The amplitude is a maximum in unsupported pipe, and a minimum for well cemented casing. The absolute reading is a function of casing size and thickness, cement bond quality, wellbore fluid, tool type and tool centering. Generally, most of these parameters will be constant so that cement bond quality can be directly related to the amplitude measured. Usually, the CBL is correlated to the open hole logs.
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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. 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 ….. 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. 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. 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. Otherwise, the analysis would become more of an educated guess than an objective fact-based report. It is the result of hole geometry and pipe centralization 2 Rheology-induced channel. In both cases, the VDL would show formation arrivals. Probably a combined effect of 1 and 2. It would affect the CBL with amplitude values ranging 10 to 30 mV, but lower than free-pipe value. Two important things here: A cement log is only a snapshot.
A cement log days after can be totally different Cement will always set. Contaminated cement is common cause when no bottom plug is present, e. Unset cement look similar to contaminated cement, but the origin is different. 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. 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.
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.
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 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.
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 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.
CBL SPE Papers related to Cement Bond Logs
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. 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 ….. 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.
CBL VDL INTERPRETATION PDF
Kajitilar Table 2 lists the limitations of conventional cement-bond logs. In an evolution similar to that of openhole acoustic logs, new designs were subsequently introduced that measured signal amplitude at knterpretation near receiver and a full waveform from a far receiver. Acoustic energy is reflected at interfaces that correspond to changes in acoustic impedance the product of acoustic velocity and density. Good cement bond to casing produces a rapid damping higher impedance of this resonance; poor cement bond results in longer resonance decay lower impedance. Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read.
CBL VDL LOG PDF
Before beginning this post I would like to greet my friends The Father, Talara and Goofy who are regular visitors to the blog and I constantly express their observations. It is still widely used and often preferred to many other instruments most recent assessment of cement. These tools CET, PET also measure the thickness of casing, micro ring and cement pipe, but no concrete measure revenue to training. A cement bond log CBL is executed for inspecting the integrity of the sealing cement sheath and formation. This will ensure that formation fluids flow into the housing when the productive zone is perforated and not towards the outside of the casing.