Laboratory experiments on hydraulic fracturing were carried out to investigate the damage evolution during cyclic injection using acoustic emission monitoring. Cylindrical granite specimens with 50 mm in diameter and 100 mm in height were tested under a uniaxial vertical load of 25 MPa. A central borehole of 8 mm in diameter along the specimen long axis was drilled, and water was pumped in at a rate of 50 mm3/s. A total of 25 specimens were tested, 5 under continuous injection, and 20 under cyclic injection with different levels of maximum pressure starting from 76.9% to 101.2% of the average breakdown pressure under continuous injection. The results indicate that the level of maximum pressure during cyclic injection, is inverse to the number of cycles that lead to failure. Moreover, there was an average reduction of 25.9 dB of the maximum AE amplitudes recorded during cyclic injection. AE activity was detected from the early stages of cyclic injection, and only right before failure during continuous injection. The crack mode classification shows similar ratios of tensile to shear cracks for continuous and cyclic injection, and the 3D location of these events indicates that the 3.5% of shear cracks during cyclic injection are well distributed among the AE cloud. Finally, the hydraulic energy was higher for cyclic injection, especially for those cases with lower levels of maximum pressure. The radiated seismic energy ranges from 2.3 × 10−14 J to 6.3 × 10−11 J even for cases that failed at low number of cycles, however with increasing cycles the values are persistently low.
The stimulation of Enhanced Geothermal Systems through hydraulic fracturing, as a way to increase its permeability, has been associated with undesired levels of induced seismicity (Majer et al., 2007). An alternative cyclic hydraulic fracturing scheme was suggested with the aim to reduce induced seismicity. This concept is based on a hydro-mechanical model, and numerical simulation results show a reduction in breakdown pressure and seismicity (Zang et al., 2013). Later, this concept was implement in laboratory scale experiments that also resulted in a reduction of breakdown pressure and AE activity. Zhuang et al. (2016) reported a reduction of nearly 20% of breakdown pressure during cyclic hydraulic fracturing in granite specimens. Another study conducted with Tennessee sandstone, reported a 16% breakdown pressure reduction (Patel et al., 2017). However, no experimental study has covered the effect of maximum pressure during cyclic injection, fixed as a fraction of the average breakdown pressure under continuous injection.
In this study, laboratory experiments were carried out on cylindrical granite specimens to investigate damage evolution during cyclic hydraulic fracturing using acoustic emission (AE) monitoring for fracture detection and propagation. The effect of maximum allowed pressure during cyclic injection on the number of cycles to failure and the maximum AE amplitude was investigated. Also, the AE crack type as well as the hydraulic and seismic energies were analyzed and discussed. The results were compared with a group of tests carried out under continuous injection, and some conclusion were drafted.