Overpressure evaluation in the Agri valley by using different geophysical methods based on seismic velocity and RVSP data

Abstract Overpressure evaluation in the Agri valley by using different geophysical methods based on seismic velocity and RVSP data. Our study is a part of Seisbit research program (financed by ENI) and its purpose is to verify the potentiality and utilization of SWD Drill-Bit (Seisbit) data as measure and prediction of overpressure levels while drilling in shaly sequences. The studied area (Southern Apennine) is geologically very complex and the overpressure may be also to the tectonic stresses. After a first geological overview of the examined area, we have collected data and analyzed three wells of the same field by evaluating seismic velocity and RVSP amplitude signal. Analyzing seismic velocity we have studied, for each examined well, the trend of shale compaction recognizing normal pressure and overpressure zones. Inside these zones we have selected shale levels and for each depth we have calculated pressures (overburden pressure, pore pressure and effective pressure) as showed in the pressure diagrams. In the pressure calculations we have indirectly obtained density from seismic velocity (Sonic log, conventional VSP and RVSP Seisbit Drill-Bit data) because no direct measures of pressure and density have been recorded. Furthermore, considering sonic seismic velocity versus differential pressure, for all selected wells, we have approximately estimated 16 megapascal shift, corresponding to the amount of overpressure on the examined area. The analysis shows that Seisbit SWD data can be used as a measure while drilling comparing differential pressure and seismic velocity curves. However, a contribution due to tectonic stress has to be evaluated. Then, we have analyzed SWD signal and it is evident the attenuation of signal amplitude in some depth intervals. This attenuation is linked to drilling parameters and in particular to rate of penetration and pressure. The measure of pressure while drilling by using drill bit is the first step towards the prediction but it will require further study using reflected signal ahead of the bit.