## 1 Introduction  
With the advancement of shale oil exploration and development technology, exploiting huge shale oil resources has gradually become viable, and the shale oil resources have already played an important role in many countries [1, 2, 3, 4, 5, 6]. The continental shale is generally mixed up with silty, tufaceous or argillaceous minerals, presenting a horizontal lamellar structure with dark and light colors [7]. Affected by this lamellar structure, the mechanical properties and hydration mechanism of lamellar shale in the continental formation are significantly different from those of bedding shale. As a result, the existing borehole stability calculation methods and anti-collapse technical measures for bedding shale formation are difficult to adapt to deal with the borehole instability problem of the lamellar shale oil reservoir. Therefore, it is important to understand the hydration-mechanical properties and micro-instability mechanism of lamellar shale, to provide a basis for the study of the borehole stability prediction technology of lamellar shale reservoir. There are obvious bedding planes in shale, and the weak bedding planes lead to the shale forming significant transverse isotropic characteristic [8; 9; 10]. This characteristic causes the direction of stress loading on the shale bedding to have a significant impact on the failure of shale. For example, when the stress is parallel to the shale bedding plane, the shale forms local shear failures or tension fractures that are parallel to the bedding plane; when the stress is perpendicular to the bedding plane, only local shear failure occurs [11; 12]. Therefore, the strength, elastic characteristics, crack propagation and corresponding strain of shale all vary with the angle of stress and the bedding plane (\(\beta\)) [13; 14]. [PERSON] et al. [15] summarized previous data and found that when \(\beta\) = 30\({}^{\circ}\), the compressive strength of shale is lowest, whereas when \(\beta\) = 90\({}^{\circ}\), the compressive strength is highest, forming a \"U\" shape between compressive strength and \(\beta\)[9]. In addition, the shear failure caused by wellbore pressure relief and the fracture extension in reservoir fracturing are all affected by bedding planes, complex natural pores and fractures, showing the anisotropic failure characteristics of shale formation [16; 17; 18]. As mentioned above, although many achievements have been made in the anisotropic characteristics of shale, there are few reports on lamellar shale. Because shale is generally rich in clay minerals, the mechanical properties of shale are significantly affected by water intrusion [19]. [PERSON] et al. [20; 21] believe that surface hydration occurs on the particles of clay minerals; these minerals then exhibit hydration swelling, which generates a swelling stress. [PERSON] et al. [22] and [PERSON] et al. [23] found that the strength, Young's modulus and brittleness index of shale gradually decrease with the increase in water content. Therefore, [PERSON] et al. [24] believe that secondary fracture failure caused by self-absorption is one of the main reasons for wellbore instability in hard and brittle shale formations. Thus far, many mechanics-chemistries coupling methods have been established to study the borehole stability of the shale reservoir [25; 26; 27]. For example, [PERSON] and [PERSON] [28] established a qualitative description method for shale hydration damage based on CT scanning data of shale to analyze shale borehole stability. [PERSON] et al. [29] found that the number of weak shale planes, the difference in the occurrence of weak shale planes and different water saturations have a significant impact on wellbore stability while drilling through shale formation. Regarding shale hydration mechanics and wellbore stability research, currently, there are also few reports on lamellar shale. In summary, at present, there are relatively few studies on the anisotropic and hydration characteristics of lamellar shale.