The paper presents the results of wellbore stability modeling of a horizontal well at the Kosukhinskoye field in Western Siberia (Rosneft Oil Company). Rosneft planned to drill a horizontal well for the Tyumen Formation sediments (U3, U4). This drilling area is characterized by high risks of wellbore collapsing. Several wellbores were eliminated as a result of shear failure in the intervals of the most unstable clay deposits of the Achimov, Abalak and Tyumen ages.
Based on the results of wellbore stability calculations for offset wells, it was determined that the density of the drilling mud was not sufficient to maintain the wellbore walls in a stable state: numerous overpulls/slack offs and reamings were observed in the Achimov and Sortym deposits, as well as in the Abalak suite. As a result, casing runs were accompanied by slack offs, which led to pressure surges and the start of mud losses. The increase of mud weight when drilling several wells eventually allowed the casing to be successfully lowered and the well completed. Based on the drilling analysis results, it was also found that with the increase of the well inclination in the unstable shales of the Achimov, Abalak and Tyumen deposits (U2), the probability of shear failure increases.
To reduce the risks of complications when drilling a planned well in the problem intervals of unstable shale formations, the density of the drilling mud was recommended based on a geomechanical model that allowed to successfully drill a planned horizontal well.
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