In this paper, we describe major improvements to ultrasonic logging techniques used for cement evaluation. Specifically, we explain engineering solutions developed to tackle known limitations such as logging in heavy (ultrasonically attenuative) muds, and very large and very thick pipes. On the hardware side, new heavy mud/thick casing transducers are presented together with dedicated large-diameter logging heads for both pulse-echo and pitch-catch (flexural attenuation) techniques. On the processing side, we present a novel pulse-echo inversion scheme, based on real time inversion using accurate 3D models for metrology accuracy and a new scheme that does not require a priori knowledge of mud properties (specifically its acoustic impedance, Zmud). Through this, Zmud becomes an output parameter of this novel processing and we present field examples of such logs where we now image features such as mud deposition and segregation in deviated pipes. This new processing enables easier and more accurate interpretation of the cement sheath together with critical information on the logging fluid. The validity of the measurement across the operating range is demonstrated in this paper through results from both high precision laboratory measurements and several logs acquired during the last few years' field test campaign. Together with the fundamental sensor and processing improvements, we present engineering changes to the logging platform, enabling up to three times faster logging and accurate well site logs delivered right after rig down. Finally, we demonstrate that the combination of the pulse-echo and pitch-catch techniques, with its high azimuthal resolution, can probe the annulus up to the cement/formation or cement/second casing interface, making it a powerful technique for cement evaluation. Previous limitations (such as mud weight, casing size and thickness) have been overcome and this has been fully demonstrated in both the laboratory and the field. Thus ultrasonic cement evaluation techniques now enable accurate and critical decisions to be made–from well construction to plugging and abandonment–in all conditions, casing sizes, and mud and cement types.

Now, more than ever, the integrity of the wellbore before the recommencement of drilling operations or production of reservoir fluids, is fundamental to safe operations. Stricter well control policies have been instated in recent years, so it is critical to perform casing inspection, and cement evaluation measurement surveys to ensure adherence to regulatory codes. As exploration and development drilling activity breach new horizons, it brings new challenges to cementing operations and subsequent cement evaluation surveys. Heavy drilling muds, extra-large casing sizes, and narrow drilling windows: cement to mud weight ratios which can led to cement contamination are just a few. Until recently these constraints limited the usefulness and reliability of ultrasonic measurements under the said conditions. This paper describes a new generation of ultrasonic sensors and technology that fills the gaps left by predecessor technologies and allows operators to feel confident and at ease with the integrity of their wellbores, even under the most taxing conditions. The advantages and associated cost savings enabled through the utilization of this technology are illustrated in the case studies at the end of the paper.

Improvements in ultrasonic cement evaluation technology are increasing operational efficiency and reliability and extending the operating envelope into extreme environments with heavy muds and larger and thicker casings. Tool hardware, software, and firmware have been substantially redesigned and optimized to increase acquisition efficiency and data quality output. One key aspect is the complete redesign of various ultrasonic transducers to enable acquisition in the heaviest, highly attenuative muds. A specially designed transducer for thick casings and large rotating logging heads are pushing the thickness and diameter envelope to cover almost all possible scenarios. Yard tests were conducted to emulate extreme downhole conditions in a controlled way: A casing filled with heavy mud (2-g/cm3 synthetic oil-based mud [SOBM]) and cemented in an eccentered position inside a simulated formation was tested with two complementary ultrasonic techniques, pulse-echo and flexural wave imaging. These tests demonstrated the proper performance of both measurements despite the highly attenuative logging mud and made it possible to determine annulus material impedance, casing centralization, and location of the position of a "simulated measurement cable" (such as fiber-optics bundles) cemented inside the annulus. Through two field tests we further confirmed the efficiency improvements concerning logging speed (i.e. software/firmware). Logs have been obtained in extremely heavy mud environments and 26.7-mm (1.05-in.) thickness casing. These new developments in ultrasonic cased hole technology enable cement evaluation and pipe inspection in environments of the heaviest muds and casings of extremely large size and greater thickness, thus augmenting the industry's ability to diagnose well integrity while helping to reduce uncertainty and risk in well construction or before abandonment operations.