Using Shear Wave Anisotropy Approach to Determine State of Stress Around the Wellbore from Advanced Acoustic Well Logging; Case Study from Southern Iraq

Abstract

Abstract Drilling operations in deep shales are more challenging due to geomechanical problems such as wellbore instability. Neglecting the impact of geomechanical issues may lead to drilling drawbacks such as loss of circulation, wellbore collapse, tight hole while tripping, stuck logging tools, and subsequent fishing, stuck pipe, and sidetracking. The directional dependency (anisotropy) of the rock properties, especially the rock strength, cause variation in the wave velocities. Identifying orientation and degree of the anisotropy and its relationship with geomechanical problems is essential for further field development. In this paper, acoustic data from vertical and deviated wells in Zubair Formation, Southern Iraq were performed to provide insights on the state of stress distribution around the wellbore through the Zubair Formation. In addition, interpreting the flexural dispersion curves as the final result of cross-dipole data processing, the maximum stress direction has been obtained to illustrate the type and source of anisotropy around the borehole in this formation. Using the flexural dispersion curves and the shear wave splitting approaches, the direction and the degree of anisotropy were determined in Zubair formation. Results indicated that drilling operations have altered the state of stress around the wellbore, and the degree of alteration is a function of the magnitude and the direction of the anisotropy. The flexural dispersion curve shows that the upper member of Zubair Formation is strongly anisotropic, whereas a slight anisotropy was mapped within the middle member of the same formation. In addition, shear wave splitting analysis revealed that the stress changes along the direction striking from north to west. Thus, considerable attention should be paid to all 1D-geomechanicsl models that constructed for Zubair Formation given its strong aviation of the magnitude and direction of its strength and elastic properties.