Deflection and propagation of fluid-driven fractures at frictional bedding interfaces: A numerical investigation

Propagation of fluid-driven or hydraulic fractures deflected at bedding interfaces in layered sedimentary rocks and subsequent fluid invasion is investigated numerically using a two-dimensional boundary element model. The fracture is driven by an incompressible Newtonian fluid injected at a constant rate. The frictional stress on the interface is assumed to obey Coulomb's frictional law without cohesion. The bedding interface can be given a non-zero minimum fluid conductivity. A numerical scheme that deals with both rock deformation and fluid flow is presented and its accuracy is verified in terms of comparisons with existing results. To explore the mechanisms involved in fluid and fracture invasion into the interface, parametric studies are carried out for different elastic modulus contrasts, in situ stresses, interfacial frictional coefficients, distances from the injection point to the interface, and fluid viscosities. The results are provided as time-dependent variations of displacements, fluid pressures, contact stresses and fluid fronts. Fracture deflection and fluid invasion into the interface are found to rely essentially on local stress and deformation states at the intersection point. Fluid invasion and fracture growth may be delayed or inhibited when the interface is subjected to large confining stress or when fluid viscosity is relatively low for cases where the fluid-driven fracture originates in the softer layer. In this case, a greater layer-parallel tensile stress is produced and can lead to fracture propagation through the bedding contact. Low to medium frictional strength is found to promote fluid penetration and T-shaped fracture formation by interfacial opening. If the hydraulic fracture originates in a stiffer layer, fluid invasion into bedding contacts can occur smoothly without the occurrence of interface closure, and the fracture is thus terminated by forming a T-shaped fracture at the bedding interface. For fracture deflection into and growth along the interface in the absence of interface closure, the long-time responses resemble the solution for a fluid-driven fracture growing along a frictionless interface with vanishing toughness.