## 3 Results and Analysis
### Microstructure Characteristics of Lamellar Shale  
Figure 10 shows the CT scanning image of the dry lamellar shale sample, the 48-h hydration lamellar shale sample and the 48-h hydration lamellar shale sample after a triaxial compression test under the confining pressure of 30 MPa. As shown in Figure 10a, the dry lamellar shale sample is relatively dense, its minerals are cemented tightly and there are no obvious fractures in the dry lamellar shale sample. As shown in Figure 10c, in the top view of the sample after hydration in distilled water at 80 \({}^{\circ}\)C for 48 h, there are two parallel unequal length fractures, the one near the center is longer and the other one is shorter. The front view image shows two fractures, one wide and one narrow, and they are approximately parallel with each other. The three fractures in the left view image are also basically parallel to each other with the same width but different lengths. In the different observation views in Figure 10b, the hydration fractures in the sample are consistent with the lamellar direction, which implies that water tends to flow along the laminae, showing the anisotropic permeation characteristics of water in lamellar shale [41] and resulting in hydration swelling damage in the laminae. Thus, lots of tensile cracks along the laminae (Figure 10b), which will lead to significant plastic deformation, would develop in the lamellar shale after hydration. Therefore, the strength and elastic modulus decrease significantly when the failure of the lamellar shale sample after hydration is along the laminae under compression test, and the rock would develop from an elastic-brittle characteristic to an elastic-plastic characteristic, as mentioned in Section 3.3. The fractures in the hydrated lamellar shale sample after a triaxial compression test under the confining pressure of 30 MPa distribute parallel to the lamina direction, forming  
Figure 10: CT scanning images of the lamellar shale samples: (**a**) the dry lamellar shale sample; (**b**) the 48-h hydration lamellar shale sample; (**c**) the 48-h hydration lamellar shale sample after triaxial compression test under the confining pressure of 30 MPa. a large tensile fracture from the top and left views (Figure 10c). Compared with the tensile fracture characteristics of the lamellar shale sample after hydration in distilled water at 80 \({}^{\circ}\)C for 48 h (Figure 10b), the widths of the fractures increase and the number of them is larger after the triaxial compression test. Therefore, it can be deduced that the hydration fractures along the lamina direction can highly promote the failure of lamellar shale during a triaxial compression test, showing that with the increase in deviatoric stress, these hydration fractures would be extended and some new features can also be produced along the lamellar direction, making the rock more likely to be damaged with a plastic characteristic and weak mechanical properties. From what has been discussed above, we should pay close attention to the wellbore stability of lamellar shale formation during drilling operations using water-based drilling fluid. First, since the water tends to flow along the lamina resulting in hydration damage in the lamina that can highly decrease the rock strength, as mentioned in Sections 3.3 and 3.5, a geological analysis should be conducted in detail to understand the distribution characteristics of lamellar shale, and water-based drilling fluid should be avoided for drilling operations in lamellar shale formation if possible. Second, because lamellar shale has the lowest strength for the sample with \(\beta=30^{\circ}\) and highest strength for the sample with \(\beta=90^{\circ}\), as mentioned in Sections 3.2 and 3.3, the direction of the wellbore should be precisely designed to obtain a high stability of the wellbore, considering the lamina direction and in situ stress distribution.