Integrated Study of Shear Wave and Resistivity Anisotropy in an Inclined Well Providing Insight to the Geomechanical Model

Das, Saikat (Baker Hughes, a GE Company) | Zhang, Tingting (Sabah Shell Petroleum Co. Ltd) | Vevakanandan, Valsan (Sabah Shell Petroleum Co. Ltd) | Abd Rahim, Mohd Helmi (Sarawak Shell Berhad)



Understanding in-situ stress orientations and magnitudes is critical in building a geomechanical model which helps in planning and execution of a development and production programme for any hydrocarbon reservoirs. The azimuthal anisotropy analysis from cross-dipole acoustic data is commonly used to derive the direction of maximum-horizontal stress. However, the interpretation of the stress orientation is challenging in inclined wells where anisotropy may also be influenced by the relative angle of bedding plane to the bore hole. Integration of the data becomes of paramount importance to correctly interpret the stress distribution.

Cross dipole wireline acoustic, 3D resistivity and 6 arm calliper data were acquired in a deviated well, offshore, Malaysia. Acoustic data was processed for azimuthal anisotropy, 3D-resistivity data was processed for formation dip, azimuth, horizontal and vertical resistivity and 6 arm calliper data was used to generate borehole shape. Acoustic analysis provided the difference in fast and slow shear wave velocities and the azimuth of fast shear. The resistivity anisotropy, dip and azimuth and bore hole shape information was incorporated to interpret effect of the dipping bed in the scheme of relating acoustic anisotropy to the formation stress.

Meaningful difference in the fast and slow shear velocities (in two orthogonal direction) is observed in this well. The fast shear wave azimuth of NW-SE is consistent with the regional trend. However, the presence of laminated shale interval in the inclined bore hole imparts uncertainty in relating the anisotropy to the stress field. The formation dip and azimuth obtained from the resistivity anisotropy provided the framework of the interpretation by identifying the intervals with higher relative dip and the associated anisotropy perceived by it. Bore hole ovalization also provides the necessary input to the interpretation scheme which is supported by the existing field wide geomechanical model.

Integrating all datasets resolved potentially ambiguous interpretation of the source of azimuthal acoustic anisotropy. This approach determines the cause of the anisotropy (unbalanced stress in formation vs. dipping beds and shale transverse anisotropy). The result provides valuable information to refine the existing geomechanical model which can be used in future well placement and planning, optimum mud weight design, and constraining water injection operating limit during the life of the field.