Density Driven Instability during Proppant Injection in a Hele-Shaw Cell

We report an experimental investigation of granular matter suspended in a liquid injected between two parallel plates with a constant separation distance called a Hele-Shaw cell. These experiments are motivated by proppant injection in hydraulic fracturing of shale used to extract hydrocarbons. The cell is fully filled in with a similar ambient fluid to avoid capillary effects and pinning and dipinning dynamics of the front. We image the dynamics and deposition of the grains as they spread inside the cell as a function of injection rate and volume fraction of the grains. A finger-like instability is observed for sufficiently density difference between the particles and the ambient fluid. We will discuss the observed phase diagram as a function of the injection-rate of the fluid, its packing fraction and time. We demonstrate a correlation between the timescale for the sedimentation of the particles and for the onset of fingers as well as subsequent bifurcations. Corresponding length scales are studied as well, i.e. the finger lengths and widths as a function of the packing fraction of the injected fluid. Early analysis suggest that for a fixed packing fraction, regardless of the flow-rate, the sedimentation time determines the onset of the instability.