01What Is a Microannulus?
A microannulus is a small gap between the casing and the cement sheath. It may be filled with liquid or remain as an unfilled separation between the two surfaces. It can affect cement evaluation log responses, making interpretation more challenging.
Microannulus can also occur between the cement sheath and the formation. This article focuses on the casing–cement interface.
02Wet and Dry Microannulus: What Is the Difference?
A wet microannulus is a liquid-filled gap between the casing and the cement sheath. It may be associated with a residual drilling fluid film or conditions on the external casing surface that limit cement bonding.
A dry microannulus is an unfilled separation between the casing and the cement sheath. It may develop when the cement shrinks or when changes in pressure or temperature cause the cement to lose contact with the casing.
These conditions can affect sonic and ultrasonic measurements differently. Recognising the difference helps explain apparently conflicting cement evaluation results.
03How Can Microannulus Be Identified on Cement Evaluation Logs?
Comparing sonic and ultrasonic responses can help identify wet and dry microannulus. The response depends on the type and thickness of the microannulus.
The table below summarises indicative thickness ranges and response patterns. These support interpretation but should not be used alone to confirm zonal isolation.
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| Microannulus type | Indicative gap thickness (µm) | Sonic amplitude | Casing arrivals | Formation arrivals | Apparent acoustic impedance |
|---|---|---|---|---|---|
| Wet microannulus | <100 | Moderate | Strong | Moderate | High |
| Wet microannulus | 100–200 | High | Strong | Moderate | Moderate |
| Wet microannulus | >200 | High | Strong | Weak | Low – liquid range |
| Dry microannulus | <10 | Low | Weak | Strong | Low – liquid range |
| Dry microannulus | 10–100 | Moderate | Moderate | Moderate | Very low – gas range |
| Dry microannulus | >100 | Higher than free pipe amplitude | Strong | Minimal | Very low – gas range |
The following examples illustrate contrasting sonic and ultrasonic responses associated with wet and dry microannulus.
New-generation cement evaluation tools use additional measurements, such as flexural attenuation, and interpretation algorithms to help identify microannulus.
Comparing logs acquired with and without applied casing pressure can also provide insight into the type and estimated thickness of the microannulus. Pressure-pass interpretation deserves a separate discussion.
04What Does Microannulus Mean for Zonal Isolation?
Detecting a microannulus does not, by itself, determine that zonal isolation was not achieved.
The assessment must be made on a case-by-case basis. It must consider the type of microannulus, its estimated thickness, distribution and continuity, together with the cement bond to the formation.
At Cementing Performance, we have developed our own interpretation workflows and algorithms to identify microannulus and assess whether an interval provides a competent annular seal.
Learn more about our cement evaluation log interpretation service.
05Microannulus Remediation
Not all microannuli require remediation. When remediation is required, it can be challenging and may not be feasible.
The fluid within the gap, its thickness and continuity, formation creep closing the microannulus, pressure limitations, and the availability of downhole tools and cementing fluids together define the remediation plan.
At Cementing Performance, we combine expertise in both cementing engineering and cement evaluation log interpretation to support remedial job planning, design, execution and evaluation.
06What Can Be Improved in the Next Cementing Operation?
Addressing microannulus prevention at the primary cementing design stage is preferable to costly and challenging remediation.
Understanding the likely causes helps identify changes for subsequent cementing jobs. These may include changes to the mud removal strategy, the use of expanding agents, cleaning the external casing surface to improve cement bonding, and the use of fluid migration control slurries.