All surface seismic data has velocity information built into it in the way of stacking velocities. From these stacking velocities, estimates of interval velocities can be determined. We have developed a quantitative technique for determining formation pore pressures from the relative changes in these velocities.
From the stacking velocity data, we develop curves which we refer to as ITT (Interval Transit Time) curves. The ITT curves are generated at the proposed location and are correlated to offset location curves to calibrate velocities. Offset location pore pressures are determined and are combined with structural geology to predict anticipated pore pressures of the proposed wellbore. With these curves, in conjunction with geological information to determine structural changes, and offset logs to determine pore pressures in offsets, we can predict pore pressures to be encountered in a proposed wellbore.
To date, we have made such predictions of pore pressure in approximately 20 exploratory wells on the gulf coast prior to their drilling, with excellent results. We will graphically show results on a number of these wells comparing both the mud weights required, as dictated by hole conditions, and the actual pore pressures encountered. Our statistics within Chevron, on the Gulf Coast, indicate that proposed casing programs are adequate for the needs of an exploratory well:
29% of the time if offset casing programs are used,
65% of the time if detailed geological information is used,
>95% of the time if ITT data is also incorporated.
Significant dollars can be saved and increased success in reaching objectives can be realized when pore pressures can be anticipated prior to drilling a well. This technique offers the opportunity to do so.