Numerical simulation of combined brittle-ductile deformation processes

Combined brittle-ductile deformations are complex processes, when brittle fracturing of rocks and ductile flow occurs simultaneously during one deformation process. Such combined brittle-ductile deformation is boudinage, which is the formation of boudins during ductile extension if there is a component of lengthening parallel to a competent layer in an incompetent matrix. Another process of both basic and applied importance is hydraulic fracturing, when viscous fluid is pumped into brittle reservoir rock in order to generate fractures. The main aim of this study is to better understand these combined brittle-ductile deformations. In order to achieve this, a new numerical method has been developed together with a more detailed analysis of boudinage processes. Existing models of different combined brittle-ductile deformation focus on either the brittle-elastic processes of the fracture initiation phase or ductile processes of the post deformation phase. However, in this dissertation, a new approach is presented for the numerical modelling of deformation processes combining brittle fracture and viscous flow. The new approach is based on the combination of two meshless, particle based methods: the Discrete Element Method (DEM) for the brittle part of the model and Smooth Particle Hydrodynamics (SPH) for the viscous part. Pure shear tests were verified the model and proved that it is suitable to qualitatively simulate real Newtonian viscous behaviour. The suitability of the combined approach is demonstrated by applying it to two geological processes, boudinage and hydrofracturing. Boudinage simulations demonstrated that, as the viscosity of the incompetent layer increases, the number of boudins increase and with even higher viscosity, pinch-and-swell structures develop. Hydrofracturing simulation showed that the viscosity of the injected fluid affects the evolution of the induced crack: the lower the viscosity, the faster the crack propagates and the wider the crack becomes. Boudinage occurs in mechanically layered rocks if there is a component of lengthening parallel to a brittle layer in a ductile matrix. However, if the extension is not layer-parallel, then asymmetric boudin structures can develop in two ways. One mechanism is extension oblique to the layering; the other is shearing oblique to the layering. The shape and the rotation of these asymmetric boudins are extensively studied; however, full process of oblique boudinage has not been modelled previously. In this study, full boudinage processes during layer oblique extension were simulated using DEM simulation software. In this model, pure shear deformation is simulated with the initial setup of a competent oblique layer situated in the middle of the less competent matrix. By varying the cohesion of the competent layer, various types and shapes of boudin blocks were simulated. By varying the angle of the competent layer, the rotations of the boudin blocks change. The study of the fracture orientation in the layer oblique to the extension showed that in the case of ductile matrix material, the fractures are normal to the layer. In the case of elastic matrix material, the fractures are parallel to the orientation of the main stress. Furthermore, it was demonstrated that boudin blocks do rotate during pure shear deformation and the rotation of the boudin blocks developed in an oblique layer is systematic. Also, the higher the dip of the oblique layer, the more the blocks rotate against dip direction. Finally, the full evolution of boudin structures developed during the simple shear and sub-simple shear of an oblique competent layer in a ductile matrix was studied using DEM simulations. According to the results of the simulations, the shape and type of boudinage is mainly controlled by the cohesion of the oblique layer and the strain rate of the deformation. The separation of the blocks is influenced by the orientation of the cohesive oblique layer. Furthermore, the results of the strain analysis of the simple shear simulations showed that the bulk strain of the model is higher than the strain of boudin blocks. The major controlling factor to characterise the difference between the boudinage strain and the matrix strain is the orientation of the competent oblique layer, whereas the cohesion of the middle layer is a minor controlling factor. Moreover, it was presented that significantly different structures developed during one-phase deformation (sub-simple shear) and two-phase deformation (first pure shear, then simple shear), in that all the shapes, rotation and the separation of the blocks vary. This study has showed that the new SPH-DEM method is suitable to model coupled brittle-ductile deformations such as boudinage or hydraulic fracturing; oblique boudinage is a complex process of coupled brittle-ductile processes and these processes can be better understood using DEM simulations