Effect of frequency during cyclic hydraulic fracturing and the process of fracture development in laboratory experiments

Abstract Cyclic injection in hydraulic fracturing has been reported to reduce the level of induced seismicity compared to the conventional continuous injection. We investigate the evolution of acoustic emission (AE) activity, hydraulic energy, fracture area, and fracture development for cyclic injection schemes with different cycle time durations (td) at a repeated maximum cyclic pressure through laboratory experiments. These test were compared with a case under conventional continuous injection at an injection rate of 50 mm3/s. Pocheon granite samples with an outer diameter of 50 mm, height of 100 mm, and an inner borehole of 8 mm were used. The maximum pressure of 5.5 MPa for each cycle represents approximately 77% of the average breakdown pressure from continuous injection cases. Four td values of 30 s, 60 s, 120 s, and 240 s were selected. The results show an exponential decrease in the number of cycles until failure with increasing td. Similarly, the total number of AEs, the peak AE rate, and the cumulative AE energy also decreased with increasing td. These values contrasted with those for continuous injection which reported the lowest number of AEs, and the highest cumulative AE energy. The td influence was also evident on the AE occurrence, with fewer high amplitude AEs at a high td, and all lower than continuous injection. In general, the hydraulic energy and cumulative AE energy decreased with increased with increasing td, although a slight deviation was related to the total testing time. The fracture areas during cyclic injection reported a positive correlation with the cumulative AE energy. Moreover, detailed analysis of the instantaneous injection rate, and the cumulative AEs suggested a three-stage damage process, which was supported with X-ray computed tomography observations. The second stage was characterized by a steady linear injection rate increase, and its slope correlated with the fatigue life or number of cycles to failure. Finally, based on the injection rate evolution, an interpretation of the damage evolution and fracture onset during cyclic hydraulic fracturing could be made.