VSP CASE HISTORY, KANPA 1A, THE WESTERN OFFICER BASIN

Abstract

The Kanpa 1A Vertical Seismic Profile (VSP) was conducted for Shell by Schlumberger and incorporated variable time and depth sampling, different source offsets and recording in cased and uncased hole. Processing was performed using Shell proprietary programs, with particular attention to:

Editing and resampling of the data set

Separation of upgoing and downgoing waves using FK and median filters

Comparison of gain recovery based on modelling the amplitude decay of direct arrivals with an averaging process (AGC)

Sensitivity of the final VSP stack to blanking of residual tube wave noise

Zero-phase whitening of the VSP trace

Stacked VSP traces for Kanpa 1A were compared with a zero-phase seismic section and synthetic seismogram at the well. The VSP/seismic match is good and, due to poor synthetic/seismic correlation, was the basis for the final seismic/well tie. Interpretation of deep VSP data enabled the estimation of formation boundaries below the total depth of the well.

It is concluded that VSPs can be invaluable in establishing well ties where seismic is poor or when detailed correlation is required (e.g. stratigraphic traps). Reflectors "ahead of the bit" can be interpreted from VSPs based on assumed velocities and VSP/seismic tie, and the predicted thickness and seismic character of the target interval. A consistent field configuration is recommended for acquisition with attention to tube wave suppression and adequate spatial and temporal sampling. Previous processing experience is advantageous if quick and reliable VSP results are required for decisions while drilling.