Using a cement bond log, a variable density log, and ultrasonic tools provides an indispensable (but different) set of evaluative data. The objective of cement bond logs like these is to provide information to understand if we have achieved zonal isolation. If not, they can help to asses if a cement squeeze will be possible to restore it.
The challenge of variable density and cement bond log interpretation
The first paragraph covers how cement evaluation logs are mostly accepted, however…
There is a big, big BUT here…
Cement logs, i.e., cement bond logs (CBL), variable density logs (VDL), and ultrasonic tools do not show or measure zonal isolation…
With them, 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). If we see that on 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. For practical purposes, I would agree. However, certain situations can blur that assumption:
1) On the one hand, poor mud properties (flocculation, leaking & thick mud cake, etc.) can lead to a similar appearance to free pipe. (When gaps beyond the depth of investigation (usually > 0.25 mm liquid filled gaps) invisible to ultrasonic tools are created) ; and
2) On the other hand, formations debris (from the drilling process or the interaction with drilling mud), fast formations and even creeping formations can give a too 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. The cement is there, so sooner or later, isolation will be in place.
Situations where you should be worried when the log is not answering important questions:
- Gas wells.
- A significant 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 these situations?
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 a micro-annulus); or channel (if there is a gap parallel to the casing). Gas and significant differential pressure can undoubtedly create 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 entirely 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 the 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 or spacer. It would affect the CBL log 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 entirely different
- Cement will always set. If contamination does not allow setting, segregation takes place. Separation means 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, abnormal lab test (wrong temperature or heating/pressure schedule) or insufficient WOC.
Unset cement looks 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
Predominantly the effect of high permeable or abnormally pressurized formations. Basically:
a) The cement liquid phase filtrates into the permeable formation or;
b) Formation fluids invade the cement during the transition (liquid to solid) stage.
In the 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 the CBL/VDL and ultrasonic logs.
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, its 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 result of the stress condition derived from pressurizing the casing. The extension of the damage would be mainly influenced by the magnitude of the pressure, the density of the cement (compressive strength) and the compressive strength 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 affected as it will take some time 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 the casing and formation.
8) Lightweight 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 the assessment of the logs particularly tricky 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 cement evaluation 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 the space below. Hope that this has helped you understand cement bond logs and variable density logs more!