Analysis of the vertical borehole stability in anisotropic rock formations

The objectively existing in situ stress field and the physical mechanical properties of rock are closely related to the borehole stability in petroleum engineering. However, in present engineering design, rock mass is simply treated as isotropic material. This method may be acceptable for shallow rock engineering, but for deep rock engineering, with the increase of drilling depth, the anisotropic properties of rock mass become stronger and should be considered. In the past, accurate methods to predict critical fracturing or collapse pressures were unavailable. Simple isotropic stress equations have been used to some extent, but these have failed to take into account real rock properties that are clearly anisotropic. On the basis of some rock testing experiments, the vertical borehole stability in transversely isotropic media was the main focus of this study. By solving the stress distribution on the borehole wall, a new vertical borehole stability model was established. The results obtained in this study showed that the anisotropy of the rock and the horizontal stress ratio greatly affect the stress distribution and the failure plane of vertical wellbores. Neglecting this effect can lead to errors in stability predictions. Therefore, it was seen that the effect of the rock anisotropy is of practical importance in the life of a well since it can avoid borehole instability issues.