Naoki Sakiyama

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.

Robust downhole ultrasonic pitch-catch measurements of compressional and shear wave velocities

Probing acoustic wave velocities of earth formations, both in azimuth and radius, is important when evaluating anisotropy and/or heterogeneity of the formations. A conventional approach for evaluating these complex formations is to employ a sonic logging tool that operates from approximately 0.1 kHz to 20 kHz with an approximately 600 mm spatial resolution. With these standard sonic tools, evaluating formation acoustic wave velocities with an order smaller spatial resolution to detect thin beds and/or azimuthal variation of formation properties is very challenging. To overcome this challenge, this paper discusses an ultrasonic pitch-catch measurement that can be applied to a downhole acoustic tool. Downhole conditions are typically harsh, especially for ultrasonic frequencies. Thus, a robust measurement system is required that maximizes the signal-to-noise ratio. One of the ways to do this is to minimize the distance between the transmitter and the array receiver (and the receiver-receiver spacings of the array). Numerical modeling corroborated by experimental study indicates such a measurement system can primarily detect refracted compressional waves and surface mode waves related to the pseudo-Rayleigh mode. Characteristics of the pseudo-Rayleigh mode measured with such system and a way to estimate shear wave velocity from the measured mode are discussed.Probing acoustic wave velocities of earth formations, both in azimuth and radius, is important when evaluating anisotropy and/or heterogeneity of the formations. A conventional approach for evaluating these complex formations is to employ a sonic logging tool that operates from approximately 0.1 kHz to 20 kHz with an approximately 600 mm spatial resolution. With these standard sonic tools, evaluating formation acoustic wave velocities with an order smaller spatial resolution to detect thin beds and/or azimuthal variation of formation properties is very challenging. To overcome this challenge, this paper discusses an ultrasonic pitch-catch measurement that can be applied to a downhole acoustic tool. Downhole conditions are typically harsh, especially for ultrasonic frequencies. Thus, a robust measurement system is required that maximizes the signal-to-noise ratio. One of the ways to do this is to minimize the distance between the transmitter and the array receiver (and the receiver-receiver spacings of the...

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.