The detection and avoidance of shallow, subsurface drilling hazards is a critical part of deepwater well construction. Such hazards can include faults, gas-charged sediments, methane hydrates, buried channels, shallow-water flows and abnormal pressure zones. The costs of unexpectedly encountering such zones can vary widely and can lead to total well abandonment, loss of sea floor infrastructure, and, in particularly severe cases, the loss of access to an entire field. Traditional methods for the delineation of shallow hazard zones are derived from careful analysis and study of surface seismic data. However, the accurate interpretation of such shallow seismic data is complicated by the fact that the correct velocities and anisotropy are usually not known or measured. The use of borehole seismic tools in shallow formations is generally limited and by the time they are run in deeper sections, the shallower zones are behind multiple strings of casing.
In addition to the validation of velocity measurements, acquiring sonic data in these zones is important for pore pressure prediction, lithological identification, and gas detection. The acquisition of sonic data using wireline tools in these shallow sections is difficult because of borehole size and centralization limitations. Logging-while-drilling (LWD) sonic and seismic tools can be used to acquire velocity information in very large surface holes, provided that the different acoustic modes propagating are understood and that the tool and the bottom hole assembly (BHA) are configured to optimize the measurement. The physics of acquiring compressional velocity information in very large boreholes and in soft, unconsolidated sediments is studied here, with Gulf of Mexico examples of data acquisition in boreholes up to 26 in in diameter. The differences between real-time and memory capability will be discussed along with the increase in the capability of new LWD technology for this challenging application.