Experimental measurements of monopole inner/outer collar waves and estimation of P wave velocity in acoustic LWD

Journal of Petroleum Science and Engineering(2022)

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Acoustic logging while drilling (LWD) technology is extensively applied for petroleum prospecting. It utilizes the acoustic characteristics of stratum to deduce the formation properties such as porosity, permeability, and oil saturation, etc., which can help to detect the location of oil reservoirs. In acoustic logging process, the formation longitudinal (P) and transverse (S) wave velocity is the primary factor of interest. Nevertheless, an accurate identification of P/S wave velocity in different formations is hard to be performed due to the influence of the collar waves, which has become an urgent problem in LWD measurements. To study this issue more practically, a small-sized measurement system based on the propagation mechanism of collar waves is established in our laboratory. With this system, we design and perform multiple sets of monopole acoustic LWD experiments with different models. After recording the experimental data, the attenuations of inner and outer collar waves received along the axial direction are compared and analyzed. The result shows that the collar waves have an exponential decay in amplitude by the expansion of source-receiver distance, but the outer collar wave decreases more rapidly. We also conduct experiments at different source frequencies (20–400 kHz) to quantitatively describe the relationship between the amplitude and frequency. The result suggests that the inner/outer collar waves and P/S waves have a similar frequency response, and it's difficult to remove collar waves from the full waveforms of LWD signals by only changing source frequency. Therefore, we present a new data processing method for extracting the formation wave velocities according to the energy distribution of the componential waves, and then apply it in the analysis of two borehole measurements. The inner and outer collar waves are almost removed from the processed time domain waveforms, which leads to a successful verification of formation P/S wave velocities. These experimental results prove the feasibility and repeatability of our method, and propose an alternative method for the LWD data explanations.
Collar wave,Frequency response,P wave velocity,Monopole LWD
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