Coiled tubing is a metal pipe used to perform various well completion, drilling, and intervention operations. This metal pipe is flexible enough to be stored on the reel.
Coiled tubing string comes in various sizes ranging from 1” to almost 4” in diameter in some cases.
Most coiled strings are made up of low carbon steel and can come in different grades (yield strengths).
The coiled tubing rig has similarities to both wireline units and workover rigs.
The main difference between coiled tubing and wireline is that wireline is just a flexible metal cable while coiled tubing is an actual pipe.
You can pump fluid and chemicals through the coiled tubing while you can not do that with a wireline.
Service rigs use jointed pipe which means that while running in hole or out they have to stop every 10 meters or so to attach another section of the pipe.
Related: What Coiled Tubing Engineers and Technical Specialists Do?
What are the Main Advantages of Coiled Tubing?
- There is no need to stop while running in a hole or pulling to the surface because coiled tubing is one continuous metal pipe.
- This makes coiled tubing operations safer and usually more time-efficient.
- This also allows coil tubing to be run on live wells without a need to kill the well.
- This can save money and prevent potential formation damage.
Typical Coiled Tubing Surface Equipment
1. Control cab (coil cab)
The control cab is the unit from which the coiled tubing operation is run.
This is the place where the coiled tubing operator and supervisor are sitting and controlling the entire operation with hydraulic or electronic controls.
You can think of the coil cab as a data van for coiled tubing operations.
Newer coil cabs are usually bigger, have touchscreen controls and satellite dishes to transmit real-time job data.
2. Reel trailer
The reel trailer is used to transport the coiled tubing reel.
Smaller coil units might have both a control cab and a reel on the same trailer.
Depending on the road conditions and size of the reel, sometimes third-party trucking companies are used to transport the coiled tubing reel to and from the location.
3. Support trailer
Used for transporting injectors, pumping iron and blowout preventers (BOPs)
4. Fluid pumps
Almost every coiled tubing operation requires fluid. Fluid pumps are used to pump fluid down the coil and into the well. The most common fluid used in coiled tubing operations is water. Fluid pumps are usually connected to 400 bbl tanks where water used for the operation is stored. Most pumps allow pumping chemicals on the fly or premix in onboard tanks.
Depending on the pump rates and pressure, sometimes more than one fluid pump might be required for the operation. Fluid pumps can also be used to pump acid and sometimes solvents.
5. Chemical trailer
Used if large volumes of chemicals are required for the job.
Some chemical trailers have a heating system to prevent chemicals from freezing in the winter.
6. Nitrogen pumper
Some coiled tubing operations require continuous nitrogen pumping.
Even if nitrogen is not required for the job, it is usually used at the end of the operation just before the rig out to purge the coil (remove fluid from the coil).
7. Crane
Crane is used for holding the injector in place during the coiled tubing operations with conventional units.
Coiled tubing mast units have specifically designed lifts for holding the injector.
The required crane rating depends on wellsite geometry and injector size.
8. Coiled Tubing Injector
Another important piece of coiled tubing equipment is an injector.
The injector is used to push the coiled tubing into the well or pull it to the surface.
The guiding arch (gooseneck) is used as a pathway for the coil to enter the injector.
Injectors have two chains with gripper blocks that are used to apply gentle pressure on the coil and move it uphole or downhole.
It is important to have the right type/size of gripper blocks to avoid pipe damage.
9. Coiled Tubing Reel
The reel is used to store the coiled tubing string. It is usually mounted on a trailer for easy transport.
10. Coiled Tubing Pressure Control Equipment
Since coiled tubing is often run into live wells, pressure control equipment is very important.
A stuffing box (stripper) is used to prevent the well fluids from escaping into the environment. The stuffing box is positioned just below the injector.
In addition to a stuffing box, different types of blowout preventers are also used to provide additional pressure control.
Related: Top 5 Chemicals Used for Coiled Tubing Operations
Most Common Coiled Tubing Operations
- Milling operations – usually the removal of plugs, frac ports, or other obstructions in the well
- Well preparation operations – tubing-conveyed perforation (TCP) or abrasive cutting
- Cleanouts – performed on older wells or during frac screenouts to remove sand or other deposits from the well
- Frac through coil or annular fracs – performed to stimulate the well before it is put on production
- Acid stimulations – used to remove the formation damage or to stimulate the wellbore with an acid
Related: How Coiled Tubing Milling Operations Are Performed?
What is a Typical Coiled Tubing Design?
A coiled tubing string is made up of smaller CT pieces that are welded together to create a longer string.
The length of the coiled tubing string that can be spooled on a reel depends on the reel size and in some cases over 10,000 m of coiled tubing can be stored on a reel.
However, many roads in North America have the maximum allowable weights for the equipment that can travel.
These maximum road limits usually vary depending on the season and this can affect how much pipe can be transported on a reel.
Coiled tubing string can either be monowall or tapered.
Monowall as the name suggests means that the wall thickness of the coiled tubing is the same for the entire length of the string.
Monowall strings are easier to manufacture and usually cheaper than tapered strings.
On the tapered strings, on the other hand, coiled tubing wall thickness changes.
Most tapered strings are designed to have a heavier wall (thicker) at the top and lighter (thinner) at the bottom.
Tapered coiled tubing is often used on deeper long lateral wells.
Related: Coiled Tubing Fatigue
What Chemicals are Used for Coiled Tubing Operations?
• Biocides – used to kill the bacteria that can potentially damage coiled tubing by causing corrosion.
• Metal to metal friction reducers – used to decrease coil/casing friction. This allows coiled tubing to get deeper in the well.
• Hydraulic friction reducers – used to decrease pump pressures. This increases the life of the coil and saves fuel.
• Inhibitors – H2S or corrosion inhibitors are used to prevent damage to the coil.
• H2S Scavengers – similar to H2S inhibitors scavengers are used to prevent coil damage due to H2S exposure.
• Gels – pumped as viscous slugs to assist with wellbore cleaning
Related: Top 5 Chemicals Used for Coiled Tubing Operations
Coiled Tubing Simulations
Most coiled tubing companies use some type of simulation software to figure out what is required to perform a coiled tubing operation.
Using a software simulator allows companies to select the appropriate equipment for the job.
The main goal of the coiled tubing simulator is to predict what conditions will be required for the job to be successful.
Typically data required to run coiled tubing simulations is a wellbore deviation survey, casing configuration, wellhead pressure, fluid types, and pumping rates.
Based on these inputs, a good coiled tubing simulator will tell you:
- How far can you reach with the current coiled tubing size
- The maximum pick-up/set-down force you will have at the specific depth
- The weight on bit (WOB) you will have at your target depth
- The circulating pressures
- The combined downhole flow rates when pumping fluids and nitrogen simultaneously
Read next: What is Wireline in Oil and Gas?
Here is the list of all the inputs required by most coiled tubing simulation programs:
- Well survey (depth, inclination, and azimuth)
- Well geometry (size of casing/tubing/liners used)
- Coiled tubing string information
- BHA info (optional for Orpheus)
- Type of the fluid used (freshwater, nitrogen, comingled fluid)
- CT Speed (average coiled tubing speed)
- Tool pressure drop (pressure drop in the BHA if it was used for the simulation)
- Wellhead pressure
- Stripper friction
- Reel back tension
- Friction factors
- Fluid density in CT
- Fluid density in an annulus
- Liquid flow rate
Read next: 18 Most Common Coiled Tubing Applications