Wellbore instability during drilling cause substantial problems in different areas of the world, even in vertical wells. These problems occur when the near wellbore stress exceed the rock strength. To prevent these problems must establish a balance between stress and the strength of the rock, which must be restored or maintained during drilling through the proper formulation of drilling fluids, mud weights, trajectory of well and drilling practices.

Wells drilled in laminated shales or sequences of shale-sand, are classified as more unstable than those drilled in homogeneous and isotropic formations, this because the laminated materials may fail in two ways, first as intact rock where material strength is greater, and the second as sliding bedding planes, where resistance is lower. The way in which the failure of the rock is generated will be strongly influenced by the wellbore trajectory that has the with respect to the planes of weakness of the rock.

For the evaluation of the impact that can have the presence of the planes of weakness in the wellbore stability, a numerical modeling with a commercial software called the ABAQUS, which has a constitutive material model called " Joint Material", by which you may include a anisotropy in the rock material modeling, with which it represents the planes of weakness, in resistance and frictional properties. The implementation of this model to determine the mud weight that which the rock failure, either by planes of weakness or intact rock and have the sensitivity of what may be the volume of rock affected under certain conditions of mud weight.

This article presents comparison between collapse pressures obtained with analytical methodologies, the influence of the planes of weakness in the geometry of the fault and a field application.

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