Model Based Deviated VSP Reservoir Imaging Technique in tough field conditions (Case History)
By
Abu Baker H.S. Aljeelani1, Azhar Akhtar2, Samer Marmesh1, Patrick Fouchard1, Parvez Ahmad3, Shafiq Janjua2
(1) ADCO, Abu Dhabi, United Arab Emirates (2) Schlumberger Middle East S.A, Abu Dhabi, United Arab Emirates (3) Schlumberger Middle East, Abu Dhabi, United Arab Emirates
Seismic imaging in tough field conditions is always a challenge for
geoscientists. Deviated VSP images with fixed source offset are not quite often
acquired due to severe ray paths bending and oblique incident angles. In this
paper, we demonstrated that a good quality subsurface image can be obtained in
such cases using a geological model and proper field geometry correction for the
ray paths. The data set used in this experiment, was from a well, spud on land
and target bottom hole location was deviated around 38 degree towards very
shallow marine environment, making it extremely difficult for imaging the
reservoir with moving source boat standard procedure, thus, leaving a choice for
other alternative methods. Multiple ray path arrivals from source to receivers
positions in the well, were simulated utilizing ray trace model technique, field
survey geometry and well deviation data on acquired depth levels. The field
measured times were corrected for slant paths and time-frequency dependent
spherical
divergence. The corrected time based interval velocities were computed
and used in the model to generate equi-offset contours for stacking the image
along these contours. Moreover, time based residual normal move out corrections
wereapplied to deviated VSP wave field before making the final CDP image. A
comparison of final processed image of deviated VSP was made and integrated with
3D seismic in the existing area, which showed extremely good match with the
reservoir image in higher resolution. This real data example showed that wells
deviated up to an angle of 38 deg., can be used with fixed source offset to
reliably image the subsurface with lateral coverage as a function of source
offset, well deviation, geological dip and velocities. An additional by product
of offset source deviated VSP, in such cases, is mode converted P to S energy,
which can be analyzed to provide further useful information about reservoir
characterization.