To investigate the breakdown pressure and the energy transferred to the surrounding rock, numerical simulations of circular boreholes under internal hydraulic pressure are carried out. For this purpose, a micromechanical continuum damage model proposed by Golshani et al.  is used. The simulation results show that the borehole breakdown pressure and the energy transferred to the surrounding rock are dependent on the mechanical properties of the rock, borehole size and far-field confining stresses. Although the energy transferred to the surrounding rock increases with increasing borehole size, the borehole breakdown pressure decreases. The analysis also shows that breakdown does not occur even if a crack initiated at a borehole wall. In fact, the breakdown occurs when the crack growth becomes unstable. In other words, breakdown pressure appears to correspond to the onset of unstable crack growth.
Stresses are applied inside the boreholes either to produce deformations in order to determine the modulus of the rock or to induce fractures . Hydraulic fracturing is one of the techniques used to stimulate the production of oil or gas in reservoirs. This technique involves pumping a fluid under pressure into a borehole. This pressurized fluid introduced into the borehole produces stress concentration in the surrounding rock causing the development of fractures. Furthermore, hydraulic fracturing is the common method for stress measurement in geotechnical engineering application. Breakdown pressure i.e., the pressure at which fracturing occurs, is regarded as a function of the state of stress. In fact, attention is focused on the prediction of the borehole breakdown pressure and is usually the only parameter available to evaluate the operation . However, the energy transferred to the rock during pressurization of the borehole can be considered as another parameter for the evaluation of the operation. Energy can be stored in or released from the rock medium in the vicinity of a borehole subjected to internal pressure. If the internal energy exceeds the limit that the material can withstand, the energy release will occur to reestablish the internal energy level within a tolerable limit. Griffith  suggested that a potential relief mechanism is the micro-cracking. According to his theory the excess of energy is dissipated with the growth of microcracks during rock failure. When the internal energy reaches a critical limit, this level must be reduced by one or more relief mechanisms. As previously explained, the most significant relief mechanism for rocks is microcracking. The main objective of this paper is to investigate numerically the breakdown pressure and the energy transferred to the rock around a vertical borehole under breakdown pressure by using a micromechanics-based continuum damage model proposed by Golshani et al. . For this purpose three types of rocks i.e., Inada granite, Mount Isa granite and Toowoomba basalt are simulated. The effect of borehole diameter and far-field stresses on the breakdown pressure and the transferred energy will also be discussed.