Impacts of Rock Anisotropy on Horizontal Wellbore Stability in Shale Reservoir

In order to understand the effects of strength anisotropy of bedding shale on the collapse pressures of horizontal wells, the cores of different bedding angles were drilled out of the Longmaxi group, Sichuan Basin, and the macroscopic and microscopic shale characteristics were studied based on the polarizing micrographs and the scanning electron micrographs. Besides, the anisotropic shale strengths were investigated through the uniaxial compressive strength tests. Given the anisotropic physical and mechanical properties of the bedding shale, the practice that the shale was roughly simplified as an isotropic body in the previous borehole stability design made the predicted collapse pressure for maintaining borehole stability incapable of meeting the need for drilling safety. Hence, a calculation model for the stress field around the borehole in anisotropic formation was established. With a transverse isotropic formation model, the effects of the isotropic surface and the elastic parameters’ anisotropic ratios on the well circumferential stresses were analyzed; meanwhile, the Mogi-Coulomb criterion was adopted to estimate the wellbore stability, and the sensitivity analysis was carried out to consider the impacts of different mechanical properties including Young’s modulus and Poisson’s ratio on the collapse pressure. Results show that the formation anisotropy increases the circumferential stress distribution heterogeneity and worsens the wellbore stress situation significantly; the degree of anisotropy affects the collapse pressure a lot. The sensitivity analysis also show that Poisson’s ratio anisotropy does not markedly change the collapse pressure for low anisotropy degrees; however, the effect of the elastic modulus anisotropy is notable. The work is useful for the in-situ wellbore design before drilling.