UC San Diego

This paper numerically investigates inelastic behavior of sandstone for better understanding of hydraulic fracture propagation in georeservoirs. Although many numerical, theoretical, and experimental studies investigated hydraulic fracturing, not enough emphasis has been given to the inelastic behavior of rock prior and during the hydraulic fracture propagation. Current practice widely uses linear elastic fracture mechanics (LEFM) principles for prediction of hydraulic fracturing in weak sandstone. However, discrepancies between LEFM models and filed or laboratory results indicate presence of plastic deformation, such as are for example micro-cracks or acoustic emission cloud data. Therefore, this study uses J-integral for obtaining hydraulic fracture propagation criteria under the elastic-plastic stress-strain state. J-integral is calculated on the path around a DEM model in two-dimensions. A synthetic rock mass modeled in DEM has an advantage of time-stepping and stress-strain redistribution which leads to micro-cracks represented by broken bonds between DEM particles, and therefore models well elastoplastic behavior. The relationship between far-field stress magnitudes and breakdown pressures, process zone length and calculated J-integral values are presented. The relationship between crack driving forces and applied stresses is investigated to better understand the plasticity effects. The influence of stiffness of sandstone on breakdown pressures and J-integral values are also studied. Overall, results show that LEFM is not applicable for describing fracture propagation at higher confinement stresses. Inelastic J-integral increases dramatically with rock confinement, especially its plastic portion.