Carbon sense

Flow visualization in horizontal wells is being improved thanks to carbon composite rods. Elaine Maslin takes a look.

The yellow wireline mast suspends the top sheave and the pressure control equipment.  To gather DFO data, the Z-Line runs into the well through the pressure control equipment, at the top of which is the brass-colored bend restrictor funnel. Photos from Ziebel.

Running wireline production logging (PL) tools into horizontal wells has never been that easy. The tool strings with the PL sensors have to either be run on the end of coiled tubing or pulled along the horizontal sections with a well tractor, both methods having operational drawbacks (time and cost) as well as frequent difficulties in acquiring good quality, valid data sets.

Even when a wireline tool acquires good data, the sensor is only able to make a measurement at one depth point at any given time. Ziebel employs its composite carbon technology conveyance systems to make distributed temperature sensing (DTS) and distributed acoustic sensing (DAS) measurements simultaneously across the entire interval of interest in an easier and faster manner, with the high strength of carbon fiber reducing the risk of not being able to retrieve the fiber optics from the well.

The Norwegian firm’s new 4.8mm-diameter Z-Line system has seen its first commercial operational use on Statoil’s Huldra platform in the Norwegian North Sea earlier this year as part of a project to assess well conditions ahead of a plugging and abandonment (P&A) campaign next year. It was the first time Statoil has commissioned a distributed fiber optic (DFO) well intervention and it was also Ziebel’s first operation for Statoil.

The next step could be leaving a composite carbon rod or line downhole permanently in a well that is being P&A’d, says Neil Gardner, global sales and business development director for Ziebel.

Z-System

During the operation offshore Norway, the winchman used this panel to monitor the line depth, speed and tension, and the video monitor to verify that the spooling was running properly on the drum.  The panel pictured indicates that a line depth of 9684ft has been reached in the well.

Ziebel’s first DFO sensing system was the Z-System, which deploys fiber optic cables via a 15mm diameter semi-stiff carbon composite rod, using a unit very similar to a light coiled tubing (CT) unit. Once “parked” stationary in the well, the system acquires DTS and DAS along the full length of the rod. The concept for the technology was initially developed in the early 2000s, with a patent eventually being granted in 2006.

“The idea was patented to use a carbon composite rod with fiber optics inside to enter a well in order to measure parameters like temperature, pressure and others,” Gardner says.

Since then, a lot of work was done refining the product, particularly experimenting with different sizes and configurations of the rod, as well as the chemistry of the composite rod material, with the University of Aberdeen providing mathematical modeling for different composite behavior, so that it would be able to perform the tasks required of it and withstand downhole conditions.

“The idea was to be able to push the semi-stiff rod out into horizontal well sections,” Gardner says. “The horizontal well stock is increasing and will continue to do so. The difficulty with current technologies is that the sensors have to be either on the end of pipe or tractored in on a wireline. If you can do it with a carbon rod you can do away with the tractor or the coiled tubing.”

For distributed measurements, the system has to be stationary. Unlike traditional sensors, which have to be moved across the entire zone of interest in order to get a full data set, the Z-System senses along the whole length of the rod while stationary. “We can also manipulate parameters in a producing well to see how it behaves and responds, such as adjusting the choke at surface or increasing the gas injection rate, and then instantly start to see the impact of these adjustments across the whole well,” Gardner says.

Z-Line

Pictured on the deck is the wireline unit, as it spools the Z-Line out towards the well.  It runs under a lower sheave, up and over an upper sheave (not pictured) and down through the brass-colored bend restrictor funnel into the pressure control equipment, and then into the well.

However, Ziebel saw the opportunity to develop a smaller footprint system, which could be deployed on smaller platforms, such as in the southern North Sea, where a CT-like Z-System unit might not fit.

The result was the development of the Z-Line, a smaller 4.8mm-diameter carbon composite line, with a 6600lb/3000kg breaking strength, containing up to six fiber optic cables encased in a central steel tube. It comes mounted on a 1m-diameter drum, which can run on a standard wireline type set-up, and includes point pressure and temperature sensors on the bottom hole assembly. “It is smaller and more modular, which opens up many more wells to be investigated using fiber optics,” Gardner says. “Because it is 4.8mm, it is much more flexible and can be deployed like a mechanical wireline or slick-line.”

The system does preclude use on horizontal wells because you cannot push it in – it requires sinker bars and gravity to move it down into the well. But, it will go into relatively high-deviation wells, Gardner says – up to 75-80° deviation, depending on the well geometry, according to the company’s calculations.

Z-Lines could be built as long as required by well depths, but the main application of interest (well integrity surveys) are in the shallower vertical section of most wells. The length is only limited by the space available on the drum and the ability to properly control the manufacturing process, Gardner says. The firm has built a 4000m-long version and it is aiming to build the next systems to at least 6000m length. The 3-4-tonne drum of Z-Line is shipped offshore inside an add-on-drum skid, with two sheave wheels, a Ziebel sensing cabin, and the dynamic seal pressure control section.

Working from the Huldra platform, Ziebel ran the Z-Line in two wellbores to measure temperature and acoustic profiles. Work was carried out by Ziebel over two, three-day periods in April and May 2015. Each operation was completed as planned, providing data that displays the conditions of each well in its entirety, during each intervention. It allowed Statoil to observe fluid movements, confirm the integrity of the wells, and plan the final abandonment, enabling the optimization of the forthcoming P&A campaign.

“We were looking for possible unwanted fluid migrations, particular in the annular space outside the main bore,” Gardner says. “We have the ability to detect and visualize fluid traveling up or down versus time outside the central tubing, through one, two, and maybe three strings of pipe in the well. If you are trying to design a P&A program you need to know what is moving down hole. If there are undesirable fluid movements we can detect them and show the operator which sections in the well need sealing to prevent fluids migrating and pressure build-ups.”

More frequent monitoring?

Rather than permanent monitoring requiring fibers to be installed in every well, an operator can choose to run frequent Z-Line runs in wells with this intervention methodology. In this future vision the Z-Line would be kept permanently on board the platform, Gardner says. “Z-Line is light and can run in a well with a wireline unit. You would just have to drop in the drum, which is compatible with standard wireline units,” he says. The sheave wheels, the instrumentation and some other ancillary equipment is also needed, but the rest is standard, Gardner says.

Can subsea wells be accessed?

To use the system on a subsea well, a rig and a high-pressure riser are required. But, in the future, it could be done from light weight intervention vessels, Gardner says. A development would be to run pressure control equipment to the seabed, which the line can go to through open water from the boat.

“There’s quite a bit of interest in the bigger 15mm rod system for subsea applications, because it is stronger than a wireline,” Gardner says. “Today, if you do light well intervention and you are running wireline tools into a long subsea well, there’s a high degree of risk because if your tool string gets stuck or the well tractor fails, you have limited overpull strength in your wireline to pull it free. The only option is to electrically release the tool string from the cable and accept to have a large blockage negatively impacting production in the well until a rig can be brought in at a later date to retrieve it.”

Ziebel is further developing the Z-System by embedding electrical conductors within their carbon rods to enable reach into extreme horizontal sections by connecting a well tractor to the end of the rod. This development will also provide the capability to run electrical well service tools, such as the new milling or cleaning tools available today, all of which would be additional to an undiminished ability to provide the DFO measurements, Gardner says.