Ultrasonic and sonic logs are commonly used to evaluate the quality of cement placement in the annulus of a pipe and its potential to perform as a barrier. In some cases, we observe that the log response is in conflict with the expectations on the outcome of the cementing job that was executed without any major issues. The log sometimes does not see the cement! This apparent disagreement if not resolved could potentially lead to costly and unsuccessful attempts to remediate the cement sheath. We highlight reasons why sometimes logs may not see the cement and implement new workflows that not only recognize these factors but also account for it in the cement sheath evaluation. The ability to recognize the existence of a wet or dry micro-annulus is critical for cement sheath evaluation and is discussed through two new interpretation workflows using ultrasonic and sonic logs. Firstly, we introduce a new ultrasonic (TIGHT model) interpretation workflow that improves discrimination of light-weight solids from displaced muds and enables identification of dry micro-annulus situations. Secondly, we introduce a new integrated workflow that combines sonic amplitude / attenuation with ultrasonic measurements in the acoustic impedance (AI) space (Sonic vs ultrasonic acoustic impedance). Through this approach, we demonstrate how we can identify cement that is well bonded and discriminate from those with a dry or a wet micro-annulus. The workflows were applied to data acquired in wells where we compared the results of conventional interpretation approaches as well as those discussed in the two workflows. The gap caused by the detachment of cement from casing (de-bonded cement), either wet or dry micro-annulus, can affect the log responses differently and sometimes significantly. The effect of micro-annulus on logs can be recognized which then facilitates correct assessment of the cement sheath behavior. We note that the log response in the presence of a dry micro-annulus may be different to that for a wet micro-annulus. In the case of a dry micro-annulus, even with relatively smaller gaps between cement and casing, conventional approaches to interpretation of the sonic and ultrasonic logs can completely miss "seeing" the cement. We observe that the integration of ultrasonic and sonic measurements through these workflows has the potential to improve the quality and reliability of cement evaluation when cements de-bond from casing. The ability to do so with existing ultrasonic and sonic technology not only helps improve confidence on cement sheath evaluation but also has the potential to reduce unwanted and costly remediation decisions.

Ultrasonic and sonic logs are increasingly used to evaluate the quality of cement placement in the annulus behind the pipe and its potential to perform as a barrier. Wireline logs are carried out in widely varying conditions and attempt to evaluate a variety of cement formulations in the annulus. The annulus geometry is complex due to pipe standoff and often affects the behavior (properties) of the cement. The transformation of ultrasonic data to meaningful cement evaluation is also a complex task and requires expertise to ensure the processing is correctly carried out as well interpreted correctly. Cement formulations can vary from heavy weight cement to ultralight foamed cements. The ultrasonic log-based evaluation, using legacy practices, works well for cements that are well behaved and well bonded to casing. In such cases, a lightweight cement and heavyweight cement, when bonded, can be easily discriminated from gas or liquid (mud) through simple quantitative thresholds resulting in a Solid(S) - Liquid(L) - Gas(G) map. However, ultralight and foamed cements may overlap with mud in quantitative terms. Cements may debond from casing with a gap (that is either wet or dry), resulting in a very complex log response that may not be amenable to simple threshold-based discrimination of S-L-G. Cement sheath evaluation and the inference of the cement sheath to serve as a barrier is complex. It is therefore imperative that adequate processes mitigate errors in processing and interpretation and bring in reliability and consistency. Processing inconsistencies are caused when we are unable to correctly characterize the borehole properties either due to suboptimal measurements or assumptions of the borehole environment. Experts can and do recognize inconsistencies in processing and can advise appropriate resolution to ensure correct processing. The same decision-making criteria that experts follow can be implemented through autonomous workflows. The ability for software to autocorrect is not only possible but significantly enables the reliability of the product for wellsite decisions. In complex situations of debonded cements and ultralight cements, we may need to approach the interpretation from a data behavior-based approach, which can be explained by physics and modeling or through observations in the field by experts. This leads a novel seven-class annulus characterization [5S-L-G] which we expect will bring improved clarity on the annulus behavior. We explain the rationale for such an approach by providing a catalog of log response for the seven classes. In addition, we introduce the ability to carry out such analysis autonomously though machine learning. Such machine learning algorithms are best carried out after ensuring the data is correctly processed. We demonstrate the capability through a few field examples. The ability to emulate an "expert" through software can lead to an ability to autonomously correct processing inconsistencies prior to an autonomous interpretation, thereby significantly enhancing the reliability and consistency of cement evaluation, ruling out issues related to subjectivity, training, and competency.

The cement bond evaluation of wells completed with fiberglass casings has always been a challenge due to the very large difference in the acoustic behavior of fiberglass with respect to steel. This problem was faced in Kuwait when ultrasonic image logs were recorded for some wells completed with fiberglass casings that gave highly erratic readings and posed significant challenges with interpretation when applying the conventional methods. It was critical to field development engineers to have the precise status of the cement bond around fiber casings to ensure integrity of casing from encroachment of formation fluids in the zone of interest. This, in turn, required that cement bond logs do accurately and precisely evaluate the cement integrity. The logging company along with drilling engineers resolved the challenge of interpretation innovatively by an integrated approach of ultrasonic and sonic data. The approach used a recently introduced platform to develop a new concept of data processing in which high-accuracy interpretation of the cement bond behind fiberglass was made possible. As has been observed through field and laboratory experiments, the conventional ultrasonic technique applicable to carbon steel pipes has been proven to be invalid in fiberglass tubulars because the velocity and acoustic impedance of fiberglass are much lower than steel; therefore, there is no resonance in fiberglass. A new method and interpretation tool was developed and applied to raw data to build on parameters specific to the fiberglass samples used in Kuwait through surface tests to identify the acoustic properties of fiberglass: acoustic impedance, attenuation factor, and velocity. Standardized processing parameters were established for consistency and accuracy to determine the actual pipe thickness, radius and cement acoustic impedance from ultrasonic measurements in many wells. The resulting logs from the new method were found to be satisfactory by field development and they were then applied to earlier drilled wells to validate the results. The advanced platform used for data processing and integration has provided a reference interpretation prototype of log response in fiberglass casings in different scenarios to accurately determine whether cement bond is poor, good, or non-existing. A further investigation of ultrasonic late waveform arrivals could elaborate unique information on casing standoff and centration inside the wellbore. A reasonable casing integrity evaluation was also feasible from the new method resulting in good estimate of valid pipe thickness and acoustic impedance. This paper illustrates the application and evolution of the new method, which enables advanced data processing and integration to provide robust images even beyond cement and pipe integrity. It has been implemented in many wells, and it has provided a significant improvement in quality of logging results in fiberglass casing wells. The new interpretation model can be successfully adopted wherever there is similar material used for casings.

As oil and gas fields mature, many wells are scheduled for permanent well abandonment or permanent abandonment of a section for subsequent slot recovery. We present a novel data interpretation workflow that integrates pulse-echo inversion to accurately predict borehole fluid properties and sonic attenuation with ultrasonic measurement to accurately define the annular material present behind the casing. Per the NORSOK D-010 standard, the requirement to qualify a well section for further drilling is to evaluate any degradation of the main bore casing, along with evaluation of the well barrier element. In an example slot recovery well, pulse-echo measurement and a cement bond log were recorded in the main borehole. The data was interpreted using an industry-available cement evaluation workflow to indicate the presence of very low impedance material (gas column) between two casings up to surface. The presence of a gas column required remedial action to mitigate well integrity issues during well construction. Advanced cement interpretation was conducted using a newly developed workflow that integrates new pulse-echo inversion and sonic attenuation measurement. The novel pulse-echo inversion accurately determines azimuthal as well as depth variations in the acoustic impedance of the borehole mud. The inversion is validated using 3D models without prior knowledge of mud properties. With the help of in situ measurement of mud properties, we can now visualize features such as mud deposition and segregation in deviated pipes as well. This new processing enables easier and more accurate interpretation of the cement sheath together with providing essential information about the logging fluid. The obtained results clearly identified the presence of solids and liquids in the annular space, which helped the operator to make informed decisions for continued slot recovery and new well drilling operations, rather than spending rig time on gas column mitigation measures. Advanced cement evaluation measurement and interpretation techniques reduce risks and enable accurate and critical decision making for well construction, intervention, and plugging and abandonment in many conditions that were previously ambiguous. Novel three-parameter inversion for pulse-echo measurements delivers unmatched accuracy for identifying the annulus material, as well as critical information on the logging mud, leading to a high level of confidence on cement placement. This aspect is especially critical in ensuring correct evaluation in complex new well designs including vertical, deviated, or horizontal pipes with a variety of mud types.