Numerical Modelling of Hydraulic Fracturing in Cohesionless Sand: Validation Against Laboratory Experiments

Summary In this paper, a new hydraulic-fracturing model is introduced for cohesionless sand, which is also applicable to weak sandstone for- mations with high permeability and low shear strength. Phenomena such as shear-band development and shear-enhanced permeability are of paramount importance during hydraulic fracturing of cohesionless sand or weak sandstones, which make the fracturing response quite different from what it is conventionally believed to be in competent rocks. The smeared approach in simulating hydraulic fracturing has been implemented in the proposed model within the continuum mechanics framework. Both matrix and fracture flow have been considered in this model. Tensile- and shear-fracture development and their fluid flow were simulated. The cubic law and Touhidi-Baghi-ni’s shear-permeability model (Touhidi-Baghini 1998) were used to capture the permeability evolution and to model flow in tensile and shear fractures, respectively. Shear fracturing of geomaterials involves intense localization of deformation and strain softening, which is a discontinuous phenomenon, resulting in mesh dependency of the results in the continuum model. The fracture-energy-reg-ularization method was used in this model to reduce the mesh-size dependency of the energy dissipated during fracture propagation. The smeared-fracture approach has been validated against lab- oratory hydraulic-fracturing experiments with reasonable agreement. Consistent with the experiments, the results of the numerical model indicate that tensile fractures are formed in a very small area around the injection point despite the application of high injection pressure compared with the minimum boundary stress. It is found that shear fracturing and shear-permeability evolution are the most important mechanisms that influence and control the fracturing response. The dominant fracturing mechanism is found to be governed by the high permeability and low shear strength of the material.