Alain Dumont

LWD Sonic Tool Design For High-Quality Logs

The design of a logging-while-drilling (LWD) sonic tool is always a challenge; the acoustic propagation along the tool cannot be ignored, nor can effects on measurements due to tool presence. It is well known that collar arrivals can interfere with compressional waves in fast formations. The interaction of the collar with other modes such as Stoneley and quadrupole must also be considered while designing an LWD acoustic tool. Because the LWD tool occupies a significant area of the borehole cross section, it is known that the presence of a sonic tool in a borehole shifts a dispersion curve in the slowness frequency domain. In this paper, we present an approach to optimize tool design for minimizing tool effects on measurements or making tool presence effects predictable for enabling a broadband use of the acquired data. Experimental results validate the design of the tool, and real log examples illustrate the quality of the acquired waveforms.

Semblance Criterion Modification to Incorporate Signal Energy Threshold

SUMMARY The semblance criterion widely used for slowness estimation is examined using the theory of statistical hypothesis testing and shown to be equivalent to that for detecting propagating signals of any energy at a specified slowness. We propose to suppress weak collar arrivals on semblance maps for logging while drilling (LWD) data by incorporating a minimum signal energy threshold in the aforementioned theory and using it to derive a couple of modifications of the semblance. Other simpler modification inspired by these are also proposed on heuristic grounds. All proposed modifications perform comparably on real LWD data and are effective at suppressing weak unwanted arrivals while preserving the desired ones.

Classifying and removing unwanted arrivals propagating through a drill collar for semblance processing

Understanding the characteristics of an acoustic wave propagating through drill collars is important for formation evaluation with logging-while-drilling (LWD) sonic tools. Knowing the frequency-slowness information of different types of waves propagating through the collar, the unwanted wave propagations can be minimized by processing and robustly identifying the formation compressional and shear arrivals. Extensional modes of the steel drill collar in water are generally dispersive and range from approximately 40 μs/ft to approximately 120 μs/ft depending on the frequency band. A fundamental torsional mode of the drill collar is nondispersive, but its slowness is sensitive to the geometry of the drill collar. Depending on the geometry and shear modulus of the material, the slowness of the torsional mode can be slower than 100 μs/ft. For identifying the slowness of the formation arrivals, the different slownesses of the waves propagating through the collar need to be identified separately from those of t...

Classifying and removing monopole mode propagating through drill collar

Understanding characteristics of the acoustic wave propagating through drill collars is important for formation evaluation with logging-while-drilling (LWD) sonic tools. Knowing the frequency-slowness information of different types of the wave propagating through the collar, we can minimize the unwanted wave propagating through the collar by processing and robustly identify formation compressional and shear arrivals. Extensional modes of the steel drill collar are generally dispersive and range from 180 μs/m to 400 μs/m depending on the frequency band. A fundamental torsional mode of the drill collar is nondispersive, but its slowness is sensitive to the geometry of the drill collar. Depending on the geometry and shear modulus of the material, the slowness of the torsional mode can be slower than 330 μs/m. For identifying slowness of the formation arrivals, those different slownesses of the wave propagating through the collar need to be identified separately from those of the wave propagating through formations. Examining various types of the acoustic wave propagating through a drill collar, we determined that the waves can be properly muted by processing for the semblance of waveforms acquired with LWD sonic tools.