Cohesive fracture mechanics based analysis to model ductile rock fracture

Most of the hydraulic fracture modeling in the oil industry continues to rely on empirical models, or on numerical methods based on linear elastic fracture mechanics (LEFM). Generally these methods give reasonable predictions for hard (brittle) rock hydraulic fractures. However, for ductile rocks, such as clay or weakly consolidated sandstones (low cohesion granular material), LEFM-based methods typically give conservative predictions on fracture geometry. One of the reasons the LEFM-based methods fail to give accurate predictions for these materials is that the fracture process zone ahead of the crack tip should not be neglected (and can be significant) in ductile rock fracture analysis. Based on the size of the fracture process zone and its effect on crack extension in ductile rock, the fundamental mechanical difference of LEFM and cohesive fracture mechanics-based methods is discussed in this paper. A pore pressure cohesive zone model (CZM) has been developed and applied to predict hydraulic fracturing for injection wells. The cohesive zone method is a numerical tool developed to model crack initiation and growth in quasi-brittle materials considering the material softening effect. The pore pressure CZM has been applied to investigate the hydraulic fracture with different rock properties. The hydraulic fracture predictions of a three-layer water injection case has been compared using pore pressure CZM with revised parameters, LEFM-based pseudo 3D model, a Perkins-Kern-Nordgren (PKN) model, and an analytical solution.