Investigation of through-casing borehole acoustic reflection imaging with a dipole source: Modeling and field application

Abstract

In view of most wells are completed by casing pipe in oil & gas exploration and development process, it is imperative to extend the application of borehole acoustic reflection imaging (BARI) from open to cased boreholes. To understand the characteristics of signals in borehole acoustic imaging, we study the radiated and reflected elastic waves in an open and cased borehole through numerical simulations. The results show that, for a broadband dipole source, the radiated SH-waves have the highest amplitudes and broadest radiation angles in the cased borehole, which is similar to the observation in an open borehole. It is believed that the cement and casing pipe would prevent to a certain extent the energy excited by the inner borehole sources from propagating into the outside formation. However, our modeling results clearly show the existence of the cement and casing will not influence the amplitude of the radiated SH-waves significantly. In fact, the decrease of the borehole radius, which is caused by cased borehole completion here, will increase the amplitude of the radiated SH-waves. As a result, the signal-to-noise ratio (SNR) of reflected waves relative to direct waves in the cased borehole is higher than that in the open borehole. Meanwhile, the cement bond quality does not heavily affect the SNR of the reflected waves and the SNR increases with the decrease of source frequency. Interestingly, as the cutoff frequency of the dipole flexural-wave mode in the cased borehole increases rapidly with the decrease of the formation S-wave velocity, the amplitude of the direct wave also decreases significantly. This results in an increase of SNR of the reflected waves, which is beneficial for extracting the reflected waves. The field data example certifies our theoretical analysis. Our results provide a theoretical foundation for the through-casing BARI with a dipole source.