Coiled Tubing and Drill String Assemblies

Coiled tubing is a long metal pipe that is usually 1 to 3.25 in (25 to 83 mm) in diameter and supplied spooled on a reel. It is used for interventions in oil and gas wells, often as production tubing in depleted gas wells.

It has several advantages over wireline such as the ability to pump chemicals through it, and being self-contained. It also allows a lot more control over the hole than drill pipe and can be used for operations such as circling or deliquification.

Coiled Tubing

Coiled tubing is a type of pipe that is used in many well interventions, such as drilling, completions, and workovers. It is a continuous metal pipe that is welded together to form a long string of tube. This is then spooled onto a reel and transported to the site of the operation.

The advantages of coiled tubing over other well intervention methods include its ability to circulate treatment fluids in the hole, its strength and rigidity, and the fact that it does not require the use of wireline techniques. This makes coiled tubing a safer and more time-efficient method of working on a well.

There are several different sized coiled tubing strings available, depending on the needs of a particular job. These can range in length from 2,000 ft to more than 30,000 ft (600 m to 9,000 m).

Some coiled tubing strings are monowall or tapered and have an outside diameter of 0.75 – 4 inches, whereas other string sizes may be larger with an outside diameter of 4.5 – 2 in. These types of tubes are produced by using plates that are rolled into a cylindrical shape, then welded together to form the string of pipe.

Typically, a coiled tubing unit contains a control cab and a reel trailer, both of which are usually on the same trailer. The control cab is where the operator sits and controls the operation with hydraulic or electronic equipment. The trailer can be small or large, depending on the size of the coil rig and how much pipe it has to haul.

A coiled tubing system may also be outfitted with a specialized drill, which is used to drill through fines and paraffin, which can block the tubing. This is done Coil Pipe Assemblies to enable the insertion of fluids to dissolve plugs and scale.

Another common use of coiled tubing is to convey well logging tools from the surface down into a wellbore and past restrictions. This can be done by pulling the logging equipment down through the pipe, or by pushing it through the well.

Coiled tubing is a great alternative to wireline, particularly for highly deviated and horizontal wellbores. It also enables the logging crew to fish for lost equipment, which can be very difficult using wireline methods.

Injector Head

The Injector Head is one of the most critical components of a coiled tubing unit. It incorporates special profiled chain assemblies to grip the coiled tubing string from the reel, and a hydraulic drive system that provides tractive effort for running and retrieving the coiled tubing string from the well bore.

The main injector frame 19 includes a number of gripper block assemblies 31 which include plier-like halves 33, 35 joined at the rear at a pivot point 37. These gripper block assemblies are rotated between an open, gripping position and a closed, disengaged position as the tubing is slid through the vertical run 29 of the main injector frame.

These gripper block assemblies are driven by a drive means located on the main injector frame and preferably comprise a chain drive. A drive sprocket 41 is driven by a hydraulic motor 43 in conventional fashion and the chain is carried along between the drive sprocket and an idler sprocket 47 mounted within the main injector frame.

As the chain is shifted by the drive sprockets, the gripper blocks are rotated by the drive means and push against the coiled tubing. A weight indicator and stripper / packer are bolted directly underneath the drive chains and typically include a load cell that senses the tubing’s weight.

When the weight of the coiled tubing is increased by overpull from the coiled tubing injection or when the tubing is compressed downhole due to hole friction or downhole obstruction, a deflection is detected by the load cell and the weight indicator needle is pushed against the bottom of the load cell. This deflection will give an indication of the pressure on the coiled tubing in the wellbore and will allow the operator to make adjustments in the flow rate of the coiled tubing injector.

Another important benefit of the injector head is that it can be removed from a string of coiled tubing to allow personnel to access the bottom end of the coiled tubing without moving the coiled tubing off the working platform 14. This allows assembly or disassembly of a bottom hole assembly on the downhole end of a coiled tubing string while the coiled tubing is spooled onto a tubing reel and inserted into the coiled tubing injector head.

Bottom Hole Assembly

The lower portion of the drillstring, consisting of (from the bottom up in a vertical well) the bit, bit sub, mud motor, stabilizers, drill collar, heavy-weight drillpipe, jarring devices (“jars”) and crossovers for various threadforms, must provide force for the bit to break the rock (weight on bit), survive a hostile mechanical environment and provide the driller with directional control of the well. It also usually includes mud motors, directional drilling and measuring equipment, measurements-while-drilling tools, logging-while-drilling tools and other specialized devices.

The design and placement of each component in the drillstring affects the bit’s trajectory, thereby affecting the directional tendency of the wellbore. A drill string’s directional behavior is determined by several factors, including the direction and magnitude of the bit’s side force; these forces are influenced by a number of other factors, such as the formation type, drilling fluid added mass, fluid pressures, borehole inclination, weight on bit and the layout of stabilizers.

Stabilizers are used to maintain the angle of the borehole and reduce bottomhole vibration. They can be used in a wide variety of applications, such as water wells, gas wells and oil wells.

Depending on the application, the driller or drilling engineer will determine what stabilizers are required for their specific requirements. For example, float subs are commonly used when drilling water wells and cross over subs are often used in gas and oil wells.

There are also a number of other components that can be used to increase the performance of the bottom hole assembly. These can include shock absorbers, which are designed to reduce or eliminate vertical oscillations of the drillstring.

In addition to the above, BHAs can be designed to be steerable; these are called rotary assemblies and they can be controlled from the surface. Unlike a dropping assembly, which contains only one stabilizer, a rotary assembly can have additional stabilizers run higher on the drillstring to control bit tilt and build rates.

The first stabilizer in a rotary assembly is generally placed 30-45 feet behind the bit; this allows the drill string to hang a little while gravity pulls the bit toward the low side of the borehole. Once this is reached, the bit tilts up and builds angle.

Tool String

The Tool String is an assembly of wireline tools connected to a wireline that are used to deliver surface controlled impacts (jar action) upwards or downwards, to manipulate devices within the well bore. It includes Coil Pipe Assemblies a Rope Socket, Stem / Weight Bar, Spang Link / Mechanical Jar, Knuckle Joint, Running / Pulling Tool and other devices that are necessary for efficient surface control during running and pulling operations on wireline / slickline.

Tool strings are made of different materials and designed to perform a wide range of functions. These include delivering surface controlled impact (jar action) to drill collars, sondes, and other downhole hardware, as well as powering steering tools that enable measurements-while-drilling operations in a well.

In addition, tool strings are sometimes used to remove fill material from a well bore. This method is called reverse circulation. It is a common technique for removing large amounts of frac sand from a well bore, and it can be effective in some applications.

However, a number of drawbacks can occur during reverse circulation, including insufficient steering accuracy and low borehole quality. Therefore, reverse circulation is not always the best choice for every situation.

One way to solve these drawbacks is to develop a more compact tool string that can be used in more difficult well conditions. These string designs can be created by removing connections between components, or engineering more compact mechanisms.

The benefit of this is that the tool string can be used in deeper wells without sacrificing the effectiveness and safety of the system. These string designs are also more affordable.

In addition, these string designs are less likely to fail during operation because they are made from lower carbon steel. Additionally, they are able to withstand abrasion and corrosion in harsh environments. These benefits make them a valuable tool in many downhole drilling applications.