Shear-wave Anisotropy in Very Porous Oil-bearing Sands: Applications in Perforation Strategy and Production Optimization

By

 Theodore Klimentos¹, Taher El-ZefZaf², Mohamed AbdelFatah², Maher Omara²

(1) Schlumberger, Cairo, Egypt (2) General Petroleum Company (GPC), Cairo, Egypt

 In this case-study, shear-wave anisotropy data was acquired by dipole shear sonic logging over several porous sandstone formations, intersected while drilling a well in the Gulf of Suez. The cross-dipole shear-wave data from the dipole shear sonic imager tool was processed to obtain oriented fast and slow shear waves. Anisotropy was then used to determine the orientation and magnitude of the principal horizontal stresses. Several highly porous sandstone zones exhibited significant shear-wave anisotropy. This observation indicates that the shear-wave anisotropy, which occurs in sands of about 30% porosity is more likely attributed to a significant stress imbalance. So far, the general perception has been that shear-wave anisotropy occurs more often in tighter rocks, e.g., carbonates or low porosity sands. The observed shear-wave anisotropy azimuth has a NW-SE orientation, which is consistent with the known tectonic regime of the Gulf of Suez stress trend, i.e., the Clysmic-fault trend. Due to anisotropy, there is a significant difference between the magnitudes of the minimum and maximum horizontal stress. The anisotropy information proved to be very valuable in optimizing the perforation and production strategy due to the fact that sanding was suspected to occur. Sanding analysis was thus performed prior to the test and maximum critical drawdowns were calculated taking into consideration the shear-wave anisotropy. The main technology value obtained from this study is that it clearly demonstrates the presence of significant shear-wave anisotropy in shallow deposited, oil-bearing and highly porous sands, which to the authors' knowledge has very seldom been reported in the literature, if ever. Therefore, this observation may assist in promoting and enhancing the usage and benefits of the shear-wave anisotropy technology in highly porous-sands.