Role of shale thickness on vertical connectivity of fractures: application of crack-bridging theory to the Austin Chalk, Texas

Abstract

Contrasting material properties of alternating chalk and shale layers control vertical connectivity of fractures. Our field observations within the Austin Chalk, Texas indicate that: (1) the majority of vertical fractures occur in chalk layers and abut against contacts with shale layers, (2) thicker shale layers have greater resistance to fracture propagation than thinner shale layers. From these observations we hypothesize both the resistance of shale to fracturing and the thickness of shale layers may inhibit fracture propagation across the shale and into the next chalk layer.We model crack propagation within a three-layered system (brittle chalk:fracture resistant shale:brittle chalk). The modeled crack extends across the shale, but closing tractions applied to the crack segment within the shale layer simulate resistance of shale to fracturing. The crack-tip lies a short distance within the unfractured chalk layer simulating a coplanar flaw with potential to propagate. If the stress intensity factor at the flaw exceeds the chalk fracture toughness, the crack propagates, thereby bridging and eventually rupturing the shale layer. For any chalk thickness, there is a critical shale thickness above which fractures cross the shale layer and below which fractures arrest at shale.Finite Element Method (FEM) analysis evaluates the influence of shale ductility within the chalk: shale: chalk system. Although remote and fluid pressure driven fractures produce identical stress intensity factors in elastic chalk/shale systems, lower driving stresses are required to propagate fluid pressure driven fractures through a system ductile shale layers than fractures under remote tension.