Mechanical Factors Controlling the Development of Orthogonal and Nested Fracture Network Geometries

Orthogonal fracture networks form an arrangement of open well-connected fractures which have perpendicular abutment angles and sometimes show topological relations by which fracture sets abut against each other, thus forming a nested network. Previous modelling studies have shown that orthogonal fractures may be caused by a local stress perturbation rather than a rotation in remote stresses. In this study, we expand on the implications of these local stress perturbations using a static finite element approach. The derived stress field is examined to assess the development of implemented microfractures. The results show that the continuous infill of fractures leads to a gradual decrease in the local tensile stresses and strain energies, and, therefore, results in the development of a saturated network, at which further fracture placement is inhibit. The geometry of this fully developed network is dependent on the remote effective stresses and partly on the material properties. Saturated networks range from: (1) a set of closely spaced parallel fractures; (2) a ladder-like geometry; and (3) an interconnected nested arrangement. Finally, we show that our modelling results at which we apply effective tension, are equivalent to having a uniformly distributed internal pore fluid pressure, when assuming static steady state conditions and no dynamic fluid behaviour.