## 4 Results
### Fracture Process  
This study focused on the hydraulic fracturing process, so L3 and L4 stages are described in detail below. According to the evolution of the dilation, the AE counts, b-value, and ultrasonic velocity, these two stages can be further divided into four phases: pore pressure increasing phase (H0), pre-fracture phase (H1), fracture nucleation phase (H2), and post-fracture phase (H3). In phase H0, the axial and confining pressure remained constant, and the pore pressure gradually increased. The rock dilation gradually increased and the velocities slowly decreased (Figure 5c). It is worth noting that the water injection pressure suddenly dropped by 2 Mpa when it reached 5.84 MPa. However, no obvious acoustic emission events were recorded at this time, and no significant changes were observed in velocities. The evolution of axial pressure, confining pressure, and dilation demonstrated that all of these decreased slightly before the decrease in the injection pressure (Figure 9a,b). Three-dimensional UT images (Figure 9d) show that an obvious low-velocity zone was below the borehole (shown by the white dotted line box) at the initial time. This indicates that there may have been defects or microcracks below the borehole. Nevertheless, with the increasing pore pressure, the velocities in this area increased at Time B but dropped again at Time C.  
Figure 8: Comparison between the location of acoustic emission events (magenta circle) in the H2 phase and CT scan images after rock fracture. The orange dotted line represents the boundary between region Ra and Rb. In phase H1, the second AE activity was initiated when the injection pressure reached 75% of its peak. Figure 5c shows that the sample underwent drastic variations in velocities in this phase. As shown in Figures 10 and 11, the AE events in this phase were clustered around the lower borehole. To better explain these variations, phase H1 has been further divided into two sub-phases: phase H1-1 and phase H1-2. Phase H1-1 corresponds to the increasing injection pressure from 12.33 Mpa to its peak 16.25 Mpa (Figure 10). The value of b increased from 1.93 to 1.99 (Time D-F), then dropped to 1.94 when \(P_{\text{in}}\) approached its peak (Time G). In combination with the UT image AE locations in Figure 10d, it can be inferred that a localized deformation had occurred at the bottom of the borehole. The increasing AE activity and dilation and decreasing velocity demonstrated that intensive microcracks were in progress. The most striking result to emerge from the UT images was that the velocity drop was restrained to some extent by the entry of water at the region marked by the dashed white box in Figure 10d. Figure 9: Mechanical response and velocity images of rock in H0 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 (the initial time refers to the time when H0 starts, and the times A–C correspond to the times marked by black dotted lines in the left subfigure). The orange dotted line in the left picture marks the moment when pore pressure starts to drop. The orange dotted line in the subfigure (**d**) represents the boundary between region Ra and Rb. The white dotted line box represents the low-velocity zone below the borehole. In Phase H1-2, accelerated energy release caused the decline in the axial pressure and pore pressure (Figure 11). Injection-induced stress variation was reflected by the b-value (Times H-K). Increasing AE activity and dilation demonstrated that the generation of the microcracks was accelerating. From the UT images, we can see that the velocity in the Ra region was higher than that in Rb. This can be explained as the higher water saturation in Ra than Rb.