While applying acoustics is not a new science, inherent uncertainties with these techniques are still not addressing challenges that limit confidence in well integrity programs. The Caspian region's significant challenges for cement evaluations include heavy mud and thick casing, as well as the high-pressure/high-temperature (HP/HT) nature of gas condensate wells, which reduces the contrast in acoustic impedance. Accordingly, difficulties have remained in the interpretation of conventional cement bond logs, which has led many operators to be suspicious of well integrity technologies.

This paper focuses on the application of ultrasonic cement evaluation technology in the Caspian Sea, and compares results between advanced ultrasonic applications and traditional cement bond logs in heavy mud. The workflow is presented to integrate the advancement of this technology and to eliminate the uncertainties in well integrity analysis. Increasing confidence for further drilling of a high-pressure gas reservoir has been achieved by combining these various measurements that enable a definitive analysis of zonal isolation.

The main objective of this well assurance program was to ensure zonal isolation and shoe integrity in order to drill ahead to perform formation integrity tests (FITs). However, obtaining high-resolution cement data in heavy, 2.16-sg, oil-based mud (OBM) was the biggest concern due to the limitations of standard ultrasonic technology. The wide disparity in acoustic impedance, combined with the low contrast between heavy mud and the cemented section, makes evaluation of cement quality and zonal isolation doubtful. Although well conditions challenged the standard measurements, the cement evaluation objective was achieved with the new technology by ensuring 360° azimuthal coverage in permeable sand zones capable of unwanted hydrocarbon production – i.e., preventing sustained casing pressure (SCP). Moreover, a strong and continuous 40-m cement bond prevented crossflow from charging zones through the wellbore and also acted as a barrier against corrosion. Enhancement of pulse-echo technology has proved that it can be applied in a highly attenuative environment to achieve high-fidelity data.

Highly acoustic attenuative mud is a major challenge for acoustic ultrasonic technology to achieve a quality answer product for well integrity. To mitigate this problem, a new tool was developed with a highly sensitive low-noise transducer, and with special programmable (both voltage and frequency) firing circuitry, to enhance the transducer signal at the resonance frequency of the casing. The various features of the processing algorithm are also improved, based on the numerous laboratory and field measurements.

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