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.

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.

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