Breaking down the coil tubing operation
The easiest way to understand coiled tubing is to separate the components of the operation and understand what function each part plays in the overall operation. There are four main parts of a coiled tubing operation: injector head, reel, control cabin, and the coiled tubing.
The injector head is the workhorse. Due to its weight, the injector head is suspended, usually by crane, at the top of the wellhead. The injector head is responsible for holding and regulating the speed at which the tubing goes in or out of the hole. Hydraulic motors, controlled by the operator in the control cabin, operate the injector head. The grip maintained on the coil enables the injector head to “snub in” or force a pipe or tubular into a well against wellbore pressure.
The reel holds the coiled tubing. The tubing reel operates similarly to a garden hose reel. Like the swivel that connects a water hose from a spigot to the hose on the reel, coiled tubing reels have a high-pressure swivel. The swivel connects the high-pressure line from the pump truck to the end of the coil which rotates around the reel. Spooling up the coil is an important part to fitting the entire string back on the reel. This process is accomplished with the assistance of the levelwind, guiding the tubing on and off the reel.
The control cabin contains the gauges and knobs necessary for the coil operator to monitor and adjust the equipment as the job progresses. The operator keeps his eye on the weight gauge, located front and center in the console, any time the tubing is moving. Too much weight down can cause the tubing to buckle. Pulling too much can cause the tubing to part.
The coiled tubing operation can’t be completed without the coiled tubing itself. Not every string of coiled tubing is the same. It is made in several different sizes and varying thicknesses; the most common sizes range from 1¼” to 2⅜” inch, referring to the outer diameter (OD) of the tubing. With the deep wells and long laterals, tapered strings have become commonplace. A tapered string is thicker at the up-hole end and thinner at the down hole end while keeping a constant outer diameter. Tapered strings are made possible by a special process when the coiled tubing is built.
Understanding the manufacture of coiled tubing
Coiled tubing strings are quite long, often in the 20,000-foot range. The string is not one continuous piece of steel, but rather a series of steel pieces called strips. These strips of flat steel are about 2,000 feet in length and are made in a variety of thicknesses and widths. The strips are then welded together at 45-degree angles to minimize stress at the weld points. The welds are then x-rayed and inspected for any imperfections. As the strips are welded together, they are rolled onto a wooden spool creating a continuous strip that will make up the string. The flat string is then fed into a machine where it runs through a series of rollers to form it into the final circular tube shape. The outside of the weld is cut off leaving a smooth finish on the outside. The final step takes the coiled tube through a cooling bath before it is spooled onto a temporary reel where it remains until it is ready to be spooled onto a coil unit. Coiled tubing is currently only manufactured in three plants in the world, located in Houston, Texas.
The age of a coiled tubing string is a critical part of the coiled tubing operation. After several uses, the string will experience plastic deformation, meaning the metal is bent so far it won’t return to its original shape. (Think of bending a paperclip in half. Continuing to bend that paper clip it eventually breaks.) Through years of research, programs have been developed to track the fatigue of a coiled tubing string.
Advantages of coiled tubing services
One of the greatest advantages of using coiled tubing over other services is its ability to work under pressure. If a well is producing, the well does not have to be stopped or killed to perform a coiled tubing operation, unlike a workover rig. Since coiled tubing has a constant OD, the seal around the coil is never broken. When the well head pressure starts to rise, pressure is applied from the control cabin, forcing the elements to be compressed to form a tighter seal against the coil to prevent the pressure from leaking.