An experimental investigation on deformation and failure behavior of carbonaceous Garau shale in Lurestan Basin, west Iran: Application in shale gas development

The safety and efficiency of commercial shale gas development strategies are tightly related to accurate evaluation of rock deformation and brittle failure processes. In contrast to extensively developed siliciclastic shale gas resources in the world, the mechanical behaviors of the carbonaceous shales and their causal mechanisms are not thoroughly understood. In this study, a geomechanical characterization program including unconfined and confined deviatoric compression experiments together with complementary compositional and microstructural analyses was therefore carried out to explain the stress-strain behaviors of the carbonaceous organic-rich Garau shales in Iran. Deformation and progressive fracturing process of the collected rock samples at varying confinement states were evaluated and discussed in terms of well-established mechanistic concepts and theoretical models. In addition, the applicability of some of widely used expressions for assessing rock brittleness was also investigated. According to the obtained results, in spite of substantially varying deformation and failure characteristics, all collected samples over the range of applied confinement states up to 40 MPa follow the typical brittle failure process. However, the degree of brittle failure in terms of brittleness is considerably variable and dependent to the established definition. Furthermore, the diagnosed range of stress-strain characteristics are dominantly correlated with variations of carbonate depositional textures. Accordingly, as the stiff fine-grained micritic fabric of a carbonaceous shale gradually transforms to relatively large sparitic texture, rock generally tends to fail at relatively lower strength but higher degree of brittleness. This relation can be advantageous to an accurate determination of the relative brittleness successions in carbonaceous shale gas layers for practical field operations.