Abstract

The importance of the geomechanical modeling and wellbore instability analysis is increasing in the petroleum industry with recent growth of drilling plans in unconventional reservoirs. Recently, a large majority of field developments are occurring in shale oil and shale gas reservoirs as a novel source of energy. Many of these reservoirs are thin and highly layered, anisotropic and naturally fractured; these make their geomechanical modeling and wellbore instability analysis remarkably crucial; subsequently a good understanding of elastic properties of the formation such as Young's modulus, Poison's ratio and in-situ stresses are necessary for accurate results.

Bakken Formation is one of the major unconventional plays in North America which is identified by three distinct members: layered upper and lower shale and fractured middle dolomite. The thin beddings within the formation have made them extremely anisotropic, which originates from the platey shaped clay particles, high kerogen content plus the existence of natural fractures thus the wellbore instability analysis and modeling of this formation has become highly challenging. For this study elastic properties along with the petrophysical values are collected and reported from several numbers of core plugs along with wireline log data. Then, these values are used as input parameters in poroelasticity equations to calculate the magnitude of the principal stresses. Finally instability analysis especially in the inclined section of the well is carried out.

Finite Difference 3D numerical modeling was conducted under Mohr-Coulumb failure criterion with anisotropic assumptions to evaluate the deformations around the wellbore to locate instable regions. In addition, the effects of different inclination angles of the wellbore versus depth were studied. Results indicated a strong relation between the magnitude of the wellbore deformations with the the drilling inclination angle changing from vertical to horizontal which takes place in the direction of maximum horizontal principal stress. Stress contours are plotted around the wellbore shows zones of higher stress concentration and deformation. Various type of failure regions are developed as well.

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