This paper introduces new real-time logging-while-drilling (LWD) sonic technologies and discusses the developments that were necessary to bring them about. Leaky compressional (leaky-P) and quadrupole shear were previously memory-mode products available only after drilling. Both are now available in real-time.
To use existing surface processing techniques in the downhole environment, a new processing board was designed utilizing multiple processing units and a greatly improved memory access scheme. This was necessary because of the advanced processing method that is employed for quadrupole. To be applicable for LWD, the processing method was optimized to require little or no user input.
The processing method for leaky-P and quadrupole shear runs on the tool while drilling and sends the respective slowness values to surface in real-time via mud-pulse telemetry. Because the slowness of each is frequency dependent, a quality control (QC) indicator is also sent uphole in real-time to give the user confidence in the processed result. An optimized method of producing the QC indicator was made allowing QC to be done within the constraints of the downhole environment.
Validation of the real-time processing was done by comparing it to memory data. The results showed good agreement; leaky-P and Quadrupole showed a real-time to memory data match within 1%. The real-time QC indicator offered valuable confirmation that the real-time answer was correct.
Compressional and shear measurements are inputs to wellbore stability control, surface seismic correlation, shallow gas identification, and various other applications. If the formation being logged is very slow, leaky-P and quadrupole shear are needed to determine the formation compressional and shear slowness. Now that both are available in real-time, more can be known about the well while drilling.
The logging-while-drilling (LWD) tool used is a multimode tool that utilizes two separate transmitters (Auchere et al., 2013). This is done to achieve better signal-to-noise ratio (SNR) for the quadrupole mode in large holes and soft formations. By using a special type of piezo ceramic in a dual-layer configuration, it is possible to excite a borehole quadrupole mode with higher SNR compared to the traditional monopole-type piezo ceramic for LWD sonic tools. Because the new type of ceramic also excites a stronger collar arrival, it is not suitable for monopole P&S measurements.
