Transient failure of a borehole excavated in a poroelastic continuum

Abstract

In this paper, the poroelastodynamic theory is employed to study the transient failure of a suddenly excavated borehole in a poroelastic continuum subjected to a non-hydrostatic far-field stress state. By introducing four scalar potential functions, the solutions for the stresses and pore pressure are given as the product of an exact treatment of the full fluid-solid coupling through field expansions with emphasis on the effect of solid-fluid acceleration. Transient tensile and shear failure responses are investigated based on an overbalanced drilling for two types of boundary conditions: a permeable surface and an impermeable surface. It's found that the permeable borehole has a larger tensile failure area than the impermeable borehole in the direction of maximum in-situ stress, and meanwhile, four symmetric shear failure areas are observed near the borehole for both internal boundary conditions. Influences of poroelastic parameters on the transient failure responses of a permeable borehole are analyzed in a detailed parametric study. The failure responses for both classical quasi-static poroelastic and poroelastodynamic theories are computed and compared to study the importance of the inertial effect in very early times. Noteworthy is that the borehole is more unstable in the poroelastodynamic theory compared to the classical poroelastic theory because the inertial effect enhances the transient response in the early times, increasing the minimum principal stress to a tensile state in the direction of maximum in-situ stress so as to result in transient generation of both tensile and shear failures in that direction. The results in this paper provide fundamental insights on the borehole instability under some dynamic bottom-hole conditions.