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.