Cased Hole Acoustic Logging - A Solution To A Problem

Whether due to hole conditions or to reduce rig time, openhole logging is sometimes foregone over certain intervals of a well. However, the depth-derived formation data from these and subsequently logged intervals require the use of acoustic velocity measurements over as large a depth range as possible to confirm the depth-time relationship with surface seismic data. With the introduction of array-type acoustic logging tools, acquiring full-waveform data in the cased section of a wellbore is now possible whilst pulling out of the hole following a deeper logging suite. These waveform data can be processed to extract the acoustic velocities from the cased interval, thereby foregoing the requirement to log the interval in open hole, and thus saving the associated time and costs. This case study focuses on waveform acquisition from array-type acoustic instruments run in a number of wells in both the southern and northern sectors of the North Sea. The postprocessing techniques and acquisition constraints will be reviewed. Acoustic data acquired over the same intervals in both open and cased wellbores will be used to demonstrate the accuracy of the measurements. Finally, synthetic seismic techniques will be used to demonstrate the response validity in cases where poor cement would preclude the use of other acoustic devices.