3D Geomechanics Modeling and Shale Anisotropy for Wellbore Stability and Horizontal Well Optimization, Middle Nazym Field, Western Siberia, Russia

Abstract

Abstract JSC RITEK was planning to drill a horizontal well in oil shales of the Bazhenov Formation in the area of wells 100H-101H in the Middle Nazym field, Western Siberia. This area is characterized by high risks of borehole instability. Several wellbores have been already lost mostly because of wellbore wall shear-failure in the intervals of unstable shales at the base of the Frolov Formation, just above the Bazhenov Formation. 3D geomechanical modeling was undertaken to optimize horizontal well planning and drilling and minimize the risks of wellbore instability in the area. Core sampling and testing and acoustic logging with registration of Stoneley wave were carried out on the pilot well to evaluate mechanical properties of rocks, identify intervals of vertical (VTI) anisotropy and calibrate 1D Mechanical Earth Model. Well data and seismic inversion results were used to build structural model, populate properties and run 3D reservoir geomechanical modeling with calculation of full stress tensor in each grid cell. The 3D modeling results were applied to wellbore stability analysis for the planned horizontal well. High borehole instability in the 90-m interval at the base of the Frolov Formation is resulted from combination of factors: mechanical properties of the rocks, horizontal stresses distribution and well trajectory. The Frolov shales in this interval have the lowest uniaxial compressive strength and highest VTI anisotropy, and, consequently, the higher magnitude of horizontal stresses with respect to the adjacent intervals. The increase of well deviation and angle of attack from 40° to 80° in the unstable shale interval results in drop of uniaxial compressive strength of the rocks and narrowing of safe mud weight window. To minimize drilling risks, trajectory, construction and mudweight program were optimized for the planned horizontal well 101H-BIS. Additional casing section of 7”, borehole deviation in a range of 55°-75° and mud weight of 1.57 g/cm3 have been suggested for the high-risk interval of unstable weak shales at the base of the Frolov Formation. Vertical anisotropy analysis on the basis of core testing and acoustic well log data proved for the first time the Bazhenov Formation contains both anisotropic and isotropic highly brittle intervals that is relevant for the completion planning. 3D geomechanical modeling taking into account VTI anisotropy of shales provided new estimation of horizontal stresses distribution that reflects heterogeneous response of units of variable mechanical properties on the applied stress.