Cohesive zone models

This chapter presents the use of the cohesive zone model as a fracture propagation criterion in nonlinear finite element analyses with application in hydraulic fracturing. The influence of the main parameters of the cohesive zone model on the hydraulic fracturing results is examined. It is shown that the size of the process zone in hydraulic fracturing is affected by the loading branch of the cohesive constitutive model with the elastic-softening to result in much larger process zone than the rigid-softening model. The exact form of the softening branch of the cohesive model has no significant influence on the obtained results. The existence of the confining stresses increases the size of the process zone. The changes in the pore pressure, during fracturing, increase further the size of the process zone. In all cases, the larger process zone imposes a bigger fluid lag resulting in higher net pressure for propagating a fracture and the created fracture is wider.