An operator drilling an offshore exploration well in East Asia expected to encounter a key formation at about 3,000 m TVD. Because the formation was suspected of being overpressured, the operator’s objective was to set casing within 20 m of its top. That objective would have been difficult to achieve using conventional 2D surface seismic, which had a depth uncertainty of ±100 m.
The seismicVISION seismic-while-drilling service provided checkshots that enabled drilling to be geostopped within 10 m of the formation top. Checkshots were acquired at planned levels during the acoustically quiet periods while connections were made. A standard three-air-gun cluster deployed at surface was used as a source. As the wellbore neared the formation top, checkshots also were acquired at mid-stand levels.
The checkshots were processed from windowed real-time waveforms transmitted uphole by mud pulse telemetry, and real-time depth updates were calculated while drilling. A real-time corridor stack processed from the waveforms showed seismic position, and geostopping decisions were guided by depth projections to the key formation, which were extrapolated from look-ahead (reflectivity) information.
Use of the seismicVISION service enabled the operator to geostop the wellbore less than 10 m from the top of the overpressured formation. This accurate casing point selection allowed successful construction of the well to TD—without the time and expense of contingency strings.
