Mechanical controls on horizontal stresses and fracture behaviour in layered rocks

Abstract

In layered materials, the deformation style, orientation, confinement, and 3D connectivity of natural fractures is generally impacted by changes in sedimentary facies and alternations in mechanical properties. In this study we address this effect and perform a numerical sensitivity analysis. Mechanical properties, confining pressures, and interfacial frictions are varied for a three-layered model, to investigate and quantify the relation between contrasting material properties, the principal horizontal stresses and fracture behaviour (i.e. deformation style and orientation). Firstly, the results show that tensile stresses develop in the stiffer layers due to the contrasting elastic parameters. The magnitude of these stresses is dependent on the ratio between the elastic parameters of stiffer and softer layers (i.e. Estiff/Esoft and νstiff/νsoft). There are no horizontal tensile stresses, when applying a compressive horizontal confining pressure (approx. 1/5 of the applied vertical stress). Implementing an interfacial friction lower than 0.2 will result in decoupling of the layers, resulting in slip on the layer boundaries and no tensile stresses within the stiffer layers. Further, the acquired numerical results are in good agreement with previously conducted laboratory work. Finally, we discuss whether the presented results can be used for better relating contrasting mechanical properties to potential fracture deformation styles and orientations in layered outcrops or subsurface reservoirs.