## 5 Discussion  
According to the above experimental results, we found some phenomena worthy of discussion during water injecting into tight sandstone. First of all, under high differential stress, a local fracture occurred at the bottom of the borehole at the initial H0 phase. However, no significant CT abnormality was observed below the borehole before the test, which indicated that these defects were unlikely to have been generated naturally. Earlier drops in \(P_{\mathrm{a}}\), \(P_{\mathrm{c}}\), and \(\Delta\) than \(P_{\mathrm{in}}\) demonstrated that this local fracture was not caused by the water injection. Therefore, it can be considered that these defects or microcracks were caused by uneven stress distribution at the bottom of the borehole. Although the fracture was small in scale and did not cause obvious acoustic emission activity, it still increased the permeability in this area and eventually led to the flow and diffusion of the fluid. Interestingly, the entry of water reduced the rock strength and promoted the occurrence of microfractures, which, in turn, accelerated the entry of water. Hence it can be considered that there was continuous competition between micro-fracture and water flow in this region. Moreover, this competition also occurred in the fracture nucleation phase, in which the intense microcracks increased the porosity, decreased the pore pressure, and increased the effective stress. Figure 14: Mechanical response and velocity images of rocks in H3 phase: (**a**) axial (red line) and confining pressure (green line); (**b**) dilation (purple line) and water injection pressure (blue line); (**c**) b-value (red line) and AE rate (green line); (**d**) slices of the 3D ultrasonic velocity images at different times (Times P–R correspond to the times marked by black dotted lines in the **left** subfigures, and Time S corresponds to 5 min past Time R). The orange dotted line in the subfigures (**d**) represents the boundary between region Ra and Rb. Secondly, a rupture occurred earlier in the Ra region than Rb. Since the drainage condition plays an important role in fracture behaviors, the fracture mechanism in these two regions should be carefully considered. Consistent with [62], our results also verified that the nucleation dimension and duration could be enlarged by good drainage conditions. In addition, the earlier fracture in Ra proved that the rupture could be premature in good drainage conditions. In our test, the high differential stress caused the sample to dilate before water injection. Hence, the competition between the microcrack and fluid flow started at the beginning of the water injection. Due to the better drainage conditions in Ra than in Rb, diffusion of the pore pressure was faster in Ra than in Rb. The faster diffusion meant the faster reduction in the rock strength. Hence, an earlier rupture was observed in Ra than in Rb. This finding may improve the predictability of the fracture induced by the fluid injection, which has a great significance in the risk assessment of hydraulic fracturing. Thirdly, our test showed that both \"dilation hardening\" and \"seismic pump\" modes were observed in different parts of the tight sandstone. This can be also be explained by the difference in drainage conditions. Although the \"dilation hardening\" had been well-established in the region Ra, the good drainage conditions caused it to be greatly suppressed by the dilatancy-driven flowing fluid. When fracture nucleation initiated in Rb, the water stored in the pore space of Ra was pumped into the newly generated fracture at Rb, which promoted the rupture process in Rb. These findings suggest that the local hydraulic condition dominated the fracture evolution. Hence, underground hydraulic conditions must be considered when designing field hydraulic fracturing schemes. After rock failure, the macroscopic shear failure greatly improved the permeability of the tight sandstone, which led to the rapid movement of water into the damaged area in the sample.