FAQs
Q: What are FLI tools made from?
Q: What happens to a FLI tool at the end of a job in a completed well?
Q: What happens to a FLI tool at the end of a job in a well being drilled?
Q: How long does the fibre survive in the well?
Q: How reliable is depth control when using FLI?
Q: How fast does a FLI tool fall during deployment?
Q: What is the maximum well deviation that can be serviced using FLI?
Q: What are FLI tools made from?
FLI tools are made using aluminium housings. After extensive customer engagement and experimentation, we have selected 2011/T2 series (UNS A92011) aluminium alloys as our standard housing material. Aluminium is strong, lightweight and, through its widespread use, has predictable properties.
Dissolvable Alloys
At the early stages of the FLI development we experimented with making the tool body from a dissolvable alloy, so that the tool would fully dissolve downhole in a controlled manner.
As things stand today, dissolvable alloys are expensive materials to use and require careful handling. For example, a dissolvable-alloy FLI tool needs to be stored in a controlled environment – eg, so it does not start to dissolve prematurely in a wet or humid offshore environment.
Furthermore, the operational procedures become complicated at the wellsite. Should pressure-testing delays occur once the FLI tool is installed in the lubricator, the FLI tool may start to dissolve prematurely. Also, once deployed, if the downhole fluids and temperatures are different to those expected, the FLI tool may dissolve earlier than intended. Once a tool starts to dissolve it rapidly loses its strength and structure, making it very difficult or impossible to fish.
In summary, although dissolvable alloys may have applications in the future, as of now, the practical difficulties outweigh the benefits in most real-world applications.
Q: What happens to a FLI tool at the end of a job in a completed well?
In the vast majority of jobs we plan to leave the FLI tool at the bottom of the well, out of the way. FLI tools are short – typically less than 2 m in length – so they take up very little space in the rathole.
What happens if a FLI tool doesn’t get to its intended depth?
First of all, this is unlikely to happen. We plan FLI jobs carefully, working closely with you, to make sure we understand what you want to achieve. We take into account the anticipated well conditions – eg, fluid types, restrictions, deviations and likelihood of debris. If we don’t think that FLI is the right solution for your particular application, we’ll tell you.
Nevertheless we recognise that, especially in old wells, there may be unforeseen issues. All FLI tools have a standard size of external fishing neck, designed to be latched by an SB-type pulling tool, which allows the FLI tool to be recovered on slickline. We have done a number of jobs where the FLI tool was planned to be fished from the well, and we have never had a FLI tool that has not been fished successfully.
Alternatively, the FLI tool housing is made from a short, thin aluminium tube, which can be easily drilled or dissolved by spotting acid.
Q: What happens to a FLI tool at the end of a job in a well being drilled?
We have carried out many intermediate-hole FLI surveys, including cement-cure evaluations and vertical seismic profiles. At the end of the job, the FLI tool is simply left at the bottom of the hole to be destroyed when drilling the next section.
How easy is it to drill out a FLI tool?
It’s very easy! In every case to date the FLI tool has been drilled without issue and without modification to the drilling programme. In fact, given that the main housing of a FLI tool is a short, thin aluminium tube, you are unlikely to even notice it.
Q: How long does the fibre survive in the well?
We try to make sure that the fibre lasts long enough for your intended purpose, and no more. Typically that means you will need the fibre to survive from a few hours to a few days, depending upon the application.
For most surveys we use a 'standard grade' optical fibre, which is coated with an acrylate material, to minimise the cost of the FLI tool. Once the job has finished, and the data has been recovered, the fibre serves no further purpose as a sensor and is simply disposed of in the well.
Sometimes you might need FLI to work in extreme environments, or in contact with corrosive fluids, or for extended time periods. In such cases we pick the specification of the fibre to suit, usually by changing the fibre coating. However, so-called specialty fibres can be expensive, and we strongly recommend working with you to see if a specialty fibre is really required.
Q: How reliable is depth control when using FLI?
FLI can provide you with highly reliable depths. We have demonstrated repeatedly that FLI can identify the position of leaks, for example, with great precision.
We are able to provide precise depth control through a combination of the following factors:
- the FLI fibre is fixed at the wellhead
- the unspooled fibre closely follows the path of the well
- the design of the FLI tool itself greatly improves optical range-finding.
If you’d like to find out more about depth control with FLI, or about how we’ve used that in real-world applications, please get in touch.
The FLI fibre is fixed at the wellhead
We connect to the optical fibre through a pressure control fitting in the FLI launcher, which is fixed at the wellhead. If you tell us the distance from the launcher to your preferred datum – eg, rotary table – we’ll report depths below that datum.
If being fixed at the wellhead sounds like a trivial point, it’s not! With FLI, all of our fibre is inside the well at all times, either spooled on the bobbin in the FLI tool or unspooled along the well path. More importantly, all of our unspooled fibre is stationary in the well.
Using FLI is very different to using drum-spooled optical cables, in which the position of the wellhead and the length of fibre in the well can be difficult to establish accurately.
The unspooled FLI fibre closely follows the path of the well
The optical fibre is extremely thin and light, and does not stretch at all. Furthermore, as the FLI tool descends, we apply a small tension to the unspooled fibre. This helps us ensure that the fibre lays along the inside of the tubing or casing string. Also, we apply a grease to the fibre on the bobbin that helps to 'stick' the fibre to the tubing wall.
The upshot of all this is that the length of the unspooled fibre is closely tied to the distance along the wellbore, and that the fibre is 'coupled' to the tubing string, which can be particularly important for acoustic surveys.
The design of the FLI tool improves optical range-finding
Optical range-finding methods are well established to determine the distance of events along a fibre. Optical time-domain reflectometry is just one example of a range-finding method. We can use range-finding to establish the point at which an unspooled FLI fibre becomes spooled on the bobbin by measuring the light loss along the length of the fibre. This allows us to measure the depth to the top of the FLI tool when it has reached the bottom of the well. However, the design of the FLI tool itself enables us to use methods that greatly improve upon the accuracy of conventional range-finding:
1. We know the precise length of each layer of fibre wound on the bobbin
We wind the fibre on to a bobbin inside the FLI tool in layers with a precisely determined length. During a job we can 'see' the layers of fibre wound on the bobbin. This means that we can count the layers left on the spool and convert that count into an accurate length of fibre that has exited the FLI tool. In effect the layers act as markers along the fibre, without needing to resort to expensive doping techniques to fix chemical markers into the glass itself
2. The FLI tool is a highly sensitive microphone that can be found acoustically
Spooled fibre on the bobbin acts as an extremely sensitive microphone. During an acoustic survey we can detect the obvious difference in sensitivity between the spooled and unspooled fibre, from which we can then determine the location of the FLI tool.
FLI is fixed at the wellhead, but drum-spooled cables are not
With a drum-spooled optical cable, a substantial portion (perhaps even the majority) of the cable is on the drum at surface. Furthermore, as the cable unspools from the drum it is moving relative to the wellhead. This means that working out how much of the unspooled cable is in the well at any time is not trivial. People often have to rely on causing “events” at the wellhead to work out its position along the optical cable. Typical examples of such events are placing ice bags at the wellhead to cause a sharp temperature drop or tapping on the wellhead to cause a sharp acoustic response.
To complicate matters further, the length of fibre inside a drum-spooled cable is not the same length as the cable itself. Optical fibre has almost no stretch before it snaps; metal wire can stretch considerably under its own weight as it gets deeper in the well. This issue is overcome by installing “slack” in the fibre. As the fibre is mobile inside the cable, the amount of slack and its location is an additional error that has to be corrected for.
Q: How fast does a FLI tool fall during deployment?
Typical FLI tool speeds are around 8–15 m/s when falling in gas and 1–2 m/s in liquid.
Of course, there are a wide variety of variables that can affect the tool speeds. This is why we plan each job carefully, using a proprietary FLI tool drag simulator. Our simulator has been calibrated against many real-world jobs, and has been shown to predict reliable descent times, as well as helping to identify potential problems and mitigations.
For more information about our approach to job planning, please contact us.
The design of the FLI tool improves optical range-finding
Optical range-finding methods are well established to determine the distance of events along a fibre. Optical time-domain reflectometry is just one example of a range-finding method. We can use range-finding to establish the point at which an unspooled FLI fibre becomes spooled on the bobbin by measuring the light loss along the length of the fibre. This allows us to measure the depth to the top of the FLI tool when it has reached the bottom of the well. However, the design of the FLI tool itself enables us to use methods that greatly improve upon the accuracy of conventional range-finding:
1. We know the precise length of each layer of fibre wound on the bobbin
We wind the fibre on to a bobbin inside the FLI tool in layers with a precisely determined length. During a job we can 'see' the layers of fibre wound on the bobbin. This means that we can count the layers left on the spool and convert that count into an accurate length of fibre that has exited the FLI tool. In effect the layers act as markers along the fibre, without needing to resort to expensive doping techniques to fix chemical markers into the glass itself
2. The FLI tool is a highly sensitive microphone that can be found acoustically
Spooled fibre on the bobbin acts as an extremely sensitive microphone. During an acoustic survey we can detect the obvious difference in sensitivity between the spooled and unspooled fibre, from which we can then determine the location of the FLI tool.
Q: What is the maximum well deviation that can be serviced using FLI?
In free-fall operations, which make up the vast majority of FLI jobs, we have deviation limits similar to those for conventional wireline surveys – ie, around 60°–70°. However, the distribution of forces affecting FLI jobs is very different to wireline jobs: the FLI tool itself has little weight, being made from aluminium, but there is no cable drag. Consequently, we try to minimise the friction of the FLI tool against the pipe wall in order to keep the FLI tool moving.
Using FLI in horizontal wells
We have successfully pumped FLI tools into horizontal wells, with the longest job to date being a horizontal section of ~8,000ft. FLI tools are lightweight and can be easily pumped out to the rathole through the toe sleeve. We have had multiple tools pumped in this way, with no problems or impact on subsequent operations.