INFLUENCE OF INELASTIC ROCK BEHAVIOR IN HYDRAULIC FRACTURING

This paper examines the importance of different non-linear mechanisms, such as plastic yielding, rock dilation, cohesive zone and fluid-lag on a hydraulic fracture propagating in a low permeability formation. Rock is modelled by the elastoplastic constitutive equations of Mohr-Coulomb flow theory of plasticity for cohesive-frictional, dilatant material. The fluid flow is modelled by lubrication theory and the constant velocity flow assumption. A cohesive crack model which takes into account the softening behaviour that most rocks exhibit in the post-failure regime is employed as the propagation criterion. The problem is solved numerically by the finite element method. It is shown that plastic yielding, which leads to rock dilation, results in a fracture with significantly greater opening and therefore a smaller dry zone than the elastic fracture.