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INTRODUCTION ABSTRACT: An analytical solution of an infinitely long borehole drilled perpendicular to the material isotropic symmetry plane of a transversely isotropic poroelastic medium is provided. The far-field principal stresses have an arbitrary inclination with the borehole axis. Numerical examples are given to demonstrate the effects of anisotropy. Horizontal and inclined borehole drilling is now a common practice in petroleum industry and also for environmental applications such as site remediation. The stability/instability issues of inclined borehole hence have attracted considerable attention. Bradley (1979) laid a milestone for inclined borehole analysis by providing an analytical solution based on linear, isotropic elasticity. Later analyses took into account material anisotropy in the forms of transverse isotropy to general anisotropy (Amadei, 1983; Aadnoy, 1987; Ong & Roegiers, 1993). In most of the analyses, the pore pressure effects were either completely ignored, or dealt with under simplified assumptions. The Blot theory of poroelasticity (Detournay & Cheng, 1993) provides a rigorous coupling between the solid and the fluid responses. For borehole analysis, analytical solutions based on poroelasticity have been derived for vertical borehole (Detournay & Cheng, 1988), and inclined borehole (Cui et al., 1995a). Material properties were assumed to be isotropic. In this paper, we extend the poroelastic inclined borehole analytical solution from the case of isotropy to transverse isotropy. Due to mathematical difficulty, only a simplified version is achieved. The solution is limited to the case in which the borehole axis is perpendicular to the material isotropy plane. Only the far-field stresses are allowed to have an arbitrary inclination. Although restricted in practical applications, it is nevertheless one of the few analytical solutions available for anisotropic poroelasticity. It can serve as a benchmark for testing numerical algorithms such as the FEM (Cui et al., 1995b). Also, analytical solutions are more efficient for parametric studies. The present solution is thus utilized to gain some understanding of anisotropy effects in poroelasticity.
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.
- Research Report > New Finding (0.54)
- Research Report > Experimental Study (0.34)