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
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
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)
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
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.
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.
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.
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