The Influence of Elastic Anisotropy on Borehole Stability

Abstract

This thesis investigates the effect of elastic anisotropy on borehole stability, with the purpose of determining how significant this effect is. How this anisotropy affects borehole stresses and borehole failure through either collapse or fracturing is examined more closely. Some key findings from previous studies related to this topic were obtained through a literature search. In addition, results from MATLAB models investigating the impact of elastic anisotropy on borehole stresses and initiation of shear and tensile failure are presented and discussed.

The results from both the literature search and the MATLAB models indicate that the effect of elastic anisotropy on borehole stability is indeed significant. Changes in elastic anisotropy affect the borehole stresses, as well as the risk of collapse and fracturing of the borehole during drilling. The results also suggest that the risk of borehole failure is even higher if high elastic anisotropy is combined with a high inclination and/or an unfavorable azimuth direction. Increases in both elastic anisotropy and stress anisotropy seem to affect the tangential stress, the collapse pressure and the fracture pressure considerably, although somewhat differently. The type of faulting regime also seems to play a part as to how elastic anisotropy affects borehole stability, specifically with regards to fracture initiation.

All these results indicate that the effect of elastic anisotropy should not be neglected with regards to borehole stability. Incorporating this form of anisotropy into models for borehole stability analysis could potentially enhance the accuracy of these models. An implication for drilling operations could be more accurate estimates of the mud weight window, and thereby also a reduced risk of borehole failure.