Coiled tubing is a very long, continuous metal pipe which is spooled on a large reel. The pipe diameter varies from 0.75 to 4.5 in and the operating length ranges from 2,000 -17,500 ft. It is used for well intervention and drilling or as production tubing in depleted gas wells. It can be used to convey the perforation and wireline tools with the advantages of ability to push it into the hole and ability to pump fluids through the coil. Coiled tubing equipment consists of reel for Storing and protecting the CT string, injector to apply the force for running CT, well control equipment (WCE) to control the well pressure, power pack to generate hydraulic power, acquisition system to record the job parameter and BHAs .
Coiled Tubing Applications
The most common use of coiled tubing is circulation. In the killed well the hydrostatic pressure created by the column of fluid in the wellbore inhibits flow of the formation fluid. The solution would be using a gas, mostly nitrogen. The procedure is running the coiled tubing into the bottom of the hole while pumping the nitrogen gas which displace out the kill-fluid and consequently the hydrostatic head is reduced and the formation fluid starts flowing. Once the well is capable of flowing, the coiled tubing is pulled out of the well and the production will be resumed. This is called nitrogen lift or nitrogen kickoff.
Coiled tubing is used for pumping operations such as, clean out borehole, cementing, acid stimulation, reverse circulation, pressure control, and a variety of other operations. Coiled tubing cleanout is used for removing debris and particulates from boreholes. Cleaning fluid is pumped with abrasive substances if required. Coiled tubing can be used for reverse circulation operation in the large casings with some wellbore restriction. Coiled tubing is also used for acid stimulation. CT is ran into the wellbore and placed across the perforated intervals, and then acid is pumped. CT acid stimulation has many advantages. Since the acid is pumped through CT, it does not get contaminated with borehole debris. Furthermore, the protection of production tubing is not the serious concern because the live acid does not contact the wellbore. Cementing is another operation which can be performed by CT. Cement balanced plug can be placed very accurately in abandoned or recompleted wells. Squeeze cementing which is the remedial cementing technique to correct the issues with primary cementing can be executed through coiled tubing.
Logging and perforating
In the horizontal or highly deviated wells where the gravity is insufficient, the coiled tubing provides additional logging and perforating capability and flexibility over other carrying systems. The strength and stiffness of coiled tubing allows enduring greater compressive and tensile stresses.
Coiled Tubing Drilling
Coiled tubing drilling is the practice of using coiled tubing with downhole hydraulic motor to rotate the bit for drilling. Since the coiled tubing is the long continuous pipe, the modern drilling technique has the advantage of requiring less time for tripping in and out of the well compared to conventional drilling systems. Coiled tubing drilling can be classified into two main categories including directional and non-directional wells. Also, CTD can be further divided into under-balanced and over-balanced drilling. Stripper provides a hydraulic seal around the coil tubing string which offers higher well control than conventional drilling with drill pipe.
In shallow gas wells where the reservoir pressure may not be enough to overcome the hydrostatic pressure of the fluid, coiled tubing is the appropriate solution. The narrow inside diameter of the pipe causes the fluid to move faster. This higher velocity assists liquid to come to the surface. Otherwise accumulation of the liquid in the wellbore kills the well.