Fluid-driven fractures in granular media: Insights from numerical investigations

Abstract

We investigate the mechanisms of opening-mode fracture initiation in granular media. The study 11 is based on simulation of grain-scale fluid-grain interactions through a coupled numerical approach in which the discrete element model (DEM) is used to solve for the mechanics of a solid granular medium and computational fluid dynamics (CFD) is used to model fluid flow and drag forces. We present benchmark problems with analytical solutions and validate this numerical model against experiments of viscous-drag driven cavity in the literature. Additional simulation results show fracture initiation mechanisms in a random granular packing subjected to constant boundary stresses and to fluid injection with a localized source. The dimensionless variable F s /F sk (ratio of seepage force F s and skeletal force F sk ) incorporates the impacts of 19 physical properties and injection parameters including fluid viscosity, injection velocity, grain 20 size and effective stresses, and has been used as a criterion separating regimes of fluid invasion 21 and drag-driven fracture opening. Our simulation results show that F s /F sk in combination with τ 1 (ratio of diffusion time from hydromechanical coupling and injection time) serve as a prediction of fracture opening within granular packing. We suggest a simple criterion (F s /F sk > 1 or τ 1 > 0.17) that is valid for various types of granular media and injection conditions to determine if fracture opening will occur. Among other applications, this study is useful to predict the initiation and propagation of fractures in natural sediments.

Numerical Simulation of Channelization Near the Wellbore due to Seepage Erosion in Unconsolidated Sands during Fluid Injection

“…The results showed that the onset of erosion is mainly controlled by hydraulic gradient; Zhang et al [25] used a discrete element method code PFC2D to model the particles movement during fluid injection. Sun et al [26] investigated the mechanisms of opening-mode fracture initiation in granular media by the discrete element method and computational fluid dynamics. However, due to the expensive cost of calculation, the discrete element method can only be used to study the microscopic mechanism of channelization in a small scale.…”