Gas-Driven Fracturing of Saturated Granular Media

Multiphase flows in deformable porous materials are important in numerous geological and geotechnical applications, however the complex flow behaviour make subsurface transport processes difficult to control or even characterise. Here we study gas-driven (pneumatic) fracturing of a wet unconsolidated granular packing confined in a Hele-Shaw cell, and present an in-depth analysis of both pore-scale phenomena and large-scale pattern formation. The process is governed by a complex interplay between pressure, capillary, frictional and viscous forces. At low gas injection rate, fractures grow in a stick-slip fashion and branch out to form a simply connected network. We observe the emergence of a characteristic length-scale – the separation distance between fracture branches – creating an apparent uniform spatial fracture density. We conclude that the well defined separation distance is the result of local compaction fronts surrounding fractures, keeping them apart. A scaling argument is presented that predicts fracture density as a function of granular friction, grain size, and capillary interactions. We study the influence of gas injection rate, and find that the system undergoes a fluidisation transition above a critical injection rate, resulting in directional growth of fractures, and a fracture density that increases with increasing rate. A dimensionless Fluidisation number F is defined as the ratio of viscous to frictional forces, and our experiments reveal a frictional regime for F < 1 characterized by stick-slip, rate independent growth, with a transition to a viscous regime (F > 1) characterized by continuous growth in several fracture branches simultaneously.