Natural and artificial phenomenon such as dike formation, vein growth and mineralization, hydrocarbon extraction through hydraulic fracturing, gas outbursts in underground mines can be simulated in the laboratory by performing fluid-driven tensile fracturing experiments. For safety of the engineered events of tensile fracturing, it is necessary to monitor the deformation from its offset and visualization of the localization patterns to demarcate sub surface damage zone.
In this study, anisotropic Vosges sandstone blocks retrieved from a quarry near the city of Hangviller was utilized to perform fluid-driven tensile fracturing experiments. Prismatic samples of volume 50×50×30 mm3, with a hole of diameter 10 mm and length 30 mm with its axis intersecting the centre of the 50×50 mm2 face was prepared. A white and black random speckled pattern was then applied on the 50×50 mm2 face of the specimen. The high pressure true triaxial apparatus (TTA) used for the deformation experiment with the unique feature of a transparent sapphire glass window in contact with the specimen’s speckled surface allowing to photograph the face of the specimen during deformation, so as to a posteriori reconstruct the surface strain field using Digital Image Correlation (DIC)[1]. DIC computation was performed in the SPAM[2] software by computing the correlation function between many subset (correlation windows) of corresponding images taken at two successive time steps.
Loading experiments were conducted in the specimens with two different orientations; one in which the applied axial load was perpendicular to the bedding plane of the sandstone and another in which the axial load was applied parallel to the bedding plane. In both the situations, isotropic loading was applied with a rate of 0.4 MPa/min to 8 MPa followed by the application of vertical load at a rate of 0.2 MPa/min to 10 MPa. Internal fluid pressurization in the circular cavity was achieved by injecting water at a rate of 0.2 MPa/min until visible fractures occurred.
Macroscopic stress-strain curves were correlated with volumetric and deviatoric component of strain tensor from onset of loading and fluid pressurization to post failure. Compaction and dilatancy regions around and away from the cavity are presented. Monitoring the advancement of strain concentration from early stages to failure, and its influence on the post failure behavior is reported. The study establishes a correlation between the mechanical response with the development and evolution of kinematic structures (full-field and global) during fracturing of porous sandstone with fluid pressure.
