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.