3D Anisotropy Analysis by Using the Vertical, Deviated, and Horizontal Well in the Khufai Formation, Oman

Abstract By analyzing sonic data with quantified deviations from vertical and true stratigraphic dips obtained via an imaging tool in vertical, deviated, and horizontal wells, the established method computes up to five elastic moduli of an orthorhombic or transversely isotropic formation, while compensating for any tool-induced effects. The algorithm yields verifiable far-field compressional and shear moduli, unaffected by the near-wellbore zone altered by stress concentrations or rock's plastic deformation. In this case study, cross-dipole data is evaluated to confirm the third shear slowness component derived from Stoneley waves across three well trajectories—vertical, deviated, and horizontal—in the Khufai formation in Oman, which is primarily composed of carbonate with some shale, exhibiting Vertical-VTI/Horizontal-HTI or Orthorhombic Anisotropy. Initially, the third shear slowness for each wellbore trajectory is computed to validate the method. This component is then used in 3D anisotropy analysis to determine up to five independent elastic moduli of HTI, orthorhombic, or VTI formations, using compressional, fast, and slow shear, and Stoneley slowness measurements. The 3D Anisotropy method converts measured compressional, fast shear, slow shear, and Stoneley slownesses relative to borehole axes into anisotropic moduli aligned with the earth's anisotropy axes. This technique can produce up to five anisotropic moduli for orthorhombic or transversely isotropic (TI) formations, using sonic data that compensates for known deviations from vertical and true stratigraphic dip from imaging tools in vertical, deviated, and horizontal wells. The method also adjusts for any tool-induced effects on the borehole's Stoneley and flexural modes. The algorithm's output delivers verifiable far-field compressional and shear moduli, not influenced by near-wellbore changes like stress concentrations or rock plastic yielding. Transverse isotropic anisotropy, or polar anisotropy, is measurable through the estimation of Thomsen parameters: ε, γ, and δ. The γ parameter is derived from borehole sonic data based on vertical and horizontal shear moduli, typically obtained from cross-dipole sonic logging.