Undrained pore pressure response for an inclined well in a transversely isotropic poroelastic formation

ABSTRACT: Sedimentary rocks like shale, and coal often exhibit transversely isotropic characteristics, which directly influence undrained pore pressure response of a wellbore immediately after drilling. Based on the anisotropic undrained poroelastic constitutive law, the analytical undrained pore pressure solution is derived using Lekhnitskii and Amadei's solution with undrained poroelastic parameters. Incorporating the effects of elastic anisotropy, in-situ stress anisotropy, and poroelasticity, undrained pore pressure response for an arbitrary wellbore trajectory under arbitrary in-situ stress and bedding plane orientations can be analyzed. Model validation is made to compare to the isotropic case. Model results showed that differing from the isotropic formation, undrained pore pressure response is related to the wellbore pressure and all in-situ stress components in the anisotropic formation, highlighting the importance of anisotropic poroelasticity. A parametric study including wellbore pressure, well trajectory, elastic anisotropy, and poroelastic parameters is made, all of which can significantly influence the undrained pore pressure response. So undrained pore pressure response and corresponding effective stress state must be considered, which may have significant implications for the poroelastic formations with strong anisotropy.