Abstract
It is well known that the effect of shale hydration causes wellbore instability due to water phase invasion of drilling fluid to lamellar shale rich formations. This is because of that the mechanical properties (compressive strength, elastic modulus, etc.) of lamellar shale surrounding the borehole, which is rich in clay minerals, will decrease significantly after hydration. In this study, using the lamellar shale in the continental stratum of the southern Ordos Basin, the mechanical properties of lamellar shale were studied by compression tests considering the effect of lamellar structure and hydration from a macroscopic point of view. In addition, the mechanical mechanism was discussed combined with the CT scanning tests results from a microscopic point of view. The results demonstrate the following points. Lamellar shale has stronger anisotropy than bedding shale, the compressive strength (deviatoric stress) and elastic modulus of lamellar shale are both lower than those of bedding shale, and it is more prone to tension fracture. With the increase in the angle (β) between the lamina and the axial direction from 0° to 90°, the compressive strength of lamellar shale decreases when β < 30° and then increases, the elastic modulus of lamellar shale decreases greatly when β < 30° and then tends to flatten. With the increase in hydration time, the compressive strength and elastic modulus of lamellar shale both gradually decrease, and the rates of their decrements reduce. The mechanical properties of lamellar shale are more affected by hydration than those of bedding shale. The hydration of lamellar shale leads to the formation of new fractures and the expansion of existing fractures in the junction area between the laminae and rock matrix, resulting in easy tension fracture along the laminae of shale.
Highlights
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]
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]
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]
Summary
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, tuffaceous 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. 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]. Many achievements have been made in the anisotropic characteristics of shale, there are few reports on lamellar shale
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