Time-lapse synchrotron imaging (4D) and quantification of fracture initiation and propagation in shales

Abstract:

Time-lapse fracture propagation has been recorded in 4D images (3D + time) during stepped loading Vickers indentation tests in shale samples at sub-micron resolution (0.4 µm). In-situ synchrotron imaging was performed on four shale samples with different compositions (silicate rich, clay rich, carbonate rich and organic rich), microstructures (unlaminated, poorly laminated, thickly laminated and finely laminated), orientations (paralleling, inclined and perpendicular to the beddings) to examine the hardness, fracture propagation and fracture geometries over time and space. Meanwhile, the time-resolved 3D strain maps were generated using digital volume correlation (DVC) to investigate the initial deformation before fracturing. This study suggests that in shales the average composition provides the major control on the hardness and fracture initiation; while the material texture and the orientation of the indentation to bedding combine to control the fracture propagation direction and geometry. These characterization and quantification also highlight the influence of microstructural anisotropy on the mechanical properties of shales. It was found that microstructural features can govern the fractures at multiple scales. Fractures kink at the interface of two laminae at microscale, and the sub-branches deflected at round corns and split at the sharp ones at nanoscale. The initial deformation determined by DVC presents evenly distribution in the unlaminated sample and heterogeneous distribution in laminated sample.

The high-resolution, time- and space- resolved images acquired in this study provides unique information for quantification and modeling of fracture propagation in rocks. It also demonstrates the micro- and nano- scale fracture propagation must be considered in order to interpret the development of fracture networks in a wide range of applications. These include improving fracturing technology in shales and geothermal energy extraction, predicting the leakage through fractures during the long-term geological storage of carbon dioxide and nuclear waste, increasing the round-trip efficacy of subsurface hydrogen storage and preventing the fracture related subsurface geo-hazards.