Reinitiation or termination of fluid-driven fractures at frictional bedding interfaces

Abstract

A two-dimensional numerical model for coupled elastic deformation and fluid flow has been developed to examine reinitiation or termination of a vertical fluid-driven fracture in bedded rocks. The rocks on both sides of the bedding interface are assumed to be impermeable, and a Newtonian fluid, whose viscous dissipation cannot be neglected, is injected at a constant rate. Crack nucleation on the frictional interface is controlled by a tensile stress criterion. A fracture approaching the interface or terminating on it can generate a large bed-parallel tensile stress in the uncracked layer in excess of the local tensile strength to initiate a new fracture. The propagation of the nucleated fracture is assisted by interface sliding and pressurized fluid. The bedding interface can provide a conductive channel to connect the parent and the new fractures. Fluid partitioning among fracture branches depends on local stress states and triggers their competition to become the main fracture. The three types of fracture patterns generated numerically by the model are crosscutting through, terminating at, and offsetting at the interface. A large modulus or toughness contrast across the interface can lead to containment of the hydraulic fracture on the interface. For offsetting fractures, the presence of the bedding plane can reduce fracture permeability, and any associated high-stress barrier acts to slow or arrest further fluid-driven fracture growth. Propagating the new fracture results in high excess pressure, consistent with measured abnormal pressure in commercial fracture treatments. The predicted offset distances are on the order of centimeters, and their continued growth is perpendicular to the bedding contacts. Copyright 2008 by the American Geophysical Union.