An Anisotropy Study: Integrated Interpretation of Shear Wave and Resistivity Anisotropy in an Offshore Field in India

Abstract

Abstract Efficient development and production of hydrocarbon reservoirs depends on the accuracy of the geological and petrophysical models. Historically, these were all basic isotropic models where formation properties do not change with direction. With the recent advancement of measurement technology and processing technqiques, the models started incorporating and integrating variations of petrophysical properties with orientation: anisotropy. Anisotropic petrophysical properties include permeability, acoustic slowness, resistivity, etc. Complex models incorporating variations of petrophysical properties with orientation (anisotropy) are useful for well design and well placement for optimum reservoir drainage, borehole instability and sand production prediction, hydraulic fracturing, surface seismic migration or amplitude-versus-offset studies. Cairn Energy India Pty. Ltd. recorded acoustic and resistivity anisotropy measurements in several of their offshore wells. Acoustic data was processed for azimuthal anisotropy estimation and resistivity data was processed for formation dip, azimuth, horizontal and vertical resistivity. Whilst resistivity information has been used for accurate hydrocarbon saturation computation, the continuous dip and azimuth results provided a geological framework that was used for interpretation of acoustic azimuthal anisotropy results. Integrating both datasets resolved potentially ambiguous interpretation of the azimuthal acoustic anisotropy source. We were able to determine the anisotropy cause (unbalanced stresses in the formation, dipping fracture/faults, highly dipping beds, and shale transverse isotropy with a vertical axis of symmetry [TIV] anisotropy). This presentation shows how an integration of acoustic anisotropy results with resistivity anisotropy results can provide essential information for interpreting and resolving ambiguous causes of acoustic azimuthal anisotropy. This information could be used for avoiding mistakes in well placement and design, reducing sand production and avoiding borehole instability, for well completion planning and several other applications during the life of the field.