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“LOOK AHEAD” APPLICATIONS OF VERTICAL SEISMIC PROFILES AS A

Vertical seismic profile (VSP) technology have allowed data acquisition with good accuracy of reflectors from 500 to 700 below deepest VSP level in wells in the Burgos Basin (Figure 1); wells as deep as 5400 m with temperatures up to 428°F have been logged successfully. The VSP makes it possible “look ahead” into zones that are being considered for drilling and shows a good correlation with surface seismic data. When combined with compressional slowness data from the latest technology dipole sonic tools, it is possible to observe the compaction trend and the pressure increment, when it is shown in a compressed scale. By integrating the VSP and dipole low frequency sonic data, it is possible to detect over-pressured zones and drilling risks in advance. This helps to optimize costs and to make decisions while drilling deep and offshore wells.
This paper focuses on three wells as case studies. In all cases, a VSP look ahead survey was acquired in the intermediate phase and the results were compared to surface seismic data. After the well was drilled, the VSP data was compared to a synthetic seismogram. The results indicate that the VSP was accurate in depth and confirms the predictive value of the VSP look ahead application for litho-structural changes.
Recent applications show the value of the VSP survey as a litho-structural evaluation tool. The original litho- structural model was used to consider a thin shale layer through a producing field. After the VSP was acquired, it was evident that the shale was actually at least twice as thick as what was originally shown in a model. A check shot velocity survey was recorded later in a deeper wellbore to confirm the VSP look ahead results. The exploration team changed the litho-structural model and subsequent exploration well locations were more accurate. Another well drilled later provided additional confirmation of the new model.
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