Abstract

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 × 120 × 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.

1. Introduction

The growth of natural faults is controlled by several factors including: tectonics stress, strain rate, pore pressure and host rocks. The impact of faults and their potential reactivation on oil and gas reservoirs have been investigated [1, 2]. Compactive and dilatant deformation in porous rocks is a crucial problem in fault development, geotechnical engineering and reservoir/aquifer management. Active tectonics and extraction of fluids from buried porous host rocks modify the pore pressure in a reservoir/aquifer, causing variations of the effective stress possibly leading to faulting and inelastic deformation that can in turn adversely affect oil production as in the case of the deep water reservoir in the Campos Basin [3]. In contrast with sandstone lithologies, limestones and marbles undergo brittle to plastic transition at room temperature for confining pressures accessible in laboratory [4, 5], because calcite requires relatively low shear stresses to initiate mechanical twinning and dislocation activity. The mechanical behaviour of carbonate rocks of a wide range of porosities has been documented by many previous studies [6–8].

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