The goal for well integrity testing is to make sure that the well has solid construction (for example, mechanical components are functioning properly) and that leakage is not occurring or likely to occur in the future. Leaks can pose environmental risks and harm to the surrounding areas, such as contamination of underground aquifers that may be tapped for municipal water purposes.
Engineers have a large selection of tests to examine the integrity of the well. Some tests are used for particular circumstances, and some are fairly universal in their application. Let’s take a look at a few of the more common tests used.
Adapted from Richard Leeth, Photo Courtesy of the Kansas Department of Health and Environment
Cement (small dot pattern) between the rock formation (pebble pattern) and the casing pipe (white area inside the cemented region) prevents fluids from migrating up the outside of the casing, and also isolates the wellbore from surrounding formations.
Cement Bond Log
A good cement job is key to the safe and environmentally sound operation of a well. Cement supports the casing, prevents fluids from migrating up the outside of the casing and leaking to the surface, and also isolates the wellbore from surrounding formations. A poor cement job can create channels along the pipe or behind the cement as shown below.1Leeth, R. (2015, September 2). Properly run and interpreted, cement-bond logs (CBL) provide highly reliable estimates of well integrity and zone isolation. Kansas Department of Health and Environment Geology & Well Technology Section Fall 2015 Well Logging Workshop. https://www.kdheks.gov/geo/download/seminar_2015/5%20Workshop%20-%20Leeth%20-%20cement-bond%20logs.pdf
A cement bond log is constructed from acoustic wireline tool data. The tool is lowered into the well and transmits and receives acoustic signals using multiple transmitters and receivers in a variety of arrangements.
Cement bond logs are used to check the integrity of the cement job and ensure enough cement was used to fill the annular space and that the cement is adhering to the casing. Acoustic logs help evaluate the mechanical integrity and quality of the cement bond in the well.2Leeth, R. (2015, September 2). Properly run and interpreted, cement-bond logs (CBL) provide highly reliable estimates of well integrity and zone isolation. Kansas Department of Health and Environment Geology & Well Technology Section Fall 2015 Well Logging Workshop. https://www.kdheks.gov/geo/download/seminar_2015/5%20Workshop%20-%20Leeth%20-%20cement-bond%20logs.pdf3Cement bond logs. (2015, June 25). In PetroWiki. https://petrowiki.spe.org/Cement_bond_logs
When the acoustic wave generated by the transmitter reaches the casing wall, part of the signal is refracted down the casing, part travels through the drilling mud at the edge of the borehole and the rest is refracted in the annulus and within the formation, received back and recorded.4Cement bond logs. (2015, June 25). In PetroWiki. https://petrowiki.spe.org/Cement_bond_logs Before we look at a cement bond log record, let’s look at the range of quality of a casing cement job.
A casing cement job can result in the following situations5Cement bond logs. (2015, June 25). In PetroWiki. https://petrowiki.spe.org/Cement_bond_logs:
- free pipe,
- poorly cemented (bond to casing only),
- partial bond, or
- good bond
Now let’s see what kind of cement bond log signal corresponds to these situations. To start with, let’s simplify the cement bond log and the acoustic tool by making an analogy to striking a bell. If the bell is hanging freely (the free pipe and poorly cemented case), it will resonate, but if the bell has someone’s hands or a cloth wrapped around it (good or partial bond with the rock formation), the energy from the hammer is dispersed into the surrounding materials. The cement works like those hands to dull the signal response.
Watch this video of free ringing on a cymbal:
Video: Jeffrey Olson
Watch this video of dampened ringing on a cymbal:
Video: Jeffrey Olson
Now, let’s look at how to use the acoustic tool to test the cement job quality. A typical cement bond log presentation includes a number of curves, but here we are going to limit ourselves to one. Amplitude is the primary bond measurement and is used to provide a qualitative interpretation of the cement-to-formation interface and other information. The expression of our cow bell analogy is that the acoustic energy is rapidly lost in regions where cement bond quality is good between the casing and the formation (low amplitude) and acoustic energy is not dispersed in partial bonding or free pipe cases (high amplitude).6Cement bond logs. (2015, June 25). In PetroWiki. https://petrowiki.spe.org/Cement_bond_logs
Ring that bell…
Free Pipe: no cement bond between the casing and cement
With a free pipe (no cement between the casing and the rock formation), there is no acoustic coupling with the formation and the signals are not dispersed into the surrounding rock. Most of the transmitted acoustic energy is confined to the casing and the borehole fluid, and as a result, a free-pipe acoustic signal is high-amplitude.7Cement bond logs. (2015, June 25). In PetroWiki. https://petrowiki.spe.org/Cement_bond_logs
I can still hear the bell…
Poorly cemented: bond to casing only
I can barely hear that bell…
Partial Bond
I can’t hear that bell…
Good Bond: cement is properly bonded to casing and to the formation
If the casing is well cemented, the signals are absorbed by cement and dispersed into the surrounding rock formations. By the time the signal reaches the sensor, it has lost most of its energy, and the amplitude is low.8Cement bond logs. (2015, June 25). In PetroWiki. https://petrowiki.spe.org/Cement_bond_logs
Engineering Analysis and Interpretation
Now that you know some categories to describe the cement bond quality, and you have an idea of what corresponding amplitude you might see on a cement bond log, let’s do some analysis and interpretation! Below is a cement bond log showing you an amplitude track and five different intervals to interpret. Select the best interpretation for each interval based on your knowledge.
Photo Courtesy of Richard Leeth, Kansas Department of Health and Environment
Amplitude curve from a cement bond log with five different intervals labeled A-E.
Look at the amplitude record for Intervals A and D. How would you interpret the cement job in those two intervals of the well?
Look at the amplitude record for Interval B. How would you interpret the cement job in this interval of the well?
Look at the amplitude record for Interval C. How would you interpret the cement job in this interval of the well?
Look at the amplitude record for Interval E. How would you interpret the cement job in this interval of the well?
Now that you’ve made an effort to analyze the cement bond log, let’s take a look at the log with an engineering interpretation on it.
Once the wellbore is in production, engineers continually use routine surveillance and maintenance procedures to maintain integrity given corrosion conditions can occur over time and use. These procedures must be followed throughout the life of the project, and at the end of the when the aging well is no longer economically sustainable and must be decommissioned and capped. Such monitoring tools include probes, measuring the quantity of water production from the well, tests for concentrations of chlorides, total dissolved solids, and other compounds.