Comparison of the classical and fracture mechanics approaches to determine in situ stress/hydrofracturing method

The earth’s crustal stress determination is a crucial item in civil, petroleum, and mining engineering and also in energy development fields. The hydraulic fracturing (HF) is one of the standard methods for estimating in situ stress, which is often analyzed with classical approach for stress estimations. The classical approach neglects the presence of pre-existing cracks in rock mass; however, the hydraulic fracturing is a phenomenon of fracture propagation in which the fracture mechanics approach is more appropriate to consider more complex items in stress estimations. In this study, the in situ stress tests are conducted in Azad Pumped Storage Power Plant (Azad PSPP) in the west of Iran and then the classical approach and four available analytical fracture mechanics methods are applied to determine the in situ stresses. Moreover, numerical modeling based on the displacement discontinuity method (DDM) is used to model hydraulic fracturing process and the results are compared with analytical methods. A comprehensive analysis on the length of pre-existing crack is done as an influential parameter on stress magnitudes. The results indicate that the magnitudes of in situ stresses calculated by the fracture mechanics approach are often greater than the stress magnitudes calculated by the classical approach. Also, the effect of crack inclination angle and pre-existing crack length on the magnitudes of σ_H is studied. In addition, the distribution of fluid pressure along the cracks and its influence on the stress intensity factor in the crack tip are analyzed, and consequently, the estimation of in situ stress is investigated.