Brittleness and microcracks: A new approach of brittleness characterization for shale fracking

Abstract

With the recent global development of shale gas, geomechanical characterization of shale plays is now at a critical point, for which brittleness prediction is a key element. Since brittleness cannot be measured directly, it needs to be estimated from measurable rock properties. However, there is no unique quantitative theory to relate brittleness to rock properties. Therefore, different heuristic arguments linking elastic constants and mineralogy to brittleness have been proposed. Although, the dependence of brittleness on microcracks is experimentally and theoretically established in rock mechanics, quantitative evaluation of brittleness is not yet satisfying. In this study we investigated the hypothesis that brittleness of shale is proportional to its crack distributions. By this means, equations relating brittleness to other measurable properties of solids and natural rocks is proposed. Theoretically, the mathematical-physical derivation of this equation is based on the energy partitioning between the crack inclusion and the rock body. Experimentally, micro-CT imaging, ultrasonic velocity measurement and uniaxial compression tests were measured on shale samples in their native state as received from subsurface shale reservoir. The experimental data is used to calculate the newly proposed index of brittleness and these new results indicate that the orientation of crack distribution is one of the most critical property controlling the anomalous brittle behavior of the investigated rocks. Our calculation of the brittleness index and the visual examination of the crack distributions aided by micro-CT imaging shows that the shale samples oriented horizontally to the bedding plane are more brittle compared with the one that is oriented vertically to the bedding plane.