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An oil and gas company is drilling horizontal wells in Lower Permian carbonate reservoir in the Field X, Russian Platform. The area is characterized by high risks of borehole instability in Upper Permian overburden shales that overlie Kungurian anhydrite and salt deposits and Artinsky fractured carbonate reservoir rocks. Deviated well trajectory in 245 mm (9 5/8”) section contribute to higher risks of borehole instability in overburden shales. 1D and 3D near-wellbore geomechanics modeling taking into account shear slip along bedding planes of weakness was carried out for deviated wells to optimize mud weight and reduce risks of wellbore instability. The plane-of-weakness failure criterion is adopted to identify the onset of rock sliding along a weak plane (bedding or fracture) in the anisotropic wellbore stability model. The influence of bedding orientation and wellbore trajectory on the borehole stability is considered in the model. Shale formation rock strength anisotropic parameters, i.e., cohesion, friction angle and tensile strength for bedding planes of weakness, were deduced from the intact rock matrix parameters as a normal practice of geomechanical modeling, as no core test data were available from the shales. According to post-drill geomechanical analysis, borehole damage computed using the wellbore stability model with bedding planes of weakness is in agreement with drilling events (cavings, overpulls) occurred under insufficient mud weight in 245 mm section of deviated well A. In contrast, poor correspondence between modeled borehole breakouts and drilling events was obtained by using the conventional wellbore stability model. Deviated well trajectory and angle of attack at 45°± 5° in 245 mm section represent settings that are favorable for shear slip along bedding planes contributing to borehole instability. According to the modeling results, stable mud weight window in 245 mm section of well A narrows in the model with planes of weakness if compared to the model with intact rock only. Well trajectory optimization was recommended to keep angle of attack below 30° in shales and increase deviation in the underlying anhydrite and salt deposits. The anisotropic wellbore stability analysis applied for the first time in deviated wells of the Field X has shown that modeling of bedding plane of weakness in shales was essential to quantify the effect of shale strength variation with angle of attack on wellbore stability, and has to be considered in geomechanical modeling for drilling the wellbore safely and cost effectively.

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