Cement Evaluation Logs

A cement evaluation log is a dataset used to assess the quality of cement around the casing in a welbore. This log can be crucial for determining whether the cement has adequately sealed the well, providing both zonal isolation and structural integrity. The primary purpose of a cement evaluation log is to evaluate the effectiveness of the cement job in preventing fluids (like oil, gas, and water) from different zones from mixing or leaking into other formations or to the surface.

The cement evaluation log can assist in identifying issues such as channels or voids in the cement sheath and the presence of a microannulus—a small gap between the casing and the cement that can compromise well integrity. These evaluations are vital for ensuring that the well operates safely, effectively, and stays environmentally sound over its lifespan.

Cement logging operations have seen a significant increase in activity recently, primarily driven by the rise in abandonment work. Cement evaluation is particularly important for abadnonments because they ensure that the well is securely sealed and there are no environmental or safety risks. Here are the key reasons why these logs are critical during the abandonment process:

• Environmental Protection: Proper well abandonment requires that all hydrocarbon-bearing zones be permanently isolated to prevent any leakage into other formations or to the surface. Cement evaluation logs help confirm that the cement barriers are intact and effective in sealing off these zones.
• Regulatory Compliance: Many regulatory bodies require comprehensive documentation that wells are abandoned according to specific standards, which often include ensuring effective zonal isolation through cement integrity. Cement evaluation logs provide the necessary evidence that these standards have been met.
• Identifying Potential Issues: Before a well can be abandoned, it’s crucial to identify any issues such as channels, voids in the cement, or the presence of a microannulus. These defects can compromise the long-term integrity of the well seal. Cement evaluation logs help detect these problems, allowing for corrective actions to be taken before the abandonment is finalized.
• Long-term Well Integrity: The logs provide a snapshot of the cement’s condition at the time of abandonment, ensuring that the well remains stable and sealed over the long term, thereby mitigating risks of contamination or subsurface migration of fluids.
• Cost Management: By accurately assessing the condition of the well and the effectiveness of the cement job, these logs help avoid future expenditures related to potential environmental cleanup or additional remedial cementing if the well were to leak post-abandonment.

Techniques used in cement evaluation logs include acoustic tools, such as sonic or ultrasonic tools, flexural waves and electromagnetic tools, an overview of each will be included in this article.

What are the main challenges in cement evaluation logs?

Operators may choose to not run cement evaluation logs in every scenario, relying instead on other indicators such as pressure tests. There are are a number of challenges in effective data acquistion and effective interpretation for cement evaluation logs including:

1. Microannulus Sensitivity: The cement log can be sensitive to the presence of a microannulus, a small gap between the casing and cement sheath that can develop as the cement shrinks upon setting. This sensitivity can lead to ambiguous readings, suggesting poor cement quality where it may be adequate.
2. Tool Centering: The accuracy of a CBL logs depends significantly on the centralisation of the tool within the casing. Poor centralization can result in inconsistent acoustic coupling, leading to unreliable log readings.
3. Acoustic Impedance Matching: Situations where the acoustic impedance of the cement is similar to the surrounding fluids (like mud or contaminated cement) can make it difficult for traditional sonic based tools to distinguish between solid cement and liquid-filled gaps. This can falsely indicate poor cement bonding.
4. Operational Complexity and Costs: Running cement logs requires specific operational conditions and expert analysis for accurate interpretation. The process can be expensive and time-consuming, particularly in challenging environments like horizontal or highly deviated wells.
5. Requirement for Pressure: Traditional acoustic cement evaluation techniques often require the casing to be pressurized to differentiate between a solid cement job and issues like microannulus. This additional step can complicate operations and increase costs.
6. Cement Contamination: During the cementing process, the cement can become contaminated by the drilling fluid (mud), which can significantly alter its acoustic properties. This contamination makes it difficult to accurately assess cement integrity using standard acoustic tools, as the impedance of the contaminated cement may closely resemble that of the surrounding mud.
7. Multiple Casing Strings: Where multiple casing strings are present this poses challenges for the following reasons:

• Acoustic Signal Complexity: Multiple casing strings create complex acoustic environments where the signals used in tools like sonic and ultrasonic cement evaluation logs can have multiple reflections and interference patterns. This complexity can obscure the clarity of the signal received, making it difficult to interpret the data accurately.
• Interference from Additional Steel Layers: The presence of additional steel layers due to multiple casings can alter the acoustic impedance of the wellbore environment. This alteration affects the transmission and reflection of acoustic waves used in tools like Cement Bond Logs (CBL) and ultrasonic logs, which rely on detecting differences in acoustic impedance to assess cement quality.
• Evaluation Tool Limitations: Standard cement evaluation tools are generally optimised for single casing strings. When multiple casings are present, these tools may struggle to penetrate all layers effectively, leading to potential misinterpretation of the cement bond quality behind the outermost casing string.
• Increased Risk of Channeling and Poor Zonal Isolation: The complexity of having multiple casing strings can lead to increased opportunities for cement channeling and other anomalies that compromise zonal isolation. Evaluating the integrity of cement in these scenarios becomes more critical and challenging.

Many if not all of these challenges can be overcome with effective job planning, tool selection and interpretation by an experienced domain consultant. There are services which are now available which can overcome the challenges of multiple casing strings (these will be dealt with in a seperate post and not within this article).

What are the different types of cement evaluation logs and how do they work?

Cement Bond Logs (CBL) and Variable Density Logs (VDL)

Cement Bond Logs (CBL) and Variable Density Logs (VDL) are essential tools used to evaluate the integrity of cement jobs in oil and gas wells, ensuring that cement has properly bonded to the casing and to the formation, providing effective zonal isolation. Here is an overview of what these logs are and how they are acquired:

Cement Bond Logs (CBL)

CBLs measure the acoustic impedance of cement behind casing to assess the bond between the casing and the cement. A good bond is indicated by lower acoustic amplitudes because the sound waves are effectively dampened by the cement. A useful analagy to visualise this is to consider a piece of copper piping being hit with a metal spanner, with no lagging present the ringing sound would be louder (higher amplitude) than with no lagging material present. Conversely, higher amplitudes indicate poor bonding or sections where cement is absent.

A CBL tool emits an acoustic signal that travels down the wellbore. The signal reflects off the casing and any cement behind it. The amplitude of the reflected signal is recorded, providing a measure of cement quality.

Variable Density Logs (VDL)

VDLs provide a visual representation of the acoustic signal measured by the CBL tool. They are essentially a waveform display of the acoustic signals received, showing how these signals vary with depth in the well.

As part of the same tool that acquires CBL data, VDLs capture the full waveform of the reflected acoustic signal. The waveform’s shape and amplitude variations help in interpreting the quality of cement to formation bond, identifying areas with channels or voids.

A VDL log waveforem represents the cement to formation bond, if you see straight lines (train tracks) this is indicative of little or no cement.

Both CBL and VDL are typically acquired using a wireline deployed into the well after the cement has cured. The tool is centralized in the casing to ensure accurate measurements and to avoid tool eccentricity, which can distort the acoustic signals.

Ultrasonic Cement Evaluation Logs

The ultrasonic imaging tool features a rotating sub with a single ultrasonic transducer. This transducer continuously emits short pulses of acoustic energy towards the casing as it rotates, providing a detailed 360-degree view of the casing circumference, which improves the assessment of the cement bond and the detection of channels or voids. The transducer acts as both the transmitter and the receiver.

The ultrasonic waves travel through the casing and interact with different interfaces. Depending on the material behind the casing (e.g., cement or formation), the waves are either absorbed or reflected differently. The initial reflection occurs at the mud-to-casing interface, which is typically the strongest due to the significant acoustic impedance contrast.

The transducer receives echoes from the casing and any cement behind it. The nature of these echoes—how quickly they decay and their amplitude—provides insights into the material properties behind the casing. A rapid decay of the echo suggests good cement bonding, which dampens the acoustic energy effectively. Conversely, a slow decay indicates poor or absent cement.

The received waveforms are digitized and processed. The analysis focuses on identifying the acoustic impedance of the materials, the thickness of the cement sheath, and the integrity of the casing itself.

Flexural Wave Ultrasonic

An example of a flexural wave ultrasonic tool is the SLB Isolation Scanner (a.k.a. IBC). The tool offers several advantages over previous ultrasonic technologies used for cement and casing evaluation, particularly in challenging conditions such as lighteweight cement (low acoustic impedence), heavy mud and thick casing environments. Here are the key advantages:

• Enhanced Measurement Capabilities: The IBC combines pulse-echo ultrasonic measurement with flexural wave measurement. This dual approach allows for better discrimination between solid, liquid, and gas phases in cement evaluation behind the casing.
• Improved Depth of Investigation: With an investigation depth of up to 3 inches beyond the casing, the IBC offers a deeper insight into the condition of the cement sheath and the surrounding formation. This depth is crucial for assessing the effectiveness of zonal isolation and identifying potential pathways for fluid migration.
• Operation in Extreme Conditions: Traditional tools such as Cement Bond Logs (CBL) and Variable Density Logs (VDL) struggle with the attenuation of acoustic signals in heavy mud and thick casing scenarios. The new ultrasonic tool is designed to operate effectively in such environments, providing reliable data where older technologies fail.
• Real-Time Data and Simplified Interpretation: The tool enables real-time generation of Solid-Liquid-Gas (SLG) maps during logging operations. This capability allows for immediate on-site evaluation of cement quality, significantly simplifying interpretation and enhancing the decision-making process.
• Flexural Wave Attenuation Measurement: This feature allows the tool to distinguish between different material densities behind the casing. It is particularly effective in identifying lightweight, foam, or contaminated cements that have similar acoustic impedances to drilling mud.

The primary difference between the USIT (Ultrasonic Imaging Tool) and IBC (Isolation Bond Tool) lies in their design, capabilities, and the detailed measurements they can provide, which influence their application in evaluating cement quality in wells. Which one should you choose? Skip to the technical advisory section at the end of this article to see some guidance on making this decision.

EMAT (Electromagnetic Acoustic Transducer) measurement for cement evaluation

EMAT technology is particularly useful in challenging environments where traditional methods might fail due to their dependence on acoustic coupling through fluids.

EMAT uses coils to induce eddy currents in the conductive material of the casing. These eddy currents interact with a magnetic field generated by permanent magnets in the tool. The interaction between the magnetic field and the eddy currents produces Lorentz forces. The Lorentz forces generate acoustic waves that propagate along the casing wall. These waves are crucial for assessing the cement’s quality around the casing.

Types of Acoustic Waves from EMAT measurements

• SH Waves (Horizontal Shear Waves): These waves move parallel to the casing surface and perpendicular to the wave direction. They are particularly sensitive to the shear modulus of materials bonded to the casing, such as cement.
• Lamb or Flexural Modes: These additional wave types can be used to detect conditions like micro-annuli (small gaps between the cement and casing).

EMAT technology is a non-contact technique that operates effectively under various well conditions, including gas-filled environments, without needing physical contact with the casing or fluid. It directly measures properties such as shear modulus, which reflect the mechanical integrity of the cement. Additionally, EMAT is highly sensitive to micro-annuli, allowing it to detect very small gaps between the casing and the cement, thereby enhancing the reliability of cement integrity assessments.

Technical Advisory

At one&zero we are often asked what is the right tool for a particular job. When it comes to well integrity and cement evaluation there are a number of options and a large amount of variables which need to feed into this decision making process.

Here we will consider the choice of CBL-VDL versus Ultrasonic versus Flexural Wave tools from SLB to exemplify the factors which feed into such decisions:

CBL-VDL (DSLT / MSIP (Sonic Scanner)

• A non-directional measurement
• Provides an average cement quality based on the dampening of the acoustic waveform (only a single value of CBL for any particular depth – non directional, no thickness, internal id, fluid type etc).
• Qualitative cement-formation bond from VDL only.
• Sensitive to fast formations and potentially unreliable
• It is usually advisable to run both to provide a backup measurement, and give confidence to any final interpretation. Although this is not a necessity for the correct functioning of the tool.

Ultrasonic Iamging Tool (USIT)

• 360 degree coverage of the casing/ behind the casing
• A cement map behind casing is provided which can be used to differentiate between fluid type behind the casing/ micro-annulus etc (may require a pressure pass)
• Provides acoustic measurements of the ID of the casing
• Provides a thickness measurement of the casing
• Can provide internal/external damage indications
• Sensitive to fluid (particularly heavy muds)
• Thick casings can reduce the operating frequency and need to be confirmed within the operating range of the tool

Isolation Scanner (IBC)

Advanced Capabilities: The IBC integrates multiple transducers and includes flexural transducers in addition to the standard ultrasonic transducer found in USIT. This allows IBC to provide both standard USIT measurements as specified above and additional flexural attenuation measurements.

Flexural Attenuation Measurement: This feature of IBC provides an additional layer of data by measuring the energy attenuation in the flexural mode of the casing, which enhances the ability to differentiate between solid, liquid, and gas phases in the annulus. This is particularly useful in complex scenarios where distinguishing between these phases based solely on acoustic impedance might be challenging (light cement / contamination ect)

Higher Cost and Complexity: IBC is more expensive and complex than USIT. It requires careful calibration and handling due to its multiple transducers and the detailed settings needed for optimal performance. The increased complexity can lead to a higher likelihood of operational issues.

Analysis

To decide whether to use the Ultrasonic Imaging Tool (USIT) or the Isolation Scanner (IBC) for cement evaluation based on ultrasonic measurements the following factors must be considered:

Experience with the Tools: Both USIT and IBC have specific characteristics that may suit different scenarios. The USIT has been used for a long time and is well-understood, while the IBC is newer offers advanced features which may, or may not add value, and likely a higher cost.

Tool Technology and Features: USIT has a single transducer while IBC uses three, which can potentially provide more detailed data. However, the complexity, including calibration and setup and components, of the IBC means more things could potentially go wrong.

Application Suitability: Depending on the specific requirements of the cement evaluation job (e.g., type of cement, conditions of the casing and surrounding materials), one tool may be more suitable than the other. The IBC’s additional measurements might provide better data in complex scenarios, but for standard applications, the USIT might suffice.

Data Interpretation and Decision Support: The discussion emphasizes the importance of having accurate, easy-to-interpret data. If the job requires high accuracy and detailed analysis (possibly in challenging or uncertain conditions), the IBC’s additional features might be advantageous.

Historical Data and Familiarity: If previous jobs have used one tool over the other with success, this historical precedence and familiarity can influence the decision, especially if similar conditions are expected.

In summary, the choice between USIT and IBC should be guided by the specific needs of the evaluation job, cost considerations, the complexity of the operation, and the level of detail required in the data. If cost is a critical factor and the job conditions are well-understood and not overly complex, USIT might be preferred. On the other hand, for more complex conditions where detailed analysis is crucial, the additional investment in IBC could be justified.

Many inputs to the decision to run one tool over another can be modelled based on cement, casing and fluid environmnets. If you require assitance in this process, one&zero domain advisors are here to help.

Contact: [email protected]