Breakout Patterns in Wellbores Drilled in Anisotropic Rocks

The additional costs that arise due to wellbore instability problems in the oil industry have captured the attention of petroleum companies. At the beginning of the oil industry, most of the drilling activities were guided by simple geological and geomechanical models that were more than enough for the complication of the fields that were being developed. Nevertheless, in the last years, the implementation of directional and horizontal drilling techniques has demanded our industry to build more complex and robust geomechanical models to better understand the rock’s mechanical properties and behavior after drilling operations. The aim of this work is first to highlight the importance of appropriate geomechanical knowledge to prevent wellbore stability problems, mainly present in anisotropic rocks in the way of wellbore enlargements, to then recognize different breakout patterns around the wellbore vicinity. We based our study on the behavior of three types of shales: Mancos, Tournemire, and Bossier by using the Weakness Plane Model and the modified Hoek and Brown failure criteria for anisotropic rocks. The three shales exhibit different degrees of anisotropy and here we characterized their response under the different in-situ state of stresses and more important, having different weakness plane inclination to recognize possible breakout patterns. The results of our analysis have proved the importance of adequate implementation of the in-situ state of stresses especially when the rocks anisotropy value is not high and for specific inclinations of the weakness planes. If the in-situ state of stresses is not properly dimensioned, the effect of the weakness planes could be either difficult to recognized or wrongly attributed to intact rock failure as we have demonstrated in this work.