## 4 Experimental Results
### Characterization of Fracture Curves and Analysis of Fracture Expansion Patterns  
According to the pump pressure curve (Figure 8), the whole hydraulic fracturing process can be divided into three stages: (1) wellbore filling zone--the fracturing fluid enters the rock specimen along the wellbore, and the pressure rises slowly at this time due to the fact that the wellbore itself occupies a certain space in the initial stage; (2) pressure-holding zone--as the fracturing fluid fills up the wellbore, the pump pressure rises rapidly, and the fracture starts inside the rock when the injected pressure reaches the rupture strength of the rock; (3) post-slit zone--after the formation of a penetrating fracture inside the rock and complete cracks, the pump pressure decreases drastically and basically stabilizes at the minimum ground stress.  
As can be seen from Figures 8 and 9, the average fracture starting pressure of the four marine shale specimens is 19.45 MPa, the pumping curve pattern is basically the same, and the post-peak pressure all shows an obvious drop. According to the comparison between S-1 and S-2, the fracturing pressure increased when using large displacement and the total fluid injection volume was lower. From the morphological results, it can be seen that with large displacement fracturing, the fracture is able to penetrate through all the laminae and activate the adjacent laminae; with small displacement fracturing, due to the lower pressure fluid energy at the initial stage, it is easier for the fracturing fluid to activate the laminae connected with the wellbore and form a dominant seam; and with the increase in fracturing fluid energy, the fracture cannot break through the laminae and finally form a laminar seam. Comparing S-1 with S-3, it can be seen that by 2 h after soaking the specimen, the fracture pressure of the marine shale will be increased. Comparing S-1 with S-3, the fracturing pressure of the specimen is reduced after soaking for 2 h and the fracture morphology is more complicated. S-1 and S-4 can be obtained. With fracturing under the condition of large stress difference, the specimen is more likely to form a main fracture without activating a laminar fracture and the fracture morphology is more homogeneous.  
Figure 8: Shale pumping curve under different fracturing conditions.  
Due to the more developed laminar joints, the average fracture initiation pressure of continental shale is 17.875 MPa, which is 8% lower than that of marine shale. The pumping pressure curve of continental shale appears to be maintained at a high-pressure state, and the pumping pressure only decreases significantly after stopping pumping. After profiling the specimens, it was found that hydraulic fractures were mostly activated laminar joints, and it was more difficult to form the main fracture through the laminae.  
From the analysis of Y-1, Y-2, and Y-4, it can be seen that under the condition of a small stress difference, increasing the fracturing fluid displacement can effectively increase the fracture complexity of continental shale. For continental shale Y-3 using hydration for 2 h, the fracture pressure decreases significantly but the fracture complexity does not change significantly, which is due to the higher clay mineral content of continental shale; the water-sensitive effect is larger; and it is easier to shear damage along the laminar joints. Therefore, it is necessary to reduce the rupture pressure during the fracturing process of continental shale oil and gas by using the smothering strategy.