What is FLI?

FiberLine Intervention (FLI) is a wellbore surveying
technique consisting of three main elements:

Each of these are discussed in detail below. If you have further questions, please have a look
at our FAQs page or contact us for more information.

FLI Probe

Our FLI probe is a single-use device, containing the spooled fibre-optic line and optional electronic sensors.

FLI Probe

FLI probes are designed to be light, compact and robust, enabling easy handling at the wellsite and reliable deployment in the well.

They are light, typically weighing less than 5 kg, and compact, typically being less than 1.5m in length, and can easily be handled by one person. As standard, the probes are made using aluminium alloy and incorporate a number of features designed to protect the optical fibre as it unspools during free-fall into the well.

The FLI probe contains one or more spools of bare optical fibre up to 25,000ft (7,715 m) long.

The probe is a simple device, but with some special features. It houses one or more custom designed bobbins, on each of which we wind a bare optical fibre – i.e., the glass is protected by nothing other than its polymer coating. Our bobbin winding technique is proprietary, designed to ensure that long optical fibres can be unspooled reliably at high speeds.

The length of fibre that can be unspooled from a FLI probe depends upon its size, but typically ranges from 3800m to 7,715m, but this is not a hard upper limit. Greater fibre lengths are easily achievable, if required, with larger probes.

We can supply Active-FLI probes, fitted with electronic sensors and optical telemetry.

FLI is primarily designed to be a distributed sensing system, in both space and time – observing the wellbore simultaneously along the entire length of the unspooled fibre. Nevertheless, it is sometimes useful to enhance the distributed sensing with single point sensors, and even sources, embedded in the FLI probe itself. We refer to these as Active-FLI.

As just one example, we can supply FLI probes fitted with short-term pressure/temperature gauges, interrogated using our proprietary optical telemetry system.

About FLI

Optical Fibre Sensors

As the FLI probe falls it unspools bare optical fibre along the path of the wellbore, which acts as a distributed sensor. The fibre is intended to be left in the well at the end of the survey, with no impact upon subsequent operations.

Optical Fibre Sensors

FLI fibre is extremely sensitive to changes in its environment, especially of temperature and acoustic energy.

FLI surveys typically involve distributed temperature sensing (DTS) or distributed acoustic sensing (DAS), sometimes used in combination. The bare fibres unspooled from the FLI probe are extremely sensitive, especially when compared to fibres encased in steel tubes. The lightweight fibres adhere to the tubing or casing wall in both gas and liquid environments. We use bare multimode fibres for DTS and single mode fibres for DAS, with each fibre type being best suited to its particular method of sensing.

Examples of temperature surveys include cement job monitoring, where we track the progress of the exothermic reaction as cement cures, and ‘warm-back’ analysis of water injectors. Acoustic surveys include leak detection jobs (especially those in which the pressurised tubing or annulus is bled-down to stimulate the suspected leak), cross-well strain measurements and vertical seismic profiles. Nevertheless, FLI is at its most powerful when temperature and acoustic surveying are used simultaneously.

Bare optical fibre takes up very little space and is intended to be left in the well at the end of the survey.

Our standard coated fibre has a diameter of just 0.25 mm. It is intended to be functional only for long enough to complete a survey, after which time it serves no further purpose. Leaving this fibre in the well has been proven to have no adverse impact on subsequent well operations.

About FLI

Surface Acquisition System

FLI surveys require a very small surface footprint, both for deploying the FLI probe and acquiring the data.

Surface Acquisition System

Wherever possible, we provide you with an answer as soon as the job is complete.

One purpose of our acquisition system is to process the data as soon as we acquire it. Whenever we can we’ll give you an initial answer at the wellsite, or as soon as possible after finishing the survey. We can do this best in cases where we know what you want us to look for – for example, the location of a leak. The experience that we have gained from numerous field surveys means that we can focus in on only the particular datasets that are most likely to be informative.

We make sure that the data from the survey is suitable for subsequent analysis.

We often perform FLI surveys to give you answers to specific questions – eg, what is the location of a suspected leak? However FLI data is rich in information, and may contain answers to questions you haven’t yet asked.

Especially when used as part of multiple well campaigns, you may observe events in some wells and wonder whether similar events have been seen in other wells. Part of the reason that distributed sensing is so powerful is that it provides a comprehensive record of a survey, in both space and time.

We can hand over your data for you to store and analyse as and when you see fit. Alternatively, we can store your data for you and, if and when required, reprocess it on your behalf.

About FLI

What makes FLI special?

FLI is built around three key attributes: FLI surveys are fast,
FLI jobs have a tiny footprint and FLI datasets are rich in information.

FLI is fast. Really fast.

A typical FLI job takes only a few hours from rig-up to rig-down, which translates into substantial rig-time savings.

FLI is fast. Really fast.

Rig-up and run-in times are short.

Running FLI is very quick compared to conventional well intervention methods. The surface rig-up is minimal and, once launched, the FLI probe is typically on bottom within 10-30 minutes, depending upon well depth and fluids.

Distributed surveys are much faster than multi-station surveys.

As FLI measurements are distributed along the length of the fibre, rather than single-point, survey times are short. We do not have to make multiple stationary stops to ensure that a sensor is in close proximity to an event. The FLI fibre is right next to all of the events, all of the time. We have completed leak detection jobs in a few hours that took more than a day to perform on wireline. We have case studies in which the actual “logging” part of the operation was completed in less than an hour!

At the end of the survey we simply cut the fibre and rig down.

The FLI probe is designed to be used once and can be disposed of in the well. When the survey is complete, we cut the fibre and rig down our launcher. That’s it!

FLI has a tiny footprint.

FLI uses little space and few services at the well site.

FLI has a tiny footprint.

The FLI rig-up is very small, making it suitable for offline operations.

Compared to conventional well intervention work, the FLI rig-up is very compact. The rig-up is sufficiently small that FLI jobs can often be performed off the critical path – ie, while the rig is doing other work – incurring no rig-time whatsoever.

FLI jobs only require one person at the wellsite.

We typically require only a single wellsite engineer. With current travel restrictions and bed space being at a premium in the wake of the Covid-19 epidemic, using FLI is good for minimising the number of personnel travelling to the wellsite.

Using FLI can help to reduce carbon footprint.

For some surveys we can train local providers to run FLI, while we monitor the operation remotely. This means that we don’t need to send any of our people to the wellsite. Together with the small amount of equipment typically required, FLI surveys can contribute towards environmental targets on lowering carbon footprint.

FLI datasets are rich in information.

Speed and simplicity do not come at the expense of capability.

FLI datasets are rich in information.

From the moment the FLI probe is launched it provides information.

The free-fall descent itself is informative, especially about fluid levels and densities. We can compare the transit of the FLI probe to our pre-job simulations, monitoring descent velocities to determine fluid interfaces. If we incorporate a pressure gauge in the FLI probe itself, we can provide pressure gradients as well as the shut-in bottom-hole pressure and temperature.

FLI is a powerful distributed surveying tool.

The FLI probe deploys one or more bare optical fibres along the path of the wellbore. Using distributed acoustic and/or temperature sensing, FLI can take continuous and simultaneous measurements along the length of the fibre.

FLI provides answers immediately AND long after the survey is finished.

We provide answers at the wellsite or, depending upon the survey, shortly after the job is over. More importantly FLI captures a comprehensive dataset from each job. If questions occur after the survey has finished, even months later, we can interrogate the FLI dataset to help answer those questions. This is especially valuable in campaign work, where the accumulation of FLI records and other measurements taken during well operations becomes a powerful source of insight.


FiberLine Intervention is a stand-alone, independent intervention and surveying system which does not rely on 3rd party equipment for deployment, other than a lightweight tool lift to help guide it into the wellhead.

The probe and launch assembly, including pressure control equipment, together weigh less than around 20kg. The complete system with everything you need to survey your well will fit into the back of a truck or can be packed into a half height 8ft shipping container, minimising deck space usage.

FLI is suitable for offline operations without incurring rig-time.

The FLI rig-up is fast and simple and the footprint is tiny. The ease of moving from well-to-well makes FLI ideal for batch operations, enabling multiple wells to be surveyed in a short period of time.

Rig-up of the system is simplified through the use of a plug and play connection that is installed on top of each FLI probe. Connection to the pressure-controlled release assembly requires removal of a retaining nut, insertion of the probe, which has a standard fish neck profile, into the launcher and re-fitting the nut. The entire assembly is then ready to be lifted and installed onto the well, with a connection to a standard lubricator section and crossover.

FLI enables precise depth control and rapid data.

We connect our surface light source and data recorder to the optical fibre in the FLI probe through a pressure control fitting in the launcher, which is fixed at the wellhead. This means we know the exact start and end point of the fibre, giving us precise depth control.

Standard procedures for pressure test and opening of the well are completed ahead of deployment. The FLI probe is launched by stroking a hydraulic piston in the release assembly using a hand pump, which activates the release mechanism.

Data is captured both during the probe’s descent into the well and during the time the probe is stationary at total depth. Find out more about these types of surveys here.

FLI reduces risk and increases efficiency.

At the end of the survey, rig-down is almost immediate. We simply cut the fibre and remove the launcher from the wellhead. For most projects we leave the aluminium probe and fibre in the well. Due to their lightweight composition, and the fibre’s exceedingly small volume, they do not limit any future well operations or present any risk of extended fishing.

Compared with traditional well surveying and intervention methods, the FLI system provides significant savings in site footprint, personnel, logistics, downtime and associated costs. Reduction of personnel on site is a significant HSE advantage in these challenging times.

View our Rig-Up Demo Video

View our Technology Concept Animation

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