Vertical Seismic Profile (VSP) technology is normally used to correlate seismic time to actual drilling depths. The introduction of new technologies for wireline tools has allowed accurate acquisition data from reflectors 500 to 700 meters below the deepest VSP level to as deep as 5400 m. The tools can tolerate temperatures up to 220 deg. C. The process of acquiring such information and analyzing it will be presented in this paper.
Selected wells will be presented as case studies for the "Look Ahead" technique. In all such studies, a "Look Ahead" VSP survey had been acquired in the intermediate phase of drilling a well, and the results were compared to the surface seismic data. After the well was completed, the VSP data was then compared to a synthetic seismogram. The results showed that the VSP was accurate, in depth, for the reflectors observed in the "Look Ahead" mode. These results confirmed that the technique was a good predictor of the depth of occurrence of litho-structural reflectors when this new technology tool is used.
More recent applications also show the value of the VSP survey as a litho-structural evaluation tool. An original litho-structural model interpreted a thin shale layer throughout a producing field. After the VSP was acquired, it was evident that the real shale thickness was at least twice the thickness interpreted for the original model. Another well drilled still later further confirmed the new, thick shale model.
The use of new technologies for VSP combined with good quality procedures in acquiring and processing seismic information has proved to be helpful in optimizing costs and making decisions especially when drilling exploratory wells.