Drilling Applications of a New Logging-While-Drilling Slim Sonic Tool: Two Case Studies

Abstract A logging-while-drilling (LWD) slim compensated long-spaced sonic tool that can be run in boreholes as small as 5 3/4 in. has been found to have useful drilling applications in the Gulf of Mexico. The 4-3/4-in.-OD tool features a two-transmitter, four-receiver acoustic array. Alternate firings of the two transmitters yield two sets of four waveforms that are processed downhole with a semblance correlation technique to compute compressional and shear slowness ranging from 40 to 180 microseconds/ft. The waveforms are stored in memory for advanced processing at surface. Applications of the sonic information include real-time processing of synthetic seismograms, prediction of pore pressure, computation of acoustic porosity from compressional and shear slowness, and determination of rock mechanical properties for borehole stability analysis. The first of the two studies underscores the accuracy of the LWD slim sonic slowness measurements in predicting pore pressure and detecting the top of supernormal pressure. An LWD slim resistivity tool was run simultaneously with the LWD slim sonic tool, and the resulting resistivity data verified that the slowness measurements indeed served as valid pore-pressure predictors. Operationally, the LWD slim sonic device can be positioned close to the bit, and the measured slowness values can be pulsed uphole in real time during drilling. To enhance drilling efficiency and safety, pore-pressure software at the rig calculates formation pressure and an equivalent mud weight. The second study presents an unusual case in which LWD slim sonic data were used to confirm that a 190-ft section of casing, believed to be cemented in place, slid 90 ft down the wellbore.