Cement Integrity Logs - Part 1

4/11/2015 Crain's Petrophysical Handbook - CEMENT INTEGRITY LOGS - PART 1

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CEMENT INTEGRITY LOGS - PART 1 -- CBL / VDL

This Page è Cementing Basics Cement Log Basics Temperature Cement Bond VDL

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CEMENTING BASICSCement bond logs were run as early as 1958 with early sonic logs andthe temperature log was used to find cement top beginning in 1933.Cement integrity logs are run to determine the quality of the cementbond to the production casing, and to evaluate cement fill-upbetween the casing and the reservoir rock. A poor cement bond mayallow unwanted fluids to enter the well. Poor fill-up of cement leaveslarge channels behind the pipe that, likewise, allow the flow ofunwanted fluids, such as gas or water into an oil well. By-products ofcement integrity logs are the compressive strength of the cement, thebond 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 thewellhead. Unfortunately, in the early days of well drilling, cement was not required by law above certaindesignated depths. Many of the shallow reservoirs around the world have been altered by pressure or fluidcrossflow 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 cementplacement, the mud cake must be scraped off the borehole wall with scratchers on the casing, fluid flowfrom the reservoir has to be prevented during the placement process, and the casing has to be centralizedin 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 createdallow 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 toachieve perfect isolation between reservoir zones. Gas worm holes are especially difficult to seal after theyhave been created.

Poor bond can be created after an initial successful cement job by stressing the casing during high pressureoperations such as high rate production or hydraulic fracture stimulations. Thus bond logs are often run inthe unstressed environment (no pressure at the wellhead) and under a stressed environment (pressure at thewellhead).

Cement needs to set properly before a cement integrity log is run. This can take from 10 to 50 hours fortypical cement jobs. Full compressive strength is reached in 7 to 10 days. The setting time depends on thetype of cement, temperature, pressure, and the use of setting accelerants. Excess pressure on the casingshould be avoided during the curing period so that the cement bond to the pipe is not disturbed.

CEMENT INTEGRITY LOG BASICSToday’s cement integrity logs come in four flavours: cement bond logs (CBL), cement mapping logs (CMT),

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ultrasonic cement mapping tools (CET), and ultrasonic imaging logs (USI, RBT). Examples and uses for eachare described in this Chapter.

Before the invention of sonic logs, temperature logs were used to locate cement top, but there was noinformation about cement integrity. Some knowledge could be gained by comparing open hole neutron logsto a cased hole version. Excess porosity on the cased hole log could indicate poor fill-up (channels) or mudcontamination. The neutron log could sometimes be used to find cement top.

The earliest sonic logs appeared around 1958 and their use for cement integrity was quantified in 1962. Thesonic signal amplitude was the key to evaluating cement bond and cement strength. Low signal amplitudeindicated good cement bond and high compressive strength of the cement.

In the 1970’s, the segmented bond tool appeared. It uses 8 or more acoustic receivers around thecircumference of the logging tool to obtain the signal amplitude in directional segments. The average signalamplitude still gives the bond index and compressive strength, but the individual amplitudes are shown as acement map to pinpoint the location of channels, contamination, and missing cement. This visualpresentation is easy to interpret and helps guide the design of remedial cement squeezes. An ultrasonicversion 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 anoffshoot of the open hole borehole televiewer. The signal is processed to obtain the acoustic impedance ofthe cement sheath and mapped to show cement quality. The tool indicates the presence of channels withmore 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 inlocating corrosion, perforations, and casing leaks.

TEMPERATURE LOGS FOR CEMENT TOPIn the “good old days” before the invention of sonic logs, therewas no genuine cement integrity log. However, the location of thecement top was often required, either to satisfy regulations or forgeneral knowledge. Since cement gives off heat as it cures, thetemperature log was used to provide evidence that the well wasactually cemented to a level that met expectations. An example isshown at right. The top of cement is located where thetemperature returns to geothermal gradient. The log must be runduring the cement curing period as the temperature anomaly willfade with time.

Today, most wells are cemented to surface to protect shallowhorizons from being disturbed by crossflows behind pipe. In thiscase, cement returns to surface are considered sufficient evidencefor a complete cement fill-up.

CEMENT BOND LOGS (CBL)



Cement bond logs (CBL) are still run today becausethey are relatively inexpensive and almost everywireline company has a version of the tool. The logexample at the right illustrates the use of theacoustic amplitude curve to indicate cement bondintegrity.

The examples in this Section are taken from”Cement Bond Log Interpretation of Cement andCasing Variables”, G.H. Pardue, R.L. Morris, L.H.Gallwitzer, Schlumberger 1962.

EXAMPLE 1: CBL in well bonded cement – lowamplitude means good bond. The SP is from anopenhole log; a gamma ray curve is more common.Most logs run today have additional computedcurves, as well as a VDL display of the acousticwaveforms.

The CBL uses conventional sonic log principals of refraction to make itsmeasurements. The sound travels from the transmitter, through the mud,and refracts along the casing-mud interface and refracts back to thereceivers, as shown in the illustration on the left. In fast formations (fasterthan the casing), the signal travels up the cement-formation interface, andarrives at the receiver before the casing refraction.

The amplitude is recorded on the log in millivolts, or as attenuation indecibels/foot (db/ft), or as bond index, or any two or three of these. A traveltime curve is also presented. It is used as a quality control curve. A straightline indicates no cycle skips or formation arrivals, so the amplitude value isreliable. Skips may indicate poor tool centralization or poor choice for thetrigger threshold.

The actual value measured is the signal amplitude in millivolts. Attenuationis calculated by the service company based on its tool design, casingdiameter, and transmitter to receiver spacing. Compressive strength of thecement is derived from the attenuation with a correction for casingthickness. Finally, bond index is calculated by the equation:

1: BondIndex = Atten / ATTMAX

Where: Atten = Attenuation at any point on the log (db/ft or db/meter) ATTMAX = Maximum attenuation (db/ft or db/meter)

The maximum attenuation can be picked from the log at the depth where the lowest amplitude occurs. On



older logs attenuation and bond index were computed manually. On modern logs, these are provided asnormal output curves. Bond Index is a qualitative indicator of channels. A Bond Index of 0.30 suggests thatonly about 30% of the annulus is filled with good cement.

INTERPRETATION RULE 1: Low Amplitude = Good CementINTERPRETATION RULE 2: High Attenuation = Good CementINTERPRETATION RULE 3: High Bond Index = Good Cement

A nomograph for calculating attenuation and bond index for older Schlumberger logs is given below.

Chart for calculating cement bond attenuation and cement compressive strength



Zone isolation is a critical factor in producing hydrocarbons. Inoil wells, we want to exclude gas and water; in gas wells, wewant to exclude water production. We also do not want to losevaluable resources by crossflow behind casing. Isolation canreasonably be assured by a bond index greater than 0.80 overa specific distance, which varies with casing size.Experimental work has provided a graph of the intervalrequired, as shown at the left.

The following examples illustrate the basicinterpretation concepts of cement bond logs. Notethat log presentations as clean and simple as thisare no longer available, but these are helpful inshowing the basic concepts.

EXAMPLE 2: CBL with both good and bad cement; hand calculated compressive strength shown by dottedlines, labeled in psi; SP from openhole log. Note straight line on travel time curve and bumps indicating casingcollars.



EXAMPLE 3: This log shows good bond over the oiland water zones, but poor cement over the gas zone,probably due to percolation of gas into the cementduring the curing process. The worm holes arealmost impossible to squeeze and this well may leakgas to surface through the annulus for life, becausethe bond is poor everywhere above the gas. Asqueeze job above the gas may shut off any potentialhazard.

EXAMPLE 4: Cement bond log before and after asuccessful cement squeeze. Even though modernlogs contain much more information than theseexamples, the basics have not changed for 40 years.

CEMENT BOND LOGS WITH VARIABLE DENSITY DISPLAY (CBL-VDL)

While the important results of a CBL are easily seen on a conventional CBL log display, such as signalamplitude, attenuation, bond index, and cement compressional strength, an additional display track isnormally provided. This is the variable density display (VDL) of the acoustic waveforms. They give a visualindication of free or bonded pipe (as do the previously mentioned curves) but also show the effects of fastformations, decentralized pipe, and other problems.

But you need really good eyes and a really good display to do this. The display is created by transformingthe sonic waveform at every depth level to a series of white-grey-black shades that represent the amplitudeof each peak and valley on the waveform. Zero amplitude is grey, negative amplitude is white, and positiveamplitude is black. Intermediate amplitudes are supposed to be intermediate shades of grey.

This seldom happens because the display is printed on black and white printers that do not recognize grey.Older logs were displayed to film that did not have a grey – only black or clear (white when printed). Soforget the grey scale and look for the patterns. Older logs were analog – the wavetrain was sent uphole as avarying voltage on the logging cable. These logs could not be re-displayed to improve visual effects. Modernlogs transmit and record digitized waveforms that can be processed or re-displayed to enhance theirappearance.



The examples below show the various situationsthat the VDL is supposed to elucidate. Theseexamples are taken from “New Developments inSonic Wavetrain Display and Analysis in CasedHoles”, H.D. Brown, V.E. Grijalva, L.L. Raymer,SPWLA 1970.

INTERPRETATION RULE 1: Low Amplitude = Good Cement



INTERPRETATION RULE 2: High Attenuation = Good CementINTERPRETATION RULE 3: High Bond Index = Good Cement

EXCEPT WHEN FAST FORMATION ARRIVALS APPEAR

EXAMPLE 5: CBL-VDL in free pipe (no cement). Notice straight line and high amplitude pattern on VDL pipearrivals (railroad track pattern). Travel Time curve is constant and amplitude curve reads high. Note casing

collar anomalies on travel time and amplitude curves, and more weakly on VDL display.

EXAMPLE 6: Casing is still unbonded (high amplitude railroad tracks on early arrivals on VDL), amplitudecurve reads high, BUT late arrivals on VDL have “shape” and track porosity log shape. This indicates free pipe



laying on side of borehole and touching formation. The VDL arrivals with “shape” are the formation arrivals.Better casing centralization should be used on the next well. A cement squeeze will improve the scene but will

probably not provide isolation on the low side of the pipe.

EXAMPLE 7: Well bonded pipe (low amplitude on early arrivals on VDL, good bond to formation (highamplitude late arrivals with “shape”). Mud arrivals would have high amplitude but no “shape”.

EXAMPLE 8: At Zone A, amplitude shows good bond, but VDL shows low amplitude formation signal. Thisindicates poor bond to formation. Travel time curve reads very high compared to baseline, indicating cycle

skipping on casing arrivals – but casing bond is still good. Travel time less than base line value would indicatefast formation. If you can detect fast formations, bond is still good, regardless of high early arrival amplitude.



EXAMPLE 9: VDL on left shows poor bond but formation signal is fairly strong. When casing is put underpressure, bond improves (not a whole lot) as seen on lower amplitude early arrivals on right hand log. This is

called a micro-annulus. Under normal oil production, the micro-annulus is not too big a problem unlessbottom hole pressure is very low. Micro-annulus is caused by dirty or coated pipe, pressuring casing before

cement is fully cured, or ridiculous pressures applied during stimulation.

EXAMPLE 10: When there is no CBL-VDL made under pressure, the un-pressured version can be used tointerpret micro-annulus. High amplitude early arrivals (normally indicating poor bond) actually indicate good

bond (with micro-annulus) IF formation signals are also strong.



EXAMPLE 11: The travel time curve is lower than baseline (shaded areas, Track 1) indicating fast formationarrivals. If you see fast formation, you have a good bond to pipe and to formation. However, you cannot use the

amplitude curve (labeled “Casing Bond” on this example) to calculate attenuation, compressive strength, orbond index, because the amplitude is measured on the formation arrivals, not the pipe arrivals.



EXAMPLE 12: CBL-VDL shows the transition from normal to foam cement just above 4650 feet. The foamcement has lower compressive strength so the amplitude curve shifts to the right. Notice the use of the

expanded amplitude scale (0 to 20 mv) to accentuate the change. The compressive strength is computed from adifferent algorithm than normal cement, shown in the nomograph in below.

Nomograph for calculating compressive strength in normal and foam cement. Note



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Cement Integrity Logs - Part 1