Hydraulic fracturing is of great interest for shale gas as well as enhanced geothermal systems (EGS). We present a laboratory-scale proof of concept study for cyclic hydraulic fracturing in which an injection pressure smaller than the monotonic breakdown pressure under continuous injection is applied in a cyclic manner. A series of hydraulic fracturing tests was performed on Pocheon granite specimen having a diameter of 50 mm and a height of 100 mm containing an inner hole of 8 mm in diameter. The average breakdown pressures of granite through continuous injection tests were calculated considering inherent strength anisotropy among rift, grain and hardway planes. A preliminary investigation shows that the threshold of a number of cycles for effective cyclic fracturing is in the order of tens of cycles with injection pressure larger than 82 % of the monotonic breakdown pressure. The generated fractures were observed by means of X-ray computed tomography (CT). It is found that fractures induced by continuous injection are more like a single fracture, while there were many branch failures for cyclic injection particularly at a large number of cycles. The anecdotal evidences from this study imply that hydraulic fracturing in the field could be conducted with a smaller injection pressure when it is subjected to a reasonable number of cycles.
Hydraulic fracturing with minimum impact on the environment, i.e. induced seismicity, is of great interest in hydraulic fracturing for shale gas as well as enhanced geothermal systems (EGS). Some concepts have been presented, like fatigue hydraulic fracturing and multistage stimulation. The key point in fatigue hydraulic fracturing is the frequent lowering of the injection pressure to allow stress relaxation at the fracture tips (Zang et al., 2013). Reducing the maximum injection pressure by cyclic injection will lower the fracture breakdown pressure and, as a consequence, induced seismicity is expected to be smaller.
For multistage stimulation, instead of massive hydraulic stimulation, injection rate and pressure are controlled and stimulated reservoir will be formed stage by stage (Zimmermann et al., 2015; Meier et al., 2015).
The multistage stimulation method has been successfully applied in commercial shale gas development (Johri and Zoback, 2013; Lee and Kim, 2016). On the other hand, the fatigue hydraulic fracturing is still at concept proof stage and its validation in field scale has yet to be achieved in EGS (Zimmermann et al., 2010; Yoon et al., 2015). As the name implies, fatigue hydraulic fracturing is to fracture rocks by cyclic injection to reach a fatigue failure. A general understanding of fatigue failure is that damages accumulate in rocks during cyclic loading, consequently leading to strength decrease. However, the mechanism of rock fatigue failure has not been fully understood yet. Both rock type (mineralogically and texturally) and loading mode influence fatigue behavior of rocks (Erarslan and Williams, 2012a). Note, however, that the fatigue cycling process during mechanical loading (Erarslan and Williams, 2012a) can be very different from the fatigue hydraulic fracturing process because of the involvement of high-pressurized fluid cycles operating at fracture tips (Zang et al., 2013).