Faulting mechanism in un-cemented sediments is addressed through laboratory experiments in a high stress ring shear apparatus. The main objective is to investigate basic mechanisms involved in the deformation process of sediments during faulting. An understanding of these processes and how they affect fluid flow is important for the development of fault models and their implementation into reservoir simulators.
The experimental test program comprises three types of ring shear tests: shearing of homogenous sand, shearing of layered sand - clay sequence and shearing of unclean sand with varying clay content. Visual inspection of the samples after testing, analyses of thin sections and permeability measurements across the shear zone are used to describe shear band characteristics and properties like geometrical continuity, thickness and sealing potential. Deformation processes such as grain reorientation, clay smear and cataclasis are identified from the tests.
Tests performed for various depths show increasing shear zone complexity with increasing depth at time of faulting. At shallow burial depth, in clay rich sediments, clay smear is the most efficient mechanism for permeability reduction. At this depth, sand-sand juxtaposition shear is dominated by grain rolling causing only minor permeability reduction. At greater burial depths, permeability reduction is dominated by grain crushing.
In parallel to the laboratory experiments, numerical analyses are performed to verify the performance of the experimental set-up and interpret the laboratory results. A 3D finite element code was used to investigate the stress distribution and progressive development of shear failure within the sample during testing. The combination between the numerical and experimental results enhances the understanding of the failure mechanisms and enables an improvement of both experiment configurations and the interpretations of the test results. Future work will aim at predicting deformation products in faults with the help of numerical analyses.