INTRODUCTION

ABSTRACT:

Using a recently derived analytical solution, the stability of an inclined borehole in an isotropic poroelastic medium is examined. The analyses include collapse as well as fracturing failures. It is demonstrated that the trend of failure in relation to borehole inclination, mud pressure, and time is quite complex.

The stability of an excavated or pressurized inclined borehole is of critical interest to the rock mechanics community and in particular to the petroleum industry. The first analytical solution of inclined borehole was presented by Bradley (1979), in which the rock was assumed to be isotropic and elastic. Although the elastic analyses have been widely used, field tests and laboratory experiments showed phenomena which could not be accounted for (Fjmr et al., 1992). Factors such as poroelasticity (Detournay & Cheng, 1988), poroviscoelasticity (Abousleiman et al., 1995), nonlinear elasticity (Addis & Wu, 1993), and plasticity (Ewy, 1993), have been proposed.

This paper focuses on the poroelastic effects. For rocks permeated with fluid, the diffusion of pore pressure strongly modifies the effective stress field around a borehole. The proper theory that describes the coupling between the solid and fluid constituents is the Blot theory of poroelasticity (Biot, 1941; Detournay & Cheng, 1993).

The analysis of borehole problem based on linear poroelasticity was presented by Detournay and Cheng (1988) for a vertical borehole. A number of interesting phenomena have been reported (Detournay & Cheng, 1988; Cheng et al., 1993). Lately, the solution has been extended to take into account the stress inclination (Cui et al., 1995).

Utilizing the recently derived solution, this paper investigates the stability associated with borehole inclination and borehole mud pressure. The borehole collapse and fracturing failures axe examined. The results show that the effect of pore pressure on the stress threshold, the time, and the region of failure is quite complex.

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