The shallow section is often thought of as being in the way; the real geology and well drilling only starts when the cuttings return after the surface casing is set. The pump–and–dump shallow section is the part of all offshore wells that we know the least about‚ but it is the part that just cannot be avoided.
When things go wrong with the shallow section‚ they are inevitably costly. In the same way as the foundations of an engineering project‚ it may not be a large portion of the initial projects costs‚ but a delay caused by shallow section risks can become a very large cost.
Why do we know so little about the shallow sections and appear not to worry about our lack of knowledge? Pump–and–dump means no cuttings log exists. Large diameter holes mean log–while–drilling geophysical data is poor‚ and often near non–existent. The only log available is often the natural gamma which could be a valuable data source in shallow sections‚ but is much impoverished by the very large hole size. Seismic data is often presented with scaling and processing methods that wipe out much shallow detail.
This lack of knowledge often means that exploration geologists either believe knowingly or perhaps are just unaware of how variable the shallow section can be. The last 1million years have seen incredibly rapid climate changes and this has made shallow sections throughout the world highly variable with very variable sedimentation rates and hence ‘soil’ strengths being just as variable.
Bluebird Geoscience geologists were leaders in the development of shallow section logging‚ providing a vital source of data which led to the understanding of the shallow section.
The geophysical logging data acquired for the Ormen Lange development offshore Norway delineating the huge Storrega Slide structure helped enable the massive project to be safely developed. Projects such as ACG and Shah Deniz in the Caspian had incredibly complex geotechnical challenges before development and the shallow section logging system devised by Bluebird’s directors and detailed seabed mapping which Bluebird also specializes in were vital in establishing a safe long–term development strategy for those massive fields.
The complex of mass wasting deposits of deepwater Nile Delta were identified with these logging systems and the turbidite structures‚ ill defined by the shallow seismic surveys‚ were identified and mapped.
Angola’s deepwater has some interesting features that were also hard to map or understand with high–resolution seismic data alone. Asphalt and carbonate layers were hard but thin and had potential for refusal for conductors installation. The refusal may not be permanent as being so thin they also had the potential to fail with time. Being prepared by mapping the detail of these thin horizons‚ the conductors were designed to penetrate these thin hard layers and only refuse on a reliable foundation.
Setting surface strings and conductors in gas hydrate also has its hazards and high resolution seismic data has no potential to identify the top of any gas hydrate indurate formation. Detailed borehole geophysics equipment was devised by Bluebird’s geologists. It was used not only for geohazard purposes but also for gas hydrate exploration in India‚ China and S Korea as well as to identify thick horizons in the Gulf of Mexico.
The data from such shallow logs was also used to derive a depth cube data for Mad Dog South in the Gulf of Mexico. The ability to transform the time data with such precision to depth data was a world first on this project
With such knowledge‚ the shallow section is no longer an unknown. Although many times the over–design and the over–spend on conductors and surface strings will allow the well to be drilled without an apparent budget overrun‚ there remains the risk that such lack of knowledge causes constant overspend.