ABSTRACT: The finite element code ABAQUS is used to model a typical granite specimen subjected to either uniaxial or biaxial compression, with two pre-fabricated flaws with the geometry 2a-30-30 in which only one flaw is pressurized. The maximum and minimum principal stresses as well as the maximum shear stresses are analyzed around the flaw tips and along the bridge between the inner flaw tips of the pressurized and non-pressurized flaws. When the specimen is loaded uniaxially, the maximum principal stresses in the bridge between inner flaw tips are tensile near the pressurized flaw and decrease significantly as one moves towards the non-pressurized flaw. For the biaxial loading, mainly compressive principal stresses are observed for low hydraulic pressures; tensile stresses start to develop for larger hydraulic pressures, but only near the pressurized flaw. For both uniaxial and biaxial cases, tensile and shear cracks may occur near the pressurized flaw but are not theoretically possible near the non-pressurized flaw.
Hydraulic fracturing is becoming a common process used in the exploitation of oil and gas entrapped within shale formations as well as in the mining of heat in Enhanced Geothermal Systems (Gidley, 1989) and in Artificial Groundwater Recharge (Singhal & Gupta, 2010). While there are a number of studies on the initiation and propagation of hydraulic fractures under uniaxial loading (Miller, 2008; Chitrala et al., 2013; Gonçalves da Silva, 2016), limited experimental and numerical studies focus on the effect of biaxial loads on the stress field around pressurized and non-pressurized fractures. As extensively discussed by previous studies, one of the major concerns involved with the hydraulic fracturing applications is the path at which new crack propagate. Bobet (1997) used molded gypsum specimens with either open or closed prefabricated flaws under uniaxial and biaxial loading conditions to observe the crack initiation, propagation and ultimate coalescence of the specimens. Bobet (1997) described several significant observations, the most important of which are shown in table 1, as they are relevant to the present study.