Development of a Low-Frequency Quadrupole Shear-Wave Technology to Improve Quality of LWD Shear Velocity Measurement

Abstract

Abstract We present the result of a new development in the logging while drilling (LWD) quadrupole shear-wave technology that was recently developed to measure formation shear velocity in the LWD environment. Due to the severity of drilling noise in the low-frequency (0-3 kHz) range, initial development focused on measurements around 4 kHz, although the quadrupole attains formation shear velocity in a lower frequency range (~2-3 kHz). The 4-kHz quadrupole data is prone to dispersion, especially when drilling with a heavy mud. The dispersion can cause uncertainties in the shear velocity measurement. Realizing the potential enhancement of the measurement, we further develop the technology to perform measurements in the 2-3 kHz range or lower. The key in this new development is enhancing the quadrupole wave energy significantly above the noise level in the designated low-frequency range. Data from this new measurement have been acquired and analyzed. Comparison has been made between the new measurement and the measurement made with the 4-kHz source. Despite the common belief that drilling noise prohibits LWD acoustic measurements toward low frequencies, we obtain quite coherent quadrupole data in the low-frequency range. More importantly, the low-frequency quadrupole shows minimal dispersion and indeed attains the formation shear velocity, in agreement with the LWD quadrupole wave theory. The new development constitutes a significant improvement in the LWD quadrupole shear-wave technology.