Hydromechanical Behavior of a Synthetic Homogenous Carbonate Rock During Direct Shear

摘要

We have conducted a series of hydro-mechanical shear experiments using the CSIRO direct shear rig. There blocks were fabricated using loose calcite and quartz grains cemented by a calcium-rich fluid that permeated and filled the interconnected pore spaces to bind the grains together, i.e. this technology is known as CIPS (Calcite In-situ Precipitation System). Those intact homogenous CIPS blocks (240 x 120 x 150 mm) were sheared to final displacements of 20, 70 or 120 mm under a constant effective vertical stress of 10 MPa. Fluid-flow response across the fault zone was monitored during both shear deformation and hold periods, while keeping a constant injection pressure throughout the measurement. On reaching the final displacement (20, 70 or 120 mm), we cored a cylindrical plug of 38 mm diameter that contains fault zone and parts of intact wall rock at either end. In parallel to the experimental study, a 2D mechanical model of the shear experiments was developed using Smoothed Particle Hydrodynamics (SPH), suitable for deformation processes involving very large strains. Numerical results were compared with the experimental data for a better understanding of local deformation process, stress distribution and potential tension cracks in samples during shear. Shear results show all the tested blocks indicated strain-softening behaviour and the flow rate decreased as displacement increased. Plastic deformation is compactant and resulted in the development of localised zone of deformation that decrease the fluid transmissibility of the blocks. Furthermore, x-ray CT images show that the created deformation features in the sheared blocks resembled those observed in natural fault zones.