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Publication History:
This article is based on
"Crain's Analyzing Cement Integrity" by E. R. Crain, P.Eng.,
2002, updated annually through 2016.
This
webpage version is the copyrighted intellectual
property of the author.
Do not copy or distribute in any form without explicit
permission. |
CEMENTING
BASICS
Cement
bond logs were run as early as 1958 with early sonic logs and the
temperature log was used to find cement top beginning in 1933.
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.
Cement Integrity Log
Basics
Today’s
cement integrity logs come in four flavours: cement bond logs
(CBL), cement mapping logs (CMT), ultrasonic cement mapping tools
(CET), and ultrasonic imaging logs (USI, RBT). 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 1958 and their use for cement
integrity was quantified in 1962. 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.
In
the 1970’s, the segmented bond tool appeared. 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.
Temperature Logs for Cement Top
In
the “good old days” before the invention of sonic
logs, there was no genuine cement integrity log. 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 (CBL)
Cement
bond logs (CBL) are still run today because they are relatively
inexpensive and almost every wireline company has a version of
the tool. The log example at the right illustrates the use of
the acoustic amplitude curve to indicate cement bond integrity.
The
examples in this Section are taken from ”Cement Bond Log
Interpretation of Cement and Casing Variables”, G.H. Pardue,
R.L. Morris, L.H. Gallwitzer, Schlumberger 1962.
EXAMPLE 1: CBL in well bonded cement – low amplitude means good
bond. The SP is from an openhole log; a gamma ray curve is more
common. Most logs run today have additional computed curves, as
well as a VDL display of the acoustic waveforms.
The CBL uses conventional sonic log principals of refraction to
make its measurements. The sound travels from the transmitter,
through the mud, and refracts along the casing-mud interface and
refracts back to the receivers, as shown in the illustration on
the left. In fast formations (faster than the casing), the signal
travels up the cement-formation interface, and arrives at the
receiver before the casing refraction.
The
amplitude is recorded on the log in millivolts, or as attenuation
in decibels/foot (db/ft), or as bond index, or any two or three
of these. A travel time curve is also presented. It is used as
a quality control curve. A straight line indicates no cycle skips
or formation arrivals, so the amplitude value is reliable. Skips
may indicate poor tool centralization or poor choice for the trigger
threshold.
The
actual value measured is the signal amplitude in millivolts. Attenuation
is calculated by the service company based on its tool design,
casing diameter, and transmitter to receiver spacing. Compressive
strength of the cement is derived from the attenuation with a
correction for casing thickness. 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
as normal output curves. Bond Index is a qualitative indicator
of channels. A Bond Index of 0.30 suggests that only about 30%
of the annulus is filled with good cement.
INTERPRETATION
RULE 1: Low Amplitude = Good Cement
INTERPRETATION RULE 2: High Attenuation = Good Cement
INTERPRETATION RULE 3: High Bond Index = Good Cement
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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. In oil
wells, we want to exclude gas and water; in gas wells, we want
to exclude water production. We also do not want to lose valuable
resources by crossflow behind casing. Isolation can reasonably
be assured by a bond index greater than 0.80 over a specific distance,
which varies with casing size. Experimental work has provided
a graph of the interval required, as shown at the left.
The
following examples illustrate the basic interpretation concepts
of cement bond logs. Note that log presentations as clean and
simple as this are no longer available, but these are helpful
in showing the basic concepts.
EXAMPLE 2: CBL with both good and bad cement; hand calculated compressive
strength shown by dotted lines, labeled in psi; SP from openhole
log. Note straight line on travel time curve and bumps indicating
casing collars.
EXAMPLE
3: This log shows good bond over the oil and water zones,
but poor cement over the gas zone, probably due to percolation
of gas into the cement during the curing process. The worm holes
are almost impossible to squeeze and this well may leak gas to
surface through the annulus for life, because the bond is poor
everywhere above the gas. A squeeze job above the gas may shut
off any potential hazard.
EXAMPLE
4: Cement bond log before and after a successful cement squeeze.
Even though modern logs contain much more information than these
examples, 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 signal amplitude, attenuation, bond index,
and cement compressional strength, an additional display track
is normally provided. This is the variable density display (VDL)
of the acoustic waveforms. They give a visual indication of free
or bonded pipe (as do the previously mentioned curves) but also
show the effects of fast formations, decentralized pipe, and other
problems.
But
you need really good eyes and a really good display to do this.
The display is created by transforming the sonic waveform at every
depth level to a series of white-grey-black shades that represent
the amplitude of each peak and valley on the waveform. Zero amplitude
is grey, negative amplitude is white, and positive amplitude 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). So forget the grey scale and look for the patterns.
Older logs were analog – the wavetrain was sent uphole as
a varying voltage on the logging cable. These logs could not be
re-displayed to improve visual effects. Modern logs transmit and
record digitized waveforms that can be processed or re-displayed
to enhance their appearance.
The
examples below show the various situations that the VDL is supposed
to elucidate. These examples are taken from “New Developments
in Sonic Wavetrain Display and Analysis in Cased Holes”,
H.D. Brown, V.E. Grijalva, L.L. Raymer, SPWLA 1970.
INTERPRETATION
RULE 1: Low Amplitude = Good Cement
INTERPRETATION RULE 2: High Attenuation = Good Cement
INTERPRETATION RULE 3: High Bond Index = Good Cement
EXCEPT WHEN FAST FORMATION ARRIVALS APPEAR
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EXAMPLE 5: CBL-VDL in free pipe (no cement). Notice straight
line and high amplitude pattern on VDL pipe arrivals (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), amplitude curve 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 (high amplitude 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. This indicates 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
indicate fast 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 under pressure, 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 unless
bottom 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 to interpret 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 formation arrivals. 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, or bond 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 foam cement 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 a different
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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