reduces fracture aperture, leading to a decrease in rock mass permeability. The frequency of natural fractures also decreases with depth, resulting in less connectivity. The permeability of crystalline rocks is typically reduced to about 10 −17 -10 −15 m 2 at targeted depths for Enhanced Geothermal Systems (EGS) applications, i.e., > 3 km. Therefore, fluid injection methods are required to hydraulically fracture the rock and increase its permeability.

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The distribution of displacement along faults is a key parameter in various areas of geology such as earthquake studies, threedimensional strain restoration, fault growth, and reservoir and seal strata relations in hydrocarbon systems. It is essential therefore to understand how local conditions govern displacement distribution. We analyse dip-parallel displacement profiles of normal faults cutting five alternating limestone and shale layers and we discuss their evolution, from their nucleation to their restriction by lithological interfaces or bed-parallel faults in clays, or to their further propagation through several layers.Local displacement gradients control the shape of displacement profiles and are highly variable over the course of fault history. Accordingly, the Dmax-L relation is nonlinear. Bed-parallel faults prove stronger restrictors than lithological interfaces and the correlation of the local gradient with lithology during restriction and propagation indicates that knowledge of these gradients is required if we are to understand how faults develop in multilayer systems.Evolution of the fault displacement profile • V. Roche et al.

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The success of hydraulic fracturing treatments is often judged by the shape and size of the resulting microseismic cloud. However, it is challenging to predict the anticipated microseismic cloud prior to treatment. We use geomechanical modeling to predict the distribution of the microseismicity prior to the hydraulic fracture treatment. We analyze the likelihood of tensile and shear failure due to 1-D variations in local stresses and rock strengths, induced by layering and pore pressure, for two field cases. The deviation in the local stresses from the regional stress field is induced by vertical variations in stiffness. This promotes failure of the stronger layers instead of the weaker ones since the stronger layers can become essentially load bearing. The simulations and field studies show that (1) microseismic events tend to locate preferentially where layers reach tensile failure due to fluid injection, and the number of events tends to decrease in layers that do not reach tensile failure; (2) shear initiation can occur in different layers from those failing in tension, thereby creating additional fluid migration paths; and (3) reactivation of preexisting fractures may occur due to fluid migration, even if their orientations are unfavorable. Numerical modeling is a significant aid in understanding the interplay of regional and local stresses and associated in situ failure due to variations in rock strength, pore pressure, and stiffnesses. In a wider perspective, it gives fundamental insights into the understanding of earthquakes and fault localization, the mechanisms of fracture development, and the role of fractures on fluid circulation and on the in situ stress field.