Borehole Stability - Aspects of the Influence of the 3D Effective Stresses

摘要

This article presents three aspects of the effective in-situ stresses with relevance to borehole stability, whose insufficient consideration can lead to serious misjudgments of stability. One of these (Aspect A) is possible modification of the effective borehole loading due to loosening of the rock. Above all in initially dense rocks this can lead (along with reduction of the rock strength) to increases in the pore pressure effectiveness and so also in the mechanically operating pore pressure (MOP P~(plus)). This results in changes (mainly reductions) in the effective stresses and thus in the borehole stability. Furthermore, the influence of borehole trajectory on shear stresses parallel to the borehole axis is explained (Aspect B). In boreholes deviated from the principal directions of the in-situ stresses there are always normal stresses to the boreholes which differ from the principal stresses, and shear stresses parallel to the borehole axis, which change the level of stability. The greatest failure intensities in deviated boreholes occur with the largest shear stresses parallel to the borehole axis and/or with the largest effective normal stresses to the borehole-While the normal stresses can be influenced by variation of the mud-pressure, it is not possible to influence the shear stresses parallel to the borehole axis. Also significant for stability analyses is the knowledge of and the complete consideration of, the 3D in-situ stress state (Aspect C). With classical 2D analyses of borehole stability made only on the basis of the in-situ normal stresses perpendicular to the borehole axis and/or without consideration of the (often not quantified) intermediate in-situ stress component it is hardly possible to identify and evaluate stability problems. All three aspects are documented with practical examples. Advice is also given about suitable tools for use with these types of situations.