Geomechanical Considerations of Anisotropic Behavior of Shale Layers

One of the major reserves of hydrocarbon explorations is shale reservoirs with unconventional geomechanical characteristics. The vast energy capacity of the mentioned reservoirs has led them to become one of the major targets of investment by the oil industry. However, due to the unconventional behavior of these reservoirs including anisotropic elastoplastic characteristics of shale layers and low permeability of the medium, traditional drilling methods do not lead to an appropriate strategy of oil extraction. On the other hand, a rigorous drilling method for operation of an oil field requires a precise comprehension of the geomechanical behavior of shale layers. The substantial aspects of geomechanical characteristics of shale layers include elastoplastic properties of the layers and stress and pore pressure distribution along the wellbore. Moreover, stress regime alteration due to the drilling advancement should also be taken into account for a comprehensive evaluation of the mentioned unconventional reservoirs.

Due to the complex mechanical behavior of anisotropic formations, the effects of anisotropy on wellbore stability has been neglected in many field explorations. In such cases, the characteristics of the layers are assumed to perform as isotropic stratifications. Studies such as works performed by Higgins et al. in 2018 and Thiercelin and Plumb in 1994 show that the assumption of isotropy for anisotropic layers leads to imprecise solutions of stress analyses. The main reason for isotropy assumption for anisotropic layers is the lack of advanced anisotropy field measurements. With the improvement of P-wave and S-wave acoustic logging technology, processing the anisotropic characteristics of formations have become more practical. The mentioned technology provides a more precise understanding of the anisotropic behavior of a geomechanical system. With the advancement of measuring technology of anisotropic characteristics of formations, several studies were conducted to evaluate the effects of anisotropy in stress and deformational behavior of unconventional reservoirs. Some of these studies include works performed by Aoki et al. in 1993, Khademian et al. in 2016, Lee et al. in 2012, and Meier et al. 2015. In these studies, analytical and numerical methods were used to understand the mechanical behavior of anisotropic layers. However, in most of the mentioned studies, the single methodology was selected without the integration of both experimental and numerical evaluations. Moreover, the mentioned studies were only limited to vertical boreholes, and the effects of wellbore inclination were neglected on the geomechanical response of the drilling.

The inclination of the wellbores is very common when a vertical drilling advancement transmits to the horizontal direction. Although more studies were focused on stability analysis of vertical wellbores, few investigations have been conducted to evaluate the inclined wellbore conditions in anisotropic shale medium. More in-depth understanding is needed for evaluation of stress and deformation regime around the inclined wellbore circumstances through anisotropic shale medium.