Temperature drift Calibration In addition to specialized cement-bond devices, modern openhole array tools are designed to also provide conventional cement-evaluation measurements in cased hole. The cement-bond instrument sleeve is typically slotted to suppress and delay the tool signal that might otherwise be confused with the important casing signals. TR spacing typically ranges from 3 to 5 ft. The shorter spacing e.
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Crain, P. This webpage version is the copyrighted intellectual property of the author. Do not copy or distribute in any form without explicit permission. Cement integrity logs are run to determine the quality of the cement bond to the production casing, and to evaluate cement fill-up between the casing and the reservoir rock.
A poor cement bond may allow unwanted fluids to enter the well. Poor fill-up of cement leaves large channels behind the pipe that, likewise, allow the flow of unwanted fluids, such as gas or water into an oil well. By-products of cement integrity logs are the compressive strength of the cement, the bond index, and in some cases, the quality of the casing string itself.
Both poor bond and poor fill-up problems can also allow fluids to flow to other reservoirs behind casing. This can cause serious loss of potential oil and gas reserves, or in the worst case, can cause blowouts at the wellhead. Unfortunately, in the early days of well drilling, cement was not required by law above certain designated depths.
Many of the shallow reservoirs around the world have been altered by pressure or fluid crossflow from adjacent reservoirs due to the lack of a cement seal. Getting a good cement job is far from trivial. The drilling mud must be flushed out ahead of the cement placement, the mud cake must be scraped off the borehole wall with scratchers on the casing, fluid flow from the reservoir has to be prevented during the placement process, and the casing has to be centralized in the borehole.
Further, fluid and solids loss from the cement into the reservoir has to be minimized. Gas percolation through the cement while it is setting is a serious concern, as the worm holes thus created allow high pressure gas to escape up the annulus to the wellhead - a very dangerous situation. Poor bond or poor fill-up can often be repaired by a cement squeeze, but it is sometimes impossible to achieve perfect isolation between reservoir zones.
Gas worm holes are especially difficult to seal after they have been created. Poor bond can be created after an initial successful cement job by stressing the casing during high pressure operations such as high rate production or hydraulic fracture stimulations. Thus bond logs are often run in the unstressed environment no pressure at the wellhead and under a stressed environment pressure at the wellhead.
Cement needs to set properly before a cement integrity log is run. This can take from 10 to 50 hours for typical cement jobs. Full compressive strength is reached in 7 to 10 days. The setting time depends on the type of cement, temperature, pressure, and the use of setting accelerants. Excess pressure on the casing should be avoided during the curing period so that the cement bond to the pipe is not disturbed.
Examples and uses for each are described in this Chapter. Before the invention of sonic logs, temperature logs were used to locate cement top, but there was no information about cement integrity. Some knowledge could be gained by comparing open hole neutron logs to a cased hole version. Excess porosity on the cased hole log could indicate poor fill-up channels or mud contamination. The neutron log could sometimes be used to find cement top. The earliest sonic logs appeared around and their use for cement integrity was quantified in The sonic signal amplitude was the key to evaluating cement bond and cement strength.
Low signal amplitude indicated good cement bond and high compressive strength of the cement. It uses 8 or more acoustic receivers around the circumference of the logging tool to obtain the signal amplitude in directional segments. The average signal amplitude still gives the bond index and compressive strength, but the individual amplitudes are shown as a cement map to pinpoint the location of channels, contamination, and missing cement.
This visual presentation is easy to interpret and helps guide the design of remedial cement squeezes. An ultrasonic version of the cement mapping tool also exists. The log presentation is similar to the segmented bond log, but the measurement principle is a little different. Another ultrasonic tool uses a rotating acoustic transducer to obtain images for cement mapping.
It is an offshoot of the open hole borehole televiewer. The signal is processed to obtain the acoustic impedance of the cement sheath and mapped to show cement quality. The tool indicates the presence of channels with more fidelity than the segmented bond tool and allows for analysis of foam and extended cements. Individual acoustic reflections from the inner and outer pipe wall give a pipe thickness log, helpful in locating corrosion, perforations, and casing leaks.
However, the location of the cement top was often required, either to satisfy regulations or for general knowledge. Since cement gives off heat as it cures, the temperature log was used to provide evidence that the well was actually cemented to a level that met expectations.
An example is shown at right. The top of cement is located where the temperature returns to geothermal gradient. The log must be run during the cement curing period as the temperature anomaly will fade with time. Today, most wells are cemented to surface to protect shallow horizons from being disturbed by crossflows behind pipe.
In this case, cement returns to surface are considered sufficient evidence for a complete cement fill-up.
Cement bond logs
Crain, P. This webpage version is the copyrighted intellectual property of the author. Do not copy or distribute in any form without explicit permission. Cement integrity logs are run to determine the quality of the cement bond to the production casing, and to evaluate cement fill-up between the casing and the reservoir rock. A poor cement bond may allow unwanted fluids to enter the well. Poor fill-up of cement leaves large channels behind the pipe that, likewise, allow the flow of unwanted fluids, such as gas or water into an oil well.
What is Cement Bond Log (CBL)?
The term of CBL is particularly refer to amplitude measurement of acoustic wave. CBL logging tool is run on electric wire line cable after cementing job is done. The measured sound wave is then analyzed. The attenuation is due to the dissipation of acoustic-wave energy by cement sheath behind the casing.
Cementation Evaluation - Cement Bond Log Interpretation