Quick Cement Log Interpretation Guide

Dear members, I have recently posted in my website the below short original article as a quick guide to some of the most relevant cement coverage defects that appear in logs. All my work in http://better-cementing-for-all.org/ is original and there is no "cut & paste" from books or other pulic media and I always want them to be helpful and meninful to people. What better than my-spread to have acess the most experience people in the industry to seek for comments to improve this document.

I would appreciate if you can give me your opinion, comments or suggestions. You can do that here  (Note from moderator: best to submit here, for all SPREAD members to see) or directly in my website: http://better-cementing-for-all.org/quick-cement-log-interpretation-guide

Thanks in Advanced for your Help

L. Diaz

Quick Cement Log Interpretation Guide

For Cementers and Drillers

The objective of cement evaluation logs is to provide information to understand if zonal isolation has been achieved, and if not achieved, help to asses if a cement squeeze is possible to restore it.

But …..

That is how cement evaluation logs are mostly accepted, however … there is a big … big BUT here … cement logs, i.e., CBL/VDL, ultrasonic tools, do not show or measure zonal isolation … with then we can only see cement presence in contact or close to the casing (CBL and Ultrasonic logs), and in contact or near the formation (only VDL)…. and If we see that in average or, better, circumferentially then we infer that isolation in likely there … but, if we do not see sufficient cement then isolation cannot be verified by logs … but by interpretation, if we are lucky enough.

Everything I will mention from here assumes that the cement evaluation logs have been QA/QC’ed diligently, in other words, there are no tool setting/calibration/configuration issues affecting negatively or positively the look of the logs.

Most people would agree that there are only two situations when the log totally speaks by itself and does not require a “log whisperer”, that is: free pipe and fully bonded cement. And for practical purposes I would agree, however certain situations can blur that assumption: 1) on one hand, poor mud properties (flocculation, leaking & thick mud cake, etc.) can lead to similar looks to free pipe, when gaps beyond the depth of investigation (usually > 0.25 mm liquid filled gaps) of ultrasonic tools are created; and 2) on the other hand, formations debris (from the drilling process or from the interaction with drilling mud), fast formations and even creeping formations can give an optimistic image on the log.

My point of view

As a cementer, I rely on pumping pressure … that is the only direct measurement we have that proves we put cement in the annulus. However, there are two circumstances here:

1) Well in control (no losses no gains while cementing) and final circulating pressure matches the expected value. In this case, doesn’t matter what the log says … cement is there, so sooner or later, isolation will be in place. Cases you should be worry about, if the log is not answering important questions:

• Gas wells.
• Big pressure differential between two adjacent formations, with a natural barrier in between. Like an upper lost zone and lower high-pressure zone.
• Interval-specific stimulation candidates

Why you should worry about with these? Because in addition to lacking isolation, which means there is a path, for fluids to move they must overcome the friction pressure of the path (if there is a gap between cement and casing or annulus, like micro-annulus) or channel (if there is a gap parallel to the casing). Gas and big differential pressure can certainly do that trick.

2) Well is not in control (loses or influx during cementing), forget about pumping pressure because now its true meaning has been concealed … and you are fully in the hands of the “log shaman” .

In both cases, post-job analysis done by a qualified cementing technical person can provide significant help to explain the look of the log. But, only if sufficient pre-job (caliper, rheology of fluids, hole condition, etc.) and job data (returns, mud logging, time log, etc.) is available. Otherwise, the analysis would become more of an educated guess than an objective fact-based report.

Now, what about what we see in the log: “cement coverage defects”

1) Geometry-induced channel. It is the result of hole geometry and pipe centralization

2) Rheology-induced channel. It is the result of improper design of cementing fluids or the desperate attempts by some cementing companies to add expensive and sometimes useless pre-flushes and/or spacers

If you only have a CBL/VDL, 1) and 2) would look exactly the same, but comparatively a rheology-induced channel would tend to have a lower amplitude reading. In both cases, the VDL would show formation arrivals.

3) Contaminated cement. Probably a combined effect of 1) and 2).

Contaminated cement is a form of unset cement caused by inter-mixing with mud and/or spacer. It would affect the CBL with amplitude values ranging 10 to 30 mV, but lower than free-pipe value. In ultrasonic logs it would appear as areas/spots with lower acoustic impedance values.

 Two important things here: 

• A cement log is only a snapshot. A cement log days after can be totally different
• Cement will always set. If contamination does not allow setting, segregation takes place … meaning mass transport (gravity/settling for particles and buoyancy/floating for lighter fluids).

Contaminated cement is common cause when no bottom plug is present, e.g., liner jobs, two-stage jobs, etc.

4) Unset cement. Long cement column, lead cement, improper lab test (wrong temperature or heating/pressure schedule) or insufficient WOC.

Unset cement look similar to contaminated cement, but the origin is different. Unset cement is a design failure when the cement is placed in a lower than expected/assumed temperature or insufficient WOC. In ultrasonic logs it would appear as near-mud acoustic impedance values and would primarily affect the definition of the TOC (pressure-based estimated TOC and cement log needs to be used jointly in these situations)

5) Formation-induced defect

This is predominantly the effect of high permeable or abnormally pressurized formations. Basically, a) cement liquid phase filtrates into the permeable formation or b) formation fluids invade the cement during the transition (liquid to solid) stage.

In cement logs they will look different, for a) considering liquid volume is lost, the resulting set cement is unable to fill the annular gap resulting in radial cracks and de-bonding. This is sometimes confused with micro-annulus, because of a similar signature in the CBL/VDL. In the ultrasonic log, because all gaps and cracks are filled with fluid, the image is erratic or patchy with intermittent highs and lows acoustic impedance; the average however will be closer to good cement value.

In b) the setting process is delayed showing a similar pattern to 3) and 4) in both CBL/VDL and ultrasonic log.

A characteristic sign of formation-induced defect is a sudden change or step change in front of the permeable or high pore pressure formation. The cement log would quickly change from good to bad; it would stay bad in front of the problem zone and then quickly changing back to good in the next formation. 

6) Pressure-induced defect

This would be the result of the excessive pressure differential a) while cementing (logging fluid lower density than displacing fluid) or b) after cementing (casing pressure test, confirmation of micro-annulus – this is a thoughtless destructive test – , formation integrity test). a) is the classical definition of micro-annulus. Theoretically this micro-annulus would be either dry or wet. 

Depending on the size of it, a dry gap could have a lesser effect on CBL (VDL would show formation arrivals in either case) than a wet gap (CBL can appear almost as free pipe). For ultrasonic logs is the opposite, a dry gap would have a greater effect than a wet gap (most ultrasonic logs can see thru a wet micro-annulus as long as it remains less than 250 microns).

In b) the cement fails as a results of the stress condition derived from pressurizing the casing. The extension of the damage would be maily influenced by the maginitud of the pressure, the density of the cement (compressive strength) and the compressive strenght of the supporting formation. This is similar to 5.a) and as well mostly confused with micro-annulus. However, in general the CBL will be less afected as it will take sometime for the gap and cracks to be filled with liquid. The ultrasonic log, the image will be erratic or patchy as well but the average in this case would be closer to bad cement value.

7) Mud-induced defect

This is the case when the mud is unattended after the drilling is over, typically flocculated mud and/or low quality mud cake (permeable) for example can make any cementing fluid placement design obsolete. The problem here is that the excessive mud filtrate dynamically leaking causes a viscosity increase or gelling in the mud; this forces the cement to flow or channel circumferentially creating, once set, a big gap between cement and formation that quickly translate to immediate de-bonding of the cement attached to the casing.

This is a typical case when the well was in control and final circulating lift pressure is ok, but there is no cement in logs … however, cement is there, it is just that the gap is beyond what the logs can see. In a way the cement is floating in between casing and formation.

8) Light weight cement

The only thing I can say about low-density cement (cenospheres, foamed cement, extended systems) is that it worsens the effect of all the previous defects (from 1) to 7)); this makes particularly difficult the assessment of the logs and gives more importance to the post-job analysis and joint work with a log interpreter. 

NOTE: I have worked with countless cement log experts in my career and in all cases the interaction has been positive and a learning experience for the parties involved. … The most important thing to understand is that a “good log” is not achieved by arguing, underestimating or ignoring the assessment of the log interpreter …. A “good log” is the result of a dynamic and strong drilling team.

Please let me know your comments and suggestions in my website

Cheers

L. Diaz

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