Fluid‐driven nucleation and propagation of splay fractures from a permeable fault

Abstract

AbstractProgressive development of opening‐mode splay or branch fractures along a permeable fault in an elastic medium, subject to elevated fluid pressure from a constant influx fluid source, is studied numerically using a plane‐strain hydraulic fracturing model that couples fracture deformation and fluid flow. In situ stresses are imposed so that their resultant shear stress on the fault is lower than the frictional strength. New splay fractures are initiated based on satisfying a dual criterion for both tensile strength and fracture toughness and meeting a minimum fracture spacing requirement. Numerical results demonstrate that spatial variations in permeability along faults can cause arrest of local slip and the created slip gradient can result in splay fracture initiation at a significant distance inward from the fault tips. One splay fracture generally grows first, and the kinematic coherence in displacements at the junction between it and the fault can reduce the downstream flow rate to prevent the nucleation of other splay fractures. However, the number of splay fractures can be increased when the branch growth extent is limited to a certain size; when the fault is divided into many segments, each with a linear distributed initial aperture; and if the main fault is curved. The generation of a number of splay fractures can act to increase the permeability of the rock mass. When the splay fractures are constrained by two faults, a rhomb‐shaped fault zone involving multiple high‐angle branches forms. The development of the second‐generation splay fractures on a first‐generation one is promoted by fluid penetration. Multiple fluid‐driven splay fractures can be created under the condition that fluid pressure is below or slightly above the fault confining stress. This implies that generation of complex splay fracture patterns requires less energy than generating a single opening‐mode fracture. Multiple fluid‐driven splay fracture nucleation and growth into a horsetail pattern occur when the fault‐parallel in situ normal stress becomes more tensile. In addition, the effects of fluid viscosity and tensile fracture toughness are investigated to determine their role in splay fracture initiation and growth.