An algorithm to simultaneously inverse the mud and cement acoustic impedances from the ultrasonic pulse-echo measurement in a cased-hole

Cement bond evaluation is extremely important in oil/gas exploration and development, which is of great significance to the efficiency of oil and gas development, as well as the safe production and environmental protection. The ultrasonic pulse-echo technology can calculate the acoustic impedance of the annular medium (cement) between the casing and formation, so as to determine the material type in the annulus for cement bond evaluation. The traditional cement acoustic impedance inversion method regards the borehole mud acoustic impedance as a constant in the azimuth and axis, which is inconsistent with the field situation and will lead to inaccurate cement acoustic impedance in the calculation. Aiming to solve this problem, this paper uses the simulation data from a multilayered model to study the influence of the acoustic impedances of borehole mud and cement on the extracted characteristic slope and intercept from a fitting line on the waveforms in a semi-logarithmic coordinate. Then a scheme to simultaneously inverse the mud and cement acoustic impedances is proposed. We construct the iterative formula to continuously iterate the borehole mud and cement acoustic impedances after comparing the characteristic slope and intercept in both the simulation data and the full-scaled calibration well measurement data. Then we form a data processing process to determine the cement and borehole mud acoustic impedances at the same time. The data processing process is tested by the data from a full-scaled calibration well, and the inversion result is consistent with the actual situation of the model well, which proves the correctness of the proposed data processing process. Finally, the proposed data processing method is applied to a field data set in a well from an oilfield.