Fluid-driven fracture mechanisms in granular media: insights from grain-scale numerical modeling

Fluid injection into porous media can initiate fluid-driven fractures. Unlike cohesive and homogeneous media, fluid-driven fractures in cohesionless granular media have complex fracture and leak-off morphologies. In this study, we investigate the mechanisms behind fluid-driven fractures in cohesionless granular media by coupling the discrete element method with computational fluid dynamics at the pore/grain scale. We perform numerical simulations of the injection of a fluid into a granular medium that is saturated with a second, immiscible fluid. Results show that fluid injection can initiate a fracture orthogonal to the minimum principal stress, as expected, through viscous drag forces. However, the displacement field around the fracture is much more complex than what is expected for a linear elastic solid, showing that (1) fracture propagation requires small leak-off (a few times the fracture width) and (2) localized shear deformation ahead of the fracture tip affects fracture morphology. The simulations support a transition from a leakoff-dominated fluid flow regime to a fracture-dominated regime, with the increase in characteristic time t(1) (ratio of permeability and injection rate per unit length) and decrease in t(2) (ratio of injected fluid viscosity and minimum total principal stress). We develop a correlation of the dimensionless initiation pressure P-D,P-peak and dimensionless stress S-D (upper bound: P-D,P-peak = 5.647S(D)(0.86), lower bound: P-D,P-peak = 0.567S(D)(0.83)) based on our grain-scale simulations, experimental data and field data from the literature, which can be used to predict the fracture initiation pressure in cohesionless granular media.